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Mars' polar ice deposits are thought to preserve a record of climate throughout their evolution. In addition to the large north polar layered deposits (NPLD) at Mars' north pole, smaller ice deposits are preserved in craters nearby. These outlying deposits were potentially formed by the same mechanisms that drive NPLD formation, or may represent more local mechanisms. Distinguishing between these possibilities would help elucidate the spatial homogeneity of Martian climate processes. Here, we analyzed SHARAD radar depth profiles from 34 locations across the NPLD and 5 locations within the Korolev crater ice deposit using Fourier transform analysis and dynamic time warping to quantitatively assess the similarity between the internal layered stratigraphy of the two deposits. We identify broad stratigraphic similarities between the Korolev deposit and the NPLD, suggesting they likely formed due to the same climate forcing mechanism, with local variability also observed across the NPLD. Plain Language Summary: Mars has two large ice caps at its poles, which combined contain a similar volume of ice to Greenland on Earth. Near these large ice caps are craters that are also filled with ice, which may or may not have formed due to the same mechanisms that formed the large neighboring ice cap. Ground penetrating radar observations of ice on Mars can allow us to see layers of ice and dust that are present throughout the interior of these deposits, and represent climate events that have taken place during the deposit's formation and evolution. We analyze radar observations from two deposits near Mars' north pole and quantitatively show that these deposits have a similar pattern of layering. These results could indicate that they have shared a similar climate history, therefore implying the importance of regional‐scale climate processes on Mars in addition to local processes for forming these ice deposits. Key Points: We use Fourier transform analysis and dynamic time warping to assess the similarities between two ice deposits near Mars' north poleWe identified a periodic signal with an average wavelength of ∼45 m in radar observations of the ice mound in Korolev crater and the NPLDWe identify similar broad climate forcing for both Korolev and NPLD ice, with local variability across the NPLD also observed [ABSTRACT FROM AUTHOR]

The evolution of charge structure involved in lightning discharge of a thunderstorm over the central Tibetan Plateau is investigated for the first time, based on the data from very high frequency interferometer, radar and sounding. During the developing‐mature stage, the TP thunderstorm exhibited a tripolar charge structure evolved from an initial inverted dipole. At the mature stage, a bottom‐heavy tripole charge structure is clearly presented, with a strong lower positive charge center (LPCC) at temperatures above −10°C, a middle negative charge region between −30°C and −15°C, and an upper positive charge region at T < −30°C. As the LPCC was depleted, the charge structure evolved into a normal tripole with a pocket LPCC. The merging between different convective cells resulted in the formation of two adjacent negative charge regions located directly and obliquely above the LPCC, and horizontally arranged different charge regions were simultaneously involved in the same lightning discharge. Plain Language Summary: Tibetan Plateau thunderstorms usually exhibit special convective structures. Using the data from the accurate lightning VHF interferometer, electric field mill, fast/slow antenna and C‐band radar, the evolution of the charge structure of thunderstorms and their influence on lightning discharges are investigated. Our observation for the first time revealed the charge structure evolution of the central‐TP thunderstorm which involved in the lightning discharge, exhibiting a bottom heavy tripole charge structure with a large LPCC in the mature stage evolved from an initial inverted dipole and the usual tripole in the dissipating stage of the thunderstorm. Under different magnitudes of the LPCC, different types of lightning discharges including ‐IC, +IC and ‐CG flashes were generated, indicating the crucial effects of LPCC on the lightning discharge types. Key Points: The charge structure of the TP thunderstorm evolves from an initial inverted dipole to a mature stage tripole with a strong LPCCHorizontally distributed negative charge zones from cell merger are simultaneously involved in the discharge of a single lightning flashDifferences in the relative magnitude of LPCC leads to various types of lightning discharges [ABSTRACT FROM AUTHOR]

Monitoring the generation and movement of equatorial plasma bubbles (EPBs) in a large longitude region is crucial important for better understanding their day‐to‐day variability. Using the newly developed Low lAtitude long Range Ionospheric raDar (LARID) at Dongfang (19.2°N, 108.8°E, dip lat. 13.8°N), China, an extremely long‐range experiment for observing EPB irregularities in a range of ±9,600 km to the radar site was first carried out. The results show that EPB irregularities with ranges up to 7,000 and 9,500 km were observed by the east and west beams of LARID, respectively. By incorporating simultaneous observations from GNSS receiver and ionosonde networks, it is demonstrated that the EPBs generated from post‐sunset to sunrise over a very wide longitude of ∼140°, from Pacific to Africa could be observed by LARID. The results, for the first time, demonstrate the possibility for tracing global EPBs in real time using a few low latitude over‐the‐horizon radars. Plain Language Summary: Equatorial plasma bubble (EPB), which can cause severe ionospheric scintillation, is an important space weather phenomenon. The occurrence of EPBs exhibits complex longitude variation characteristics. Due to the fact that most of the equatorial and low latitude region is covered by ocean, it is challenging to monitor the generation and movement of global EPBs. Recently, an over‐the‐horizon (OTH) radar at low latitude, that is, the LARID, has been built for observing EPB irregularities. However, it is not clear that how far an OTH radar at low latitude can observe irregularities. This would be very important in the design of a low latitude OTH radar network for tracing global EPB irregularities. To address this issue, an extremely long‐range experiment covering a wide longitude of about 180° was performed for the first time with LARID. The successful observation of EPB irregularities from Pacific to Africa sectors demonstrates the possibility of monitoring the complex longitudinal variations of EPBs by an OTH radar, even during geomagnetic storms. The results provide meaningful insight for building a low latitude OTH radar network in future, that consists of three to four OTH radars could have the capability to obtain global EPBs in real time. Key Points: First extremely long‐range experiment for observing equatorial plasma bubbles over a large longitude was conductedEquatorial plasma bubbles with ranges as far as 9,500 km were successfully observed by an over‐the‐horizon radarThe results demonstrate the capability for tracing global equatorial plasma bubbles using a few low latitude over‐the‐horizon radars [ABSTRACT FROM AUTHOR]

Subglacial lakes have been mapped across Antarctica with two methods, radio‐echo sounding (RES) and ice‐surface deformation. At sites where both are coincident, these methods typically provide conflicting interpretations about the ice‐bed interface. With a single exception, active subglacial lakes identified by surface deformation do not display the expected flat, bright, and specular bed reflection in RES data, characteristic of non‐active lakes. This observational conundrum suggests that our understanding of Antarctic subglacial hydrology, especially beneath important fast‐moving ice streams, remains incomplete. Here, we use an airborne RES campaign that surveyed a well‐characterized group of active subglacial lakes on lower Mercer and Whillans ice streams, West Antarctica, to explore inconsistency between the two observational techniques. We test hypotheses of increased scattering and attenuation due to the presence of an active subglacial lake system that could suppress reflected bed‐echo power for RES observations in these locations, finding that entrained water is most plausible. Plain Language Summary: The bottom of an ice sheet is insulated from cold air temperatures, often warm enough to melt and pond liquid water into lakes. These lakes beneath the ice sheet have been identified by two independent measurements, first with radar methods and second with changes in height of the ice surface (altimetry). Interestingly, the two methods rarely identify the same lakes: radar generally detects lakes in the ice‐sheet interior, whereas altimetry detects active lakes near the ice‐sheet margins that fill and drain within the time series of repeated measurements (∼years). In this study, we investigate a group of active subglacial lakes at which both radar and altimetry data sets are available. We demonstrate that the radar returns from active lake reflections are much dimmer than expected based on non‐active lake signatures and investigate the physical processes controlling those dim reflections. We argue that water moves into the ice when the lake fills or drains and that is the most plausible explanation for the observational discrepancy. Key Points: Active subglacial lakes, identified by surface deformation, do not create the expected bright and specular radar reflectionEntrained water in basal ice suppresses radar power by scattering and attenuation, and it also likely alters the basal ice mechanicsUnderstanding the radar expression of subglacial water on Earth provides context for investigations of subsurface water on planetary bodies [ABSTRACT FROM AUTHOR]

Estimates of radar attenuation in the shallow Martian subsurface are retrieved from RIMFAX soundings along the Perseverance rover traverse. Specifically, analyzed data is from the Hawksbill Gap area during the rover's first drives onto the Jezero Western Fan Front. The centroid frequency‐shift method is employed to quantify attenuation in terms of the constant‐Q approximation. Results are then compared with the amplitude decay method, which—in order to calculate attenuation—requires propagation velocities retrieved from radargram analysis. By verifying that results from two separate analyses are consistent, we ensure that quantified radar properties are well constrained. First estimate of constant‐Q is 78.8 ± 11.6. For a subsurface propagation velocity of 0.113 m/ns, that equals an attenuation of −2.1 ± 0.4 dB/m at the RIMFAX 675 MHz center frequency. Results are consistent with dry sedimentary rocks, and are distinguishable from the magmatic lithologies on Jezero Crater Floor. Plain Language Summary: This study presents first estimates of radar attenuation at the Jezero Western Fan Front. Measurements were made with the RIMFAX payload instrument on the Mars 2020 Perseverance rover mission, acquired along the rover drive path. Results indicate low signal losses in the subsurface that are consistent with dry sedimentary rocks, as observed on the surface by other payload instruments. Maximum imaging depths increase compared to imaging over magmatic lithologies on Jezero Crater Floor. By using separate methods of analysis (the centroid frequency‐shift method and the amplitude decay method), we reliably quantify attenuation and maximum penetration depths at the Western Fan Front, and observe differences to the Crater Floor lithologies. Key Points: The first estimate of radar attenuation at the Jezero Western Fan Front is on average −2.1 dB/mFor an average propagation velocity of 0.113 m/ns, returned power and time‐frequency analyses yield similar resultsRadar properties are consistent with dry sedimentary rocks and are distinguishable from the magmatic lithologies on Jezero Crater Floor [ABSTRACT FROM AUTHOR]

A THz radar, with its wide bandwidth, is capable of high‐resolution imaging down to the centimeter scale. In this study, a THz radar is applied to detect hydrometeors generated in a spray chamber. The observed backscattering signals show fluctuations at centimeter scales, indicating various hydrometeor distribution patterns along the radar beam. A co‐located High‐Speed Imaging (HSI) sensor is used to measure the Drop Size Distributions (DSD) in the spray chamber. The radar sampling beam is well aligned with the HSI probes, allowing an objective comparison between the remote sensing and in situ observations. In this study, the observed radar power is compared with the power estimated from the HSI measurements. Results show great consistency, with power difference smaller than 0.5 dB. This study demonstrates the feasibility and great potential of using a THz radar for ultra‐high‐resolution observations of clouds in a laboratory facility, and in the real atmosphere. Plain Language Summary: An improved understanding of how precipitation forms in clouds calls for new observational instruments and novel measurement strategies. Radars operating at ultra‐high frequency (a.k.a. THz radars) are known for their unprecedented resolution down to centimeter scale. Here, for the first time, we applied a THz radar to detect hydrometeors generated in a spray chamber. An additional in situ probe is utilized within the chamber to validate the THz radar observations. The radar beam is directed through the same volume that is sampled by the in situ probe, thus enabling an objective comparison between the in situ and remote sensing measurements. Different hydrometeor distributions are generated in the chamber, each being jointly observed by the radar and the in situ probes. Results show that the THz radar measurements agree well with the radar power calculated from the in situ observations. This unique experiment indicates promising scenarios of applying THz radar to cloud and precipitation studies in a laboratory facility and in the real atmosphere. Key Points: A novel experiment is conducted by applying a THz radar to detect hydrometeors generated in a spray chamberGood agreement between in situ and radar observations demonstrates the unique capability of THz radar for cloud and precipitation studiesImplications of applying THz radar to design novel observational strategies for solving fundamental gaps in cloud physics are demonstrated [ABSTRACT FROM AUTHOR]

Tornadoes, as highly destructive weather events, require accurate detection for effective decision‐making. Traditional radar‐based tornado detection algorithms (TDA) face challenges with limited tornado feature extraction capabilities, leading to high false alarm rates and low detection probabilities. This study introduces the Multi‐Task Identification Network (MTI‐Net), leveraging Doppler radar data to enhance tornado recognition. MTI‐Net integrates tornado detection and estimation tasks to acquire comprehensive spatial and locational information. As part of MTI‐Net, we introduce a novel backbone network of Multi‐Head Convolutional Block (MHCB), which incorporates Spatial and Channel Attention Units (SAU and CAU). SAU optimizes local tornado feature extraction, while CAU reduces false alarms by enhancing dependencies among input variables. Experiments demonstrate the superiority of MTI‐Net over TDA, with a decrease in false alarm rates from 0.94 to 0.46 and an increase in hit rates from 0.23 to 0.81, highlighting the effectiveness of MTI‐Net in handling small‐scale tornado events. Plain Language Summary: Tornadoes, highly destructive small‐scale weather phenomena, demand accurate detection for informed decision‐making. Although meteorological radars are commonly utilized for tornado identification, current methods often suffer from false alarms or missed detections due to radar noise. In this study, we introduce the multi‐task learning‐based identification network (MTI‐Net), which not only enables tornado detection but also estimates tornado counts within radar data. We integrate Convolutional Neural Networks (CNNs) with Transformer techniques to enhance the model's ability to capture tornado information. CNNs detect local details using filters, while Transformers manage global connections through attention mechanisms. A series of experiments demonstrate significant improvements in tornado detection with MTI‐Net compared to traditional methods. Key Points: A tornado dataset with detailed radar features was created over China from 2017 to 2023Multi‐task learning was designed to simultaneously infer tornado detection and tornado number estimationIntegration of spatial and channel attention units can better extract tornado features from radar data [ABSTRACT FROM AUTHOR]

Machine learning‐based approaches demonstrate a significant potential in radar quantitative precipitation estimation (QPE) applications. In contrast to conventional methods that depend on local raindrop size distributions, deep learning (DL) can establish an effective mapping from three‐dimensional radar observations to ground rain rates. However, the lack of transparency in DL models poses challenges toward understanding the underlying physical mechanisms that drive their outcomes. This study aims to develop a DL‐based QPE system and provide a physical explanation of radar precipitation estimation process. This research is designed by employing a deep neural network consisting of two modules. The first module is a quantitative precipitation estimation network that has the capability to learn precipitation patterns and spatial distribution from multidimensional polarimetric radar observations. The second module introduces a quantitative precipitation estimation shapley additive explanations method to quantify the influence of each radar observable on the model estimate across various precipitation intensities. Plain Language Summary: Ground radars can provide continuous spatial observations over large areas with high spatiotemporal resolutions, so they form the infrastructure for precipitation monitoring and observation in many countries. Recently, deep learning (DL) techniques have shown great potential for use in polarimetric radar‐based precipitation estimates. Nevertheless, the black‐box and turn‐key characteristics of DL models make it difficult for researchers to understand the model decision‐making process and cast doubt on the reliability of the model results. This study introduces a physically explainable polarization radar‐based quantitative precipitation estimation (QPE) system built on DL technology that can explain the causes of the precipitation estimates provided by deep learning models under different rainfall amounts. An experiment indicates that our model achieves better estimates than the conventional methods. Furthermore, the explainability methodology allows for visualization of the microphysical precipitation information. Being the initial attempt to apply explainability learning in the QPE domain, the explainability results may offer valuable guidance for rainfall estimation. Key Points: A polarimetric radar‐based rainfall estimation system is developed using deep neural networksThe deep learning‐based rainfall estimates generally outperform products derived from traditional parametric relationsThe proposed deep learning interpretation method can provide physical and statistical explanations of the model decision‐making process [ABSTRACT FROM AUTHOR]

Previous theoretical simulations showed the generation of double‐layer structures of E‐region field‐aligned irregularities (FAIs). In this study, we report the double‐layer structures of E‐region FAIs observed by an all‐sky radar at low latitude Ledong (18.4°N, 109°E), China. These FAIs appeared at the altitudes ∼90 and 110 km respectively. Both layers displayed quasi‐periodic patterns with synchronized spatial features, that is, being manifested as spatially separated patches or wavelike structures over similar longitudes at the two layers simultaneously. The neutral wind observations revealed the existence of wind shears at two separated altitudes where the double FAI layers occurred. The two wind shears created two vertically separated sporadic E layers and possibly drove the generation of the double‐layer FAIs via gradient drift instability. The synchronized spatial features of the FAIs at the two layers could be modulated by gravity waves. Plain Language Summary: The E‐region field‐aligned irregularities (FAIs) were observed usually in a form of single layer. Due to the limited field of view of narrow‐beam radars which were traditionally employed for observing ionospheric irregularity, the spatial morphology and evolution of E‐region FAIs in a small region were resolved. Owing to the recently developed capability of all‐sky radars in observing E‐region irregularities, the spatial features of E‐region irregularities over a larger zonal region up to hundreds of kilometers in the horizontal plane can be derived. Further, the spatial features of E‐region irregularities in the vertical direction can be unambiguously determined. Utilizing an all‐sky radar at low latitude Ledong, China, a case of double‐layer structures of E‐region FAIs, which occurred at ∼90 and 110 km altitudes respectively, was observed. The double layers of FAIs spanned a zonal coverage ∼250 km, showing as synchronized spatially separated quasi‐periodic patches or wavelike structures in two layers. Especially, the wavelike structures synchronized in the two layers could provide solid evidence for the crucial role of gravity waves in modulating E‐region irregularities. In combination with the collocated ionosonde Es and neutral wind observations, the physical mechanisms responsible for the generation of the double‐layer E‐region FAIs were discussed. Key Points: Double layer structures of E‐region field‐aligned irregularity (FAI) were observed at low latitude by an all‐sky radarBoth layers of FAIs were quasi‐periodic type and synchronously manifested as spatially separated patches or wavelike structuresThe double‐layer FAIs were likely generated by wind shear driven gradient drift instability and modulated by gravity waves [ABSTRACT FROM AUTHOR]

The morphology and motion of auroras have been widely studied due to their indications on magnetospheric processes. Here, we report a new kind of "auroral curls," which have wavelengths in the mesoscale (∼100 km) and propagate azimuthally. Utilizing data from the Chinese Antarctic Zhongshan Station (the all‐sky imager and the high‐frequency radar), the Active Magnetosphere and Planetary Electrodynamics Response Experiment and the Defense Meteorological Satellite Program, we analyze an event occurred on 23 April 2019. We find these curls are fine structures in the poleward boundary of multiple arcs. Corresponding field‐aligned currents manifest as a series of longitudinally arranged pairs, while ionospheric flow velocities nearby oscillate with periods in the Pc 5 band. Observational evidence suggests these curls are connected with ultra‐low frequency (ULF) waves, which opens the possibility of using auroras to globally image ULF waves. Plain Language Summary: Auroras caused by precipitation of magnetospheric particles contain information about physical processes happened in the magnetosphere. In this letter, we report a new kind of auroral dynamic forms observed in Antarctica. These structures present both spatial and temporal periodic characteristics, which have similar scales with those of magnetospheric ultra‐low frequency (ULF) waves. We propose these auroral forms are connected with ULF waves, which provides a potential method to globally image ULF waves by analyzing properties of these auroras. Key Points: Azimuthally propagating "auroral curls" with mesoscale wavelengths were observed in AntarcticaThese curls are fine structures in the poleward boundary of multiple arcs formed by longitudinal‐arranged field‐aligned current pairsIonospheric flow velocities nearby oscillate with periods in the Pc 5 band, indicating connections with ultra‐low frequency waves [ABSTRACT FROM AUTHOR]

An exceptionally strong westward propagating quasi‐6‐day wave (Q6DW) with zonal wavenumber 1 in connection with the rare 2019 Southern Hemispheric Sudden Stratospheric Warming (SSW) is observed by two meteor radars at 30°S and is found to modulate and interact with the diurnal tide and gravity waves (GWs). The diurnal tide is amplified every 6 days and a prominent 21 hr child wave attributed to Q6DW‐diurnal tide nonlinear interaction occurs. Q6DW modulation on GWs is confirmed as the 4–5 day periodicity in GW variances. Simultaneously, the Q6DW appears to shift its period toward the periodicity of the modulated GW variances. Enhancement is also observed in the first results of meteor radar observed Q6DW Eliassen‐Palm flux, which may facilitate the global perturbation and persistence of this Q6DW. We conclude that the observed SSW triggered Q6DW‐tide and Q6DW‐GW interactions play an important role in coupling the lower atmospheric forcings to ionospheric variabilities. Plain Language Summary: Our work provides observational evidence for the 6‐day planetary wave‐tide and 6‐day planetary wave‐gravity wave interactions at the Earth's mesosphere and lower thermosphere. The results strongly support the theory that wave‐wave interactions are the primary mechanism coupling planetary waves to ionospheric variability and provide an additional mechanism as the 6‐day wave modulation on the gravity waves. We utilize measurements from two meteor radars to diagnose planetary wave characteristics and identify wave‐wave interactions, and compute the first‐time meteor radar observed Eliassen‐Palm flux. Enhancement is observed in the Eliassen‐Palm flux of 6‐day wave following the SSW maximum phase, which demonstrates that energy of the 6‐day wave is enhanced and therefore, facilitates the global perturbation and persistence of the 6‐day wave for an extended time period. While meteor radar observations are widely used to investigate planetary waves and tides, high meteor detection rate is required for further studying temperature perturbations and small scale waves (e.g., gravity waves). Thus, this work also highlights the capability of a modern multi‐static meteor radar system, Chilean Observation Network De meteOr Radars, in resolving oscillations of small spatial scales over a broad range of periods, and for calculating Eliassen‐Palm flux of planetary waves. Key Points: A dominating W1 Q6DW is observed at 30°S and its Eliassen‐Palm flux is enhanced during the 2019 SH SSWQ6DW amplifies the diurnal tide every 6 days and a strong 21 hr child wave is observedQ6DW modulates the gravity wave variances and its frequency appears to shift accordingly [ABSTRACT FROM AUTHOR]

Turbulence in clouds is known to enhance particle collision rates, as widely demonstrated for warm rain formation. A similar impact on ice growth processes is expected but a solid observational basis is missing. A statistical analysis of a 15‐month data set of cloud radar observations allows for the first time to quantify the impact of turbulence on ice aggregation and riming in Arctic low‐level mixed‐phase clouds. Increasing eddy dissipation rate (EDR), from below 10−4 to above 10−3 m2 s−3, yields larger ice aggregates, and higher particle concentration, likely caused by increasing fragmentation. In conditions more favorable to riming, higher EDR is associated with dramatically higher particle fall velocities (by up to 125%), under similar liquid water paths, indicative of markedly higher degrees of riming. Our findings thus reveal the key role of turbulence for cold precipitation formation, and highlight the need for an improved understanding of turbulence‐hydrometeor interactions in cold clouds. Plain Language Summary: Liquid and frozen precipitation mainly forms by collision and subsequent aggregation of small particles. Collisions between cloud particles, such as droplets and ice crystals, are thought to be increased by turbulence. While this effect has been intensively studied for liquid‐only clouds, the impact of turbulence on ice‐ice collisional growth (aggregation) and ice‐liquid collisional growth (riming) is expected but has so far been poorly quantified. We study the effect of turbulence on aggregation and riming based on a long‐term remote‐sensing data set of low‐level clouds containing both ice and liquid particles, recorded at the Arctic site of Ny‐Ålesund, Svalbard. Cloud radar observations are used to retrieve the dissipation rate of turbulent kinetic energy (i.e., the eddy dissipation rate; EDR), which is the relevant quantity driving increases in collision rates, and to characterize ice particle properties. We find evidence that higher EDR regimes enhance the aggregation of particles, and are associated with signatures of increased ice particle concentration, possibly caused by the production of particle fragments upon collision. In temperature regimes more favorable to riming, turbulence dramatically enhances the particles' fall velocity, denoting higher degrees of riming. Our findings thus highlight a key role of turbulence for the formation of precipitable ice. Key Points: Relation between turbulence and ice growth investigated based on long‐term remote sensing data set of Arctic low‐level mixed‐phase cloudsHigher eddy dissipation rate (EDR) correlates with larger ice aggregates, and possibly higher degrees of fragmentationHigh EDR is an essential component needed for the formation of rimed particles [ABSTRACT FROM AUTHOR]

Meteor radar observations provide wind data ranging from 80 to 100 km altitude, while the Michaelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer satellite offers wind data above 90 km altitude. This study aims to generate wind profiles in the mesosphere and lower thermosphere by combining the winds derived from meteor radar and MIGHTI observations over the Korean Peninsula from January 2020 to December 2021. The wind profiles derived from the two instruments are continuous at night, but they show discrepancies during the day. The atomic oxygen 557.7 nm (green line) emission intensity measured by MIGHTI peaks at approximately 100 km during the day and 94 km at night. The vertical gradient of the airglow volume emission rate is more pronounced during the day. These differences can cause day‐night differences in the MIGHTI wind retrieval accuracy, potentially leading to discrepancies during the day. Plain Language Summary: This study aims to derive vertical wind profiles in the mesosphere and lower thermosphere (MLT) by integrating wind measurements from different techniques. Neutral winds in the MLT provide a means to study the activity of various atmospheric waves originating from the lower thermosphere and their propagation to the upper thermosphere. The Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer satellite provides wind measurements above 90 km altitude. A meteor radar in Korea provides wind data in the altitude range of 80–100 km. By combining the MIGHTI and meteor radar observations, we derive extended wind profiles in the MLT. While nighttime winds driven from two different techniques show good agreement, discrepancies exist in daytime winds. Vertical gradients in airglow intensity can affect the wind retrieval from MIGHTI's airglow measurement, and this factor can be one of the sources of daytime discrepancy. Key Points: Vertical wind profiles above 80 km are derived from meteor radar and Ionospheric Connection Explorer (ICON) observations over KoreaThese observations produce continuous wind profiles at night, but discontinuity exists between two measurements during daytimeSignificant vertical variation in airglow intensity on the dayside can impact wind retrieval from ICON airglow observations [ABSTRACT FROM AUTHOR]

Understanding the diurnal variation of horizontal wind in the atmospheric boundary layer is important for weather and climate research and wind energy applications. Here we analyze the hourly data from 91 wind profiler radar sites in China and observe that the power spectral density of horizontal wind in lower troposphere approximately follows the −5/3 power law in the mesoscale range over the ocean and coastal areas. However, in inland areas, the slopes of the power spectra are significantly greater than −5/3. We characterize the temporal and spatial variations of maximum wind speed and low level jets and find that the thermal wind effect may partially contribute to the high percentage of low‐level jets observed in the southeastern coast of China and Hainan Island. While the ERA5 reanalysis reproduces wind spectrum well for time scales >1 day, its spectrum diverges significantly from that of profiler data at shorter time scales. Plain Language Summary: Understanding how the horizontal wind changes throughout the day in the lower part of the atmosphere is important for studying weather and climate and using wind energy. In this study, we looked at data from 91 radar sites in China that measure wind every hour. We found that the pattern of how the wind changes follows a specific mathematical relationship, called a power law, where the wind decreases in a particular way with decreasing spatial and temporal scales. This pattern holds true mostly over the ocean and coastal areas, but in areas further inland, the wind behaves a bit differently. We also studied how the low‐level jet (LLJ), which are fast winds at low altitudes, change over time and space. We discovered that the difference in temperature across the region contributes to the occurrence of these LLJs in coastal areas in southeast China and Hainan Island. Finally, we compare our findings with the ERA5 reanalysis, which demonstrates excellent agreement in reproducing the wind spectrum for time scales greater than 1 day. However, the spectrum derived from the ERA5 reanalysis diverges significantly from the profiler data at shorter time scales. Key Points: Wind spectrum density from wind profiler radars over China shows a less negative slope over inland sites than the −5/3 power law over oceanTemporal and spatial variation of maximum wind speed in the lower troposphere and low level jets are characterizedWind spectrum from ERA5 reanalysis is realistic (deficient) for time scales >1 day (<1 day) compared with profiler data [ABSTRACT FROM AUTHOR]

Measuring the radar backscatter characteristics of glacier ice at different frequencies and incidence angles is fundamental to predicting the glacier mapping performance of a sensor. However, such measurements at 94 GHz do not exist. To address this knowledge gap, we collected 94 GHz radar backscatter data from the surface of Rhônegletscher in Switzerland using the All‐Weather Volcano Topography Imaging Sensor (AVTIS2) real‐aperture Frequency Modulated Continuous Wave radar. We determine the mean normalized radar cross section σmean0 $\left({\sigma }_{\text{mean}}^{0}\right)$ to be −9.9 dB. The distribution closely follows a log‐normal distribution with a high goodness of fit (R2 = 0.99) which suggests that radar backscatter is diffuse and driven by surface roughness. Further, we quantified the uncertainty of AVTIS2 3D point clouds to be 1.30–3.72 m, which is smaller than other ground‐based glacier surface mapping radars. These results demonstrate that glacier surfaces are an efficient scattering target at 94 GHz, hence demonstrating the suitability of millimeter‐wave radar for glacier monitoring. Plain Language Summary: Radar sensors map glacier surfaces by transmitting a signal at a specific frequency and measuring its return strength when reflected back. This returned signal strength, called radar backscatter, is determined by the characteristics of the glacier surface and varies with radar frequency and sensor viewing angle. Millimeter‐wave radars operating at 94 GHz can acquire high resolution measurements of glaciers in most weather conditions. However, there are currently no measurements of radar backscatter from glacier surfaces at this frequency. We therefore acquired the first ever measurements of 94 GHz radar backscatter from glacier ice. The results are consistent with those expected from randomly rough surfaces, hence we conclude that the roughness of the glacier surface is the primary driver of 94 GHz radar backscatter. We also show that 3D glacier surface mapping at this frequency is more accurate than other ground‐based radars that are employed to map glacier geometries. The results overall indicate that 94 GHz radar is an effective tool for glacier monitoring and thus opens up new possibilities for studying glacier processes. Key Points: 94 GHz radar backscatter from alpine glacier ice has been characterized for the first timeSurface roughness is the primary factor in 94 GHz radar backscatter from glacier iceThe uncertainty of 3D glacier mapping using 94 GHz radar has been quantified [ABSTRACT FROM AUTHOR]

Snow depth on sea ice is an Essential Climate Variable and a major source of uncertainty in satellite altimetry‐derived sea ice thickness. During winter of the MOSAiC Expedition, the "KuKa" dual‐frequency, fully polarized Ku‐ and Ka‐band radar was deployed in "stare" nadir‐looking mode to investigate the possibility of combining these two frequencies to retrieve snow depth. Three approaches were investigated: dual‐frequency, dual‐polarization and waveform shape, and compared to independent snow depth measurements. Novel dual‐polarization approaches yielded r2 values up to 0.77. Mean snow depths agreed within 1 cm, even for data sub‐banded to CryoSat‐2 SIRAL and SARAL AltiKa bandwidths. Snow depths from co‐polarized dual‐frequency approaches were at least a factor of four too small and had a r2 0.15 or lower. r2 for waveform shape techniques reached 0.72 but depths were underestimated. Snow depth retrievals using polarimetric information or waveform shape may therefore be possible from airborne/satellite radar altimeters. Plain Language Summary: Data collected using a surface‐based radar instrument on sea ice during the MOSAiC Arctic expedition were used to develop new techniques to estimate the depth of the overlying snow. We used different polarizations of the radiation to detect the depths of the upper and lower snow surfaces, and subtracted them to give snow depth. These depths agreed well with an independently collected snow depth data set. Estimates of snow depth using two different radar frequencies were less accurate, whilst using information of the shape of the returning pulse of radiation also showed a relationship with the independent snow depths, though not as strong as the polarization method. These results indicate that polarimetry (using a new satellite mission) and/or waveform shape (using existing missions) could be used to estimate snow depth on sea ice from airborne or satellite platforms. Key Points: Novel polarization‐based snow depth estimation techniques were developed using surface‐based Ku‐ and Ka‐band polarimetric radar altimeter dataThe dominant scattering surface was the air/snow and snow/ice interface in co‐ and cross‐polarized data, respectively, at both frequenciesRadar‐derived snow depths agreed with independent measurements, with r2 up to 0.77 and accuracy of 1 cm for best‐performing techniques [ABSTRACT FROM AUTHOR]

The coupling at the interface between tectonic plates is a key geophysical parameter to capture the frictional locking across plate boundaries and provides a means to estimate where tectonic strain is accumulating through time. Here, we use both interferometric radar (InSAR) and Global Navigation Satellite System (GNSS) data to investigate the plate coupling of the Hikurangi subduction zone beneath the North Island of New Zealand, where multiple slow slip cycles are superimposed on the long‐term loading. We estimate the plate coupling across the subduction zone over three multi‐year observational periods targeting different stages of the slow slip cycle. Our results highlight the importance of the observational time period when interpreting coupling maps, emphasizing the temporal variability of plate coupling. Leveraging multiple geodetic data sets, we demonstrate how InSAR provides powerful constraints on the spatial resolution of both plate coupling and slow fault slip, even in a region where a dense GNSS network exists. Plain Language Summary: Plate coupling as a concept describes to what degree the boundaries between tectonic plates are locked and building up stress. Such accumulated stress (over hundreds to thousands of years) will eventually be released in earthquakes, and therefore provides important information about the potential for future earthquakes. Our study uses satellite data to investigate how coupling between tectonic plates along the Hikurangi subduction zone (New Zealand's largest and most dangerous plate boundary fault) changes with time. We analyzed Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data to map the areas where the plates are stuck together (coupled) and where they move past each other (uncoupled). We show that plate coupling varies significantly in space over 2, 4, and 10‐year time scales, highlighting the importance of carefully considering the observational time period when interpreting coupling maps. Key Points: Integration of high‐resolution displacement maps from radar imagery captures plate coupling at fine scalesEstimates of plate coupling depend strongly on the time period over which surface velocities are measuredTemporal variations in plate coupling highlight when and where slow slip dominates the slip budget [ABSTRACT FROM AUTHOR]

This study analyzes microphysical signatures relevant to the short‐cycle lightning activity in the inner core of Typhoon Hato (2017) before landfall in China. Observations reveal that the lightning bursts were accompanied by enhanced inner‐core convection with a behavior cycle of about 3 hr. The coupling between wavenumber‐2 vortex Rossby waves (VRWs) and shear‐forced convective asymmetries resulted in a local updraft enhancement. Furthermore, supercooled liquid water droplets invigorated a striking enhancement of graupel via riming immediately above the freezing level, further enhancing charge separation and lightning generation outside the eyewall. Furthermore, when similar phase‐locking was associated with other propagating VRWs, graupel and updraft volumes were significantly boosted, leading to short‐cycle lightning outbreaks in the inner core. Plain Language Summary: Lightning behavior is one of the well‐defined indicators of convection within the Tropical cyclone (TC) circulation. Before the landfall of Typhoon Hato (2017), we found short‐cycle lightning outbreaks in the inner core of this typhoon. The interaction between the internal structures of the storm and changes in wind speed and direction with height could strikingly reinforce convection, thereby favorable for the short‐cycle electrification enhancement. This scenario is distinct from diurnal lightning bursts within TCs, which were found in previous studies. The diurnal lightning activity is thermodynamically governed by radiation, while vortex‐environment interactions in the inner core of Hato dynamically force the short‐cycle lightning outbreaks revealed here. Therefore, the findings presented in this case study suggest that interactions between the internal structures of a TC and the environment warrant an adequate focus in the short‐range forecasting of inner‐core convection in landfalling TCs. Key Points: Short‐cycle lightning outbreaks were found in the inner core of Typhoon Hato (2017)Interactions between vortex Rossby waves and the shear‐induced convective enhancement likely led to the quasi‐periodic lightning behaviorStrengthened updrafts and the significant growth of graupel enhanced the charge separation and lightning outbreaks [ABSTRACT FROM AUTHOR]

Many studies have performed microphysical retrievals using radars of different frequencies, platforms, and methodologies. However, little is known about the consistency of retrievals derived from different radar platforms (i.e., airborne or spaceborne vs. ground‐based) and their methodologies. This study is the first to directly compare snow mass‐weighted mean diameter (Dm) retrievals from both nadir‐pointing airborne multi‐frequency radars and ground‐based polarimetric range‐height indicator radar scans along the same airborne flight track with a resolution of O(10m) $\mathcal{O}(10\,\mathrm{m})$. Dm retrievals between each method produced mean absolute errors of 0.49, 0.74, and 0.93 mm where the largest differences were between the ground and aircraft retrievals. A triple‐frequency analysis suggests the possibility that snow aggregates were generally composed of needles. These results can be used as a benchmark for comparing retrieval methodologies and highlight the continued uncertainty regarding the optimal approach for ice microphysical retrievals. Plain Language Summary: Both airborne and ground‐based weather radar measurements can be used to estimate snowflake characteristics such as their mean size. However, it is difficult to compare the radar‐estimated mean sizes between these platforms because of the vast methodological differences in how these sizes are estimated and the spatial and temporal differences resulting from each radar's geographical location. This study is the first to directly compare snowflake size retrievals using ground‐based mobile radar scans that collected data along the same path as an overhead‐passing aircraft, thus directly minimizing any error resulting from matching values between radars aboard the aircraft and the ground radar. We also used the aircraft radars' multiple frequencies to investigate what types of particles were dominant throughout the storm. Through this multi‐frequency analysis, we found that it is possible that snow particles were aggregates of thin needles that were stuck together. These results can be used by other researchers and forecasters to understand how snowflake size estimates can vary between different platforms and methodologies. Key Points: Mobile X‐band polarimetric radar Range Height Indictor scans were directly collocated with overpassing multi‐frequency airborne radars during a snowstormThree separate mass‐weighted mean diameter retrievals had mean absolute errors of 0.49, 0.74, and 0.93 mm between each otherThese results provide a one‐of‐a‐kind, high‐resolution benchmark for comparing retrieval methods across various platforms [ABSTRACT FROM AUTHOR]

Capabilities to directly assimilate radar data are implemented within the local ensemble transform Kalman filter (LETKF) and the gain‐form LETKF (LGETKF) algorithms of the Joint Effort for Data assimilation Integration (JEDI) system. The capabilities are evaluated for the analysis and forecast of a severe convection case of 20 May 2019 in the Southern Great Plains using the limited area model version of the FV3 dynamical core (FV3‐LAM) from a recent release for Short‐Range Weather Application (SRW App). The LETKF and LGETKF implementations are shown to produce analyses and short‐range forecasts comparable to those using the ensemble square‐root Kalman Filter (EnSRF) within the Gridpoint Statistical Interpolation (GSI) framework used by current NCEP operational models. In addition, LGETKF retaining only 60% variances for model‐space vertical localization performs similarly to LGETKF retaining 99% of variance and LETKF using observation error‐based vertical localization. JEDI LETKF shows better parallel scalability than LGETKF and GSI EnSRF. Plain Language Summary: The JEDI data assimilation (DA) framework under development will be used by all operational forecasting systems of the U.S. National Weather Service, including the Rapid Refresh Forecast System based on FV3, a dynamic core chosen for next‐generation NWS forecasting systems. In this study, we implemented radar DA capabilities into the JEDI with two local ensemble transform Kalman filter (LETKF) algorithms coupled with limited‐area FV3. Analyses and forecasts with radar DA better match radar reflectivity and radial velocity observations than forecasts without radar DA during the assimilation period. Three‐hour forecasts of precipitation, reflectivity, and updraft rotation show skills comparable to a DA experiment using the ensemble square‐root Kalman filter within the more mature GSI DA system. In addition, the new gain‐form LETKF, which has not been applied to radar DA before, performs quite well, although it is computationally more expensive than the regular LETKF. This work paves the way for using JEDI to assimilate radar data in operational forecasting systems. Key Points: Radar data assimilation capabilities are implemented and tested in Joint Effort for Data assimilation Integration (JEDI), a future data assimilation infrastructure for Unified Forecast System applicationsLocal ensemble transform Kalman filter (LETKF) algorithm with gain‐form vertical localization is tested for radar data assimilation and is found to perform similarly as LETKFThe enhanced gain‐form LETKF and LETKF in JEDI perform similarly as ensemble square‐root Kalman Filter within Gridpoint Statistical Interpolation for radar data assimilation [ABSTRACT FROM AUTHOR]

As a tropical cyclone (TC) intensifies, the tangential wind field expands vertically and increases in magnitude. Observations and modeling support vortex height as an important TC structural characteristic. The Tropical Cyclone Radar Archive of Doppler Analyses with Recentering data set provides kinematic analyses for calculation of the height of the vortex (HOV) in observed storms. Analyses are azimuthally‐averaged with tangential wind values taken along the radius of maximum winds. A threshold‐based technique is used to determine the HOV. A fixed threshold HOV strongly correlates with current intensity. A dynamic HOV metric quantifies vertical decay of tangential wind with reduced dependency on intensity. Statistically significant differences are present between dynamic HOV values in groups of steady‐state, intensifying, and rapidly‐intensifying cases categorized by subsequent changes in pressure. A tall vortex is always observed in cases meeting a pressure‐based rapid intensification definition. Taller vortices are also evident with slower intensification. Results suggest HOV may be a helpful predictor for TC intensification. Plain Language Summary: As a tropical cyclone gets stronger, the wind field of the storm grows taller in the atmosphere. The height‐intensity relationship is known to be present from the beginning of a tropical cyclone until its peak strength. A large data set of airborne radar observations in tropical cyclones is used to show the strength of the relationship between vortex height and current intensity. A tall vortex is always present in observed cases for periods of rapid intensification during which the minimum surface pressure at the storm center drops at a high rate. Drops in minimum surface pressure are associated with increases in wind and size in tropical cyclones. A taller vortex also favors intensification at slower rates. Improved understanding of the role that tropical cyclone height plays during intensification may be helpful for forecasting changes in intensity of these storms. Key Points: Vertical extent of the tropical cyclone wind field correlates strongly with current intensity in observed stormsDeep vertical vortex structure is always present in observed cases meeting a pressure‐based rapid intensification definitionVortex height decreases with increasing vertical wind shear, but taller storms intensify more in moderate shear [ABSTRACT FROM AUTHOR]

Attenuation of radar waves in the subsurface can be quantified with a constant‐Q approximation through time‐frequency analysis. We implement the centroid frequency‐shift method and study Radar Imager for Mars' Subsurface Exploration (RIMFAX) data acquired along the Mars 2020 Perseverance rover traverse. Attenuation is among key media properties, but quantified estimates need to account for instrument characteristics and limitations in the analysis technique. We ensure accurate constant‐Q characterization and present the first estimates of radar attenuation in the upper 5 m of the shallow Martian subsurface. Over Jezero Crater Floor, constant‐Q is on average 70.4 ± 7.7, which equals an attenuation of −2.6 ± 0.3 dB/m at RIMFAX' 675 MHz center frequency. Regions comprising the Máaz or Séítah formations have similar attenuation properties that are consistent with magmatic lithologies. Plain Language Summary: First estimates of radar attenuation in the shallow Martian subsurface are presented. Estimates are from Jezero Crater Floor, along the drive path of the Mars 2020 Perseverance rover. Attenuation is among key media properties that describe radar wave propagation and can relate to lithological properties. Estimates of signal losses are consistent with magmatic lithologies and differences between distinct regions on Jezero Crater Floor are not detected. Key Points: First estimates of radar attenuation in the shallow Martian subsurface are on average −2.6 dB/m at the Radar Imager for Mars' Subsurface Exploration (RIMFAX) 675 MHz center frequencyResults are consistent with magmatic lithologies on the Jezero Crater FloorThe centroid frequency‐shift method is adequately implemented for RIMFAX analysis [ABSTRACT FROM AUTHOR]

From meteor radar observations typically measuring neutral winds and temperatures in the upper atmosphere, we showed that under‐dense meteor plasma trails respond to geomagnetic activity. Depending on the geomagnetic activity conditions, the parameters of the under‐dense trail echoes show substantial anomalies, and their responses are immediate. As geomagnetic activity increases, the decay time decreases up to about 10% at Kp = 8, while both the returned signal power and radial velocity error increase. In addition, the types of diffuse evolution vary with geomagnetic activity, resulting in strong geomagnetic activity that immediately affects the trail. The evolution of the trail is affected not only by the ambient mesospheric neutral atmospheric properties but also by electromagnetic effects, such as strong electric fields inside meteor trails. Furthermore, our findings suggest that the geomagnetic effect should be considered when estimating atmospheric parameters from MR during a geomagnetically active period. Plain Language Summary: Through long‐term meteor radar (MR) observation (2007–2021) at King Sejong Station in the Antarctic Peninsula, we found that the diffusion process in the meteor trail immediately accelerated under enhanced geomagnetic activity. The backscattered‐returned signal powers, which are proportional to the square of the electron line density of the meteor trail, also instantly increased. These anomalous characteristics in meteor trails appear while the background atmospheric parameters remain constant. Therefore, we propose that the observed immediate responses in the MR may be associated with the strong electric field in the ambient ionosphere D/E region induced by the increased geomagnetic activity. The strong external electric field can stimulate an electric field and current inside the meteor trail, owing to the difference in electrical conductivities between the meteor trail and ambient atmosphere. Thus, the electric field, current, and charge separation inside the meteor trail can lead to faster diffusion and enhanced electron line density in the trail. The meteor plasma trails are observed in regions that are affected by electric fields caused by geomagnetic activity; therefore, when deriving neutral atmospheric parameters from MRs, the impact of geomagnetic activity must be considered. Key Points: Under‐dense meteor echo parameters at a geomagnetic latitude of 50.2°S revealed an instantaneous response to geomagnetic activityGeomagnetic activity can directly modulate the evolution of meteor trails, such as the decay time and returned signal powerEffects of geomagnetic activity should be considered when estimating neutral atmospheric parameters from meteor radars [ABSTRACT FROM AUTHOR]

Satellite observations of sea ice freeboard are integral to the estimation of sea ice thickness. It is commonly assumed that radar pulses from satellite‐mounted Ku‐band altimeters penetrate through the snow and reflect from the snow‐ice interface. We would therefore expect a negative correlation between snow accumulation and radar freeboard measurements, as increased snow loading weighs the ice floe down. In this study we produce daily resolution radar freeboard products from the CryoSat‐2 and Sentinel‐3 altimeters via a recently developed optimal interpolation scheme. We find statistically significant (p < 0.05) positive correlations between radar freeboard anomalies and modeled snow accumulation. This suggests that, in the period after snowfall, radar pulses are not scattering from the snow‐ice interface as commonly assumed. Our results offer satellite‐based evidence of winter Ku‐band radar scattering above the snow‐ice interface, violating a key assumption in sea ice thickness retrievals. Plain Language Summary: Arctic sea ice thickness is often estimated using radar pulses from satellite‐mounted Ku‐band altimeters, which retrieve the radar freeboard. This is a proxy for the height of the ice surface above the waterline. Ku‐band radar pulses are widely assumed to penetrate through the overlying snowpack and reflect from the top of the sea ice. This means that increased snow loading on a sea ice floe is expected to reduce its radar freeboard, as the snow weighs the sea ice down. We produce daily resolution pan‐Arctic radar freeboard data sets from CryoSat‐2 and Sentinel‐3 retrievals. Using these new products, we find that an increased snow load often increases the radar freeboard, suggesting that the radar pulses are not reflecting off the ice surface. This could explain why satellite‐based sea ice thickness estimates don't always match in situ observations. Key Points: We reveal synoptic‐scale positive correlations between snow accumulation and Ku‐band radar freeboard changeThese correlations indicate that the conventional assumption of full radar‐wave penetration of the snowpack does not always hold trueThis sensitivity in freeboard estimates to snow accumulation introduces synoptic variability into satellite estimates of sea ice thickness [ABSTRACT FROM AUTHOR]

Since early 2019, the Lunar Penetrating Radar (LPR) onboard Chang'e‐4 (CE‐4)'s Yutu‐2 rover has been gathering data relating to the subsurface structure of the Von Kármán crater within the South Pole‐Aitken Basin (SPA) on the lunar farside. Low‐frequency radar data have the potential of carrying geological information of about 300 m worth of strata below the traversed path. Forty‐two days' data have revealed a bifurcated structure within the layered structure beneath the CE‐4 surveying area for the first time, affecting the overlying reflectors between 90 and 310 m. This study suggests that, based on the morphological characteristics, thickness, depth (timing sequence) and direction of the newly found structure, its origin might be linked to the deposition of ejecta from the Schrödinger impact. The local stratigraphy is interpreted as consisting of distinct geological layers, corresponding to the superposition of ejecta from different impact craters, paleo‐regolith, and basaltic lava flows. Plain Language Summary: A radar (the Lunar Penetrating Radar, or LPR), built to look at the rocks below the surface, is presently traveling across an area of the lunar farside onboard a rover, called Yutu‐2 sent by China to shed some light on the striking difference between the Earth‐facing side of the Moon and its farside. So far, the radar has revealed layered structures interpreted as distinct volcanic flows, thus helping to unravel their history. The rover has been surveying the floor of the Von Kármán crater inside one of the oldest and largest impact structures of the Moon, the South Pole‐Aitken (SPA) Basin. By analyzing 42 lunar days of data, we found a new layer with a hunched upper surface and a relatively flat bottom inserted into a multilayered structure. Based on the morphology, thickness, depth, and direction of the new feature, we argue that its source could represent the ejecta deposition from one of the ancient lunar impact basins: Schrödinger. The subsurface structure revealed by the radar shows a complicated geologic history on the farside including multiple lava eruption events, the deposition of ejecta from different impact craters and weathering layers that develop during the quiescent stage. Key Points: The CH‐1 Lunar Penetrating Radar data of Chang'e‐4 reveals ∼300 m deep subsurface stratigraphy of the lunar farsideA unique bifurcated structure that resembles an ejecta discontinuity affects the overlying and underlying strataThe geologic evolution of the Von Kármán crater and the ejecta layer might be connected to the Schrödinger basin [ABSTRACT FROM AUTHOR]

Interpretation of radar sounder reflections to infer the structure and composition of the martian polar caps depends on whether bright returns correspond to single packed dust layers or a more finely layered structure. Reflections from multiple layers can create strong resonant scattering (interference) effects that impact analyses of radargram reflectors and inference of dielectric contrast. We identify resonant behavior for an areally extensive reflector in the north polar layered deposits from Shallow Radar data processed in two frequency bands. Echo strength varies by ∼2 dB between subband reflections across a region ∼400 km in extent, with the stronger echo shifting abruptly from the high‐ to low‐frequency band outside the central region of Gemina Lingula. This behavior can arise from resonant scattering between two layers of dust (0.3–0.6 m thick) separated by 0.5–3 m of ice. Such layering requires there be little postdepositional aeolian activity to preserve layer thickness and spacing. Plain Language Summary: The north polar cap of Mars is made of layers of ice and dust that are a record of the changing climate over the past few million years. These north polar layered deposits (NPLD) have been imaged with radar sounder data from the Shallow Radar (SHARAD) instrument. The radar data reveal reflectors caused by changes in the electrical properties of the layered ice and dust. However, as the radar beam is reflected off multiple layers it can interfere with itself, changing the appearance of the reflector. We identified resonance by splitting SHARAD data into two bands at different frequencies and looking for differences in the reflectors. We used two models to simulate interference and suggest plausible thicknesses for the ice and dust layers. Our simulations suggest a portion of the NPLD formed in less than 200 thousand years, at a time when the region was much less windy than it is currently. Key Points: We identify resonant radar scattering behavior in radargrams of the martian north polar layered deposits using Shallow Radar multiband dataResonant behavior can affect interpretation of radargrams and can arise from two thin dust layers separated by less than a few meters of iceWe characterize a reflector that formed during quiescent periods of sustained dust accumulation with minimal aeolian activity [ABSTRACT FROM AUTHOR]

Dual‐polarization radars can provide rich three‐dimensional (3D) information on cloud precipitation structure. To utilize the existing polarimetric radar network in operational data assimilation systems, a new polarimetric radar‐based cloud analysis method is introduced. New features include the employment of fuzzy‐logic hydrometeor classification, improved estimation of liquid and ice regions, and newly‐added number concentration estimation. The new scheme is evaluated with a typical squall line case. Results show that the 3D cloud precipitation structure and short‐term precipitation forecast is consistently improved. With extra information from the polarimetric observations, the new scheme is able to produce reasonable polarimetric signatures for analysis. More supercooled water results in more latent heat release which enhances the updraft, leading to stronger convection in the subsequent forecast. This study emphasizes the importance of correct initial liquid and ice particle condition for the prediction of deep convection. Plain Language Summary: Severe convective storms have always been one of the most the disastrous types of weather. However, due to their relatively small scale and rapid evolution, such storms are difficult to predict. Polarimetric radar networks have been established for nearly a decade and have been widely used in the monitoring and short‐term forecast of severe weather. Yet, this extra polarimetric information has not been used in the operational data assimilation system, which is key to a successful forecast. A new dual‐polarization (dual‐pol) radar‐based cloud analysis aiming to improve short‐term forecasts is therefore proposed. Results show that the new scheme is able to capture the 3D in‐cloud signature of deep convection. Consistent improvements are found for the prediction of 3D structure of deep convection and as well as for the quantitative precipitation forecast. Key Points: A modified cloud analysis model is proposed for polarimetric radar data assimilationA fuzzy‐logic method combined with ZDP is used to classify hydrometeor particles, followed by a partially double‐moment retrieval schemeExperiments with polarimetric‐data‐based cloud analysis show consistent improvement on analysis and forecast [ABSTRACT FROM AUTHOR]

Exploration of Venus in the 1970–1990s revealed that the geology of Venus, the most Earth‐like of the terrestrial planets, was decidedly un‐Earth‐like, with no plate tectonics, and no record of the first 80% of its history. A major outstanding question is whether Venus is still volcanically active today. We find that regions of Ganis Chasma have low radar emissivity values, due to low volumes of high dielectric minerals formed by surface‐atmosphere weathering on the timescales of around 10 s Ma. This confirms the presence of geologically recent volcanism in association with this major tectonic rift zone. The spatial correspondence of this emissivity signature with transient thermal anomalies suggests that Venus has been volcanically active at this site for at least the last few decades, a prediction that can be tested with space missions to Venus in the coming decade. Key Points: Radar anomalies in Ganis Chasma correspond to transient bright spots seen in infrared dataLow radar emissivity values suggest recent volcanic and tectonic activity on VenusLava flows in Ganis Chasma may have erupted over the last 30 years [ABSTRACT FROM AUTHOR]

This paper provides analyses to quantify the maturity of regolith within 24 m depth using a novel polarimetric attributes analysis of lunar penetrating radar (LPR) data. The results demonstrate that LPR signals were mainly from subsurface rock fragments of different sizes and were rarely from strata interfaces. The averages and ranges of maturities for the near‐surface (0–2 m), the fine‐grained regolith (2–12 m), and the coarse‐grained ejecta (12–24 m) are 68.98% (0%–99.82%), 82.61% (41.52%–96.56%), and 72.33% (5.91%–96.66%), respectively. Local regions with anomalously low maturities on the lunar surface may be due to the dense materials formed by local impacts or the rock fragments from distal impacts; a newly discovered sandwich structure with low maturity at a depth between 12 and 18 m is formed by the heterogeneous weathering in a paleo‐crater. Based on these new insights, we infer a heterogeneous weathering process of the regolith. Plain Language Summary: The entire lunar surface is considered to be covered by a layer of regolith. The study of regolith weathering degree almost focused on the near‐surface because the traditional techniques cannot detect the regolith at large depths. The Chang'E‐4 lunar penetrating radar (LPR) can detect the regolith structure within dozens of meters depth. However, previous publication of LPR results only use the macro layering model to interpret the regolith structure. This study innovatively extracts new properties from the LPR data acquired while the rover was turning and estimated the quantitative maturity of regolith within ∼24 m depth at lunar farside. We found that the LPR can mainly detect the subsurface rock fragments that survived weathering and rarely the interfaces of strata because the materials of different strata are mixed at interfaces and make it gradational. Our results also reveal the spatial difference of weathering in the regolith. We investigate the formations and material compositions of several interesting regions with unusual weathering degrees. Based on these new insights, we establish a model to illustrate the weathering process of the regolith at the CE‐4 landing site. Key Points: We quantify the maturity of the lunar regolith within 24 m depth by analyzing the lunar penetrating radar (LPR) data acquired during the rover turningsLPR signals were mainly from subsurface rock fragments of different sizes and were rarely from strata interfacesInferring the heterogeneous weathering process by comprehensively investigating the regions with anomalous maturities in the regolith [ABSTRACT FROM AUTHOR]

Venus highlands such as Maxwell Montes are an exploration target in the search for a past water‐rich environment. We combine Magellan and Arecibo radar data to characterize surface properties across Maxwell for orbital or landed investigations. Arecibo data reveal a region surrounding and west of the 90‐km crater Cleopatra with lower radar echoes and circular polarization ratio despite increased surface reflectivity. Modeling of microwave emissivity shows that the surface undergoes step‐like reflectivity shifts at ∼6056.2 km and ∼6,061 km radius, such that the changes in radar echo between these altitudes are not due solely to dielectric effects. We propose that fine‐grained Cleopatra ejecta mantles much of Maxwell Montes, consistent with Venus distal crater ejecta patterns and the longevity of mantling debris in the highlands. Future orbital or landed studies must consider the thickness of ejecta, source material, and effects of shock or melting on ejecta mineralogy. Plain Language Summary: Venus may once have had an ocean and perhaps habitable conditions, but the current greenhouse atmosphere is very dense, hot, and dry. New spacecraft missions to Venus will look through the clouds and probe the deep atmosphere to search for evidence of rock types that form when water is abundant, such as those that make up Earth's continents. The best places to look for these rocks are in ancient mountain ranges, but there are many impact craters on Venus that spread thin layers of fine‐grained sediment over vast regions. Earth‐based radar observations provide a unique type of information that maps these sediments as a guide to searches for evidence of a water‐rich past. We show that much of Maxwell Montes, the tallest mountain range on Venus, is at least partly covered by impact‐produced sediments that must be considered when analyzing future orbital observations. Key Points: Mountain ranges on Venus may hold evidence of a past water‐rich periodImpact craters can widely distribute fine‐grained ejecta that masks surface characteristicsMaxwell Montes, the highest range on Venus, has extensive fine‐grained ejecta deposits from Cleopatra crater [ABSTRACT FROM AUTHOR]

This study investigates warm‐season extreme precipitation events (EPEs) with the focus on the less‐attended weak convection (WeEPEs) using spaceborne radar observations. WeEPEs are compared to EPEs with intense convection (InEPEs), including their frequency/location, convective structures, and storm environments. WeEPEs and InEPEs both account for about 30% of the EPEs, highlighting the importance of WeEPEs. Overland WeEPEs mainly occur over key monsoon regions and maximize over windward sides of coastal mountains likely due to orographic enhancement. WeEPEs develop under conditions with deep moist profiles and strengthened low‐level onshore flows. Radar, microwave, and lightning proxies all suggest a weak updraft and limited ice contents in WeEPEs. However, vertical radar profiles of WeEPEs exceptionally increase (∼15 dBZ) downward below the melting level, indicating substantial raindrop growth and/or concentration increase through very active warm‐rain processes. This study provides useful guidelines for evaluating high‐resolution model simulations and satellite rainfall retrievals on extreme precipitation. Plain Language Summary: When extreme precipitation occurs, one will easily connect it to intense storms such as those producing numerous lightning flashes. However, some rainstorms display little severe signatures, therefore, they are even more challenging from the monitoring or nowcasting point of view. This study investigates the warm‐season extreme precipitation events (EPEs thereafter) with weak convection (weak upward motion indicated by satellite proxy) in the tropics and subtropics using 15 years of satellite observations. Surprisingly, EPEs with weak convection (WeEPEs) over land are as frequent (∼30%) as EPEs with intense convection, highlighting the significance of WeEPEs. Over land, WeEPEs preferentially develop over deep monsoon regions and their maxima are located over or near coastal mountains, suggesting the important role of orographic lifting/convergence in producing WeEPEs. Spaceborne radar, microwave, and lightning observations all indicate that WeEPEs have a weak convective intensity and limited ice contents above the melting level. WeEPEs are featured by vertical radar profiles persistently increase downward below the melting level (by ∼15 dBZ). Our findings collectively suggest that heavy rainfall in WeEPEs is largely caused by precipitation process in the warm‐cloud layer, instead of the melting of large amount of ice particles. Key Points: Nearly 30% warm‐season extreme precipitation events (EPEs) have only weak convection, similar fraction as EPEs with intense convectionMost weak convective EPEs (WeEPEs) are situated over the ITCZ, Asian Monsoon, and South American Monsoon, especially coastal regionsWeEPEs are featured by limited ice‐phase process and vertical radar profiles markedly increasing downward below the freezing level [ABSTRACT FROM AUTHOR]

We introduce a transfer matrix model for radio‐wave propagation through layered anisotropic ice that permits an arbitrary dielectric permittivity tensor in each layer. The model is used to investigate how crystal orientation fabrics without a vertical principal direction affect polarimetric radar returns over glaciers and ice sheets. By expanding the c‐axis orientation distribution in terms of a spherical harmonic series, we find that radar returns from synthetic fabric profiles are relatively insensitive to the harmonic mode responsible for a nonvertical principal direction; however, only for normally incident waves. Consequently, the strength of this mode might be relatively difficult to infer in glaciers and ice sheets, which in turn has implications for the ability to determine the full second‐order structure tensor, needed to infer the local flow regime, flow history, or to represent the directional viscosity structure of glacier ice for ice‐flow modeling. Plain Language Summary: The orientation of ice crystals in glacier ice locally co‐evolve with and can enhance the flow of ice. Methods that allow inferring the crystal structure inside glaciers and ice sheets are, therefore, essential for improving the accuracy and realism of ice‐flow models, and have broad implications for understanding past and present flow regimes. In this work, we introduce a new radio‐wave model and use it to investigate the extent to which radar surveys over glaciers and ice sheets can reveal the orientation information necessary to improve such ice‐flow models and construct a proxy for past flow. We show that conventional polarimetric radar surveys, which look straight down, might be poorly suited for the task; a specific but important component of the grain orientation structure cannot be seen with such surveys. We find, however, that radio waves transmitted at an angle to the surface might overcome this crucial limitation and allow the full crystal structure to be inferred. Key Points: Radar returns over glacier ice are, for normal incidence, insensitive to whether or not a vertical principal fabric direction existsThe harmonic mode responsible for a nonvertical principal fabric direction might be determined from oblique radar soundingAssuming a vertical principal fabric direction when there is none can substantially affect inferred directional viscosities of glacier ice [ABSTRACT FROM AUTHOR]

The seasonal features of the eastward jet through the Tsugaru Strait was investigated using surface velocity data between 2014 and 2019 obtained from high‐frequency radar located in the eastern part of the strait, as well as data obtained from repeated shipboard observations from 2009 to 2019. The potential‐vorticity‐front‐model was used, and the volume transport of low potential vorticity (PV) water and the intensity of the PV gap at the front in the eastern part of the strait were estimated from these data. The influence of these parameters on the seasonal flow pattern of the Tsugaru Warm Current out to the Pacific Ocean (the summer/autumn "gyre mode" or winter/spring "coastal mode") was examined. The distribution of the parameters was consistent with the theoretical understanding, showing that in addition to the volume transport, an increase in the local PV gap can also contribute to the development of the gyre. Plain Language Summary: The Tsugaru Warm Current forms part of the coastal flow around Japan and shows remarkable seasonal changes in its flow pattern, which are similar to the Kuroshio meandering. The current flows along the coast from winter to spring, but demonstrates large anticyclonic meandering in the offshore direction, known as "gyre mode," from summer to autumn. The cause of this change in the flow pattern has been thought to be the seasonal change in volume transport. In this study, we used high‐resolution temporal and spatial data obtained from a high‐frequency radar system, as well as shipboard hydrographic data collected over a period of 10 years, combined with a theory based on the relationship between coastal waves and offshore jets with fronts to identify another important factor that affects the seasonal flow pattern of the current, namely, "potential vorticity." This study is important because it helps our knowledge of the behavior of fluids flowing out of channels and straits as well as that in relation to the connection between local phenomena and larger phenomena. Key Points: Observational verification of the seasonal flow pattern theory of the Tsugaru Warm Current based on velocity data from high‐frequency radarImportance of potential vorticity gap at the front recognized as well as the volume transport of the current on the flow pattern changeIntegration and improvement of our understanding about the behavior of fluids flowing out from channels and straits [ABSTRACT FROM AUTHOR]

The first step to accurately measure global snowfall is to separate rainfall from snowfall correctly (i.e., precipitation phase discrimination). This study first evaluates the phase discrimination performance in four remote sensing datasets, including observations from ground radar, spaceborne radars, and spaceborne radiometer, relative to ground observations. Results show that the snowfall discrimination accuracy varies greatly among these datasets ranging from 42% to 96%, dependent on whether and how the temperature information are considered. For example, over half of the snowfall from the Global Precipitation Measurement Mission (GPM) spaceborne radar is actually rainfall at the surface since it detects snowfall in the air without considering the temperature information close to the surface. Second, we evaluate the discrimination performance using the temperature information from four reanalysis datasets. It is found that MERRA2 temperature close to the surface is colder than the other three datasets, leading to more rainfall being misclassified as snowfall. Plain Language Summary: Satellite remote sensing provides the only means of measuring rainfall/snowfall on the global scale. Misclassifying the precipitation phase (i.e., rainfall as snowfall, or vice versa) could lead to the estimated precipitation rate being one order of magnitude smaller or larger. Our results reveal that the snowfall discrimination accuracy varies greatly among four remote sensing datasets ranging from 42% to 96%. For example, over half of the snowfall from the state‐of‐the‐art precipitation product based on the Global Precipitation Measurement radar is rainfall at the surface without considering the temperature information close to surface. Additionally, the temperature discrepancy among different reanalysis datasets also greatly affects precipitation phase discrimination. Our results show that MERRA2 temperature close to the surface is colder than the other three major datasets, leading to more rainfall pixels being misclassified as snowfall pixels. Key Points: Snowfall determination accuracy varies greatly among four remote sensing datasets ranging from 42% to 96%More than half of the snowfall indicated by Global Precipitation Measurement Mission (GPM) dual frequency precipitation radar (DPR) is rainfall on the groundMERRA2 temperature close to the surface is noticeably colder than observed, leading to more rainfall being classified as snowfall [ABSTRACT FROM AUTHOR]

Nowcasting of convective storms is urgently needed yet rather challenging. Current nowcasting methods are mostly based on radar echo extrapolation, which suffer from the insufficiency of input information and ineffectiveness of model architecture. A novel deep‐learning (DL) model, FURENet, is designed for extracting information from multiple input variables to make predictions. Polarimetric radar variables, KDP and ZDR, which provide extra microphysics and dynamic structure information of storms, are fed into the model to improve nowcasting. Two representative cases indicate that KDP and ZDR can help the DL model better forecast convective organization and initiation. Quantitative statistical evaluation shows using FURENet, KDP, and ZDR synergistically improve nowcasting skills (CSI score) by 13.2% and 17.4% for the lead time of 30 and 60 min, respectively. Further evaluation shows the microphysical information provided by the polarimetric variables can enhance the DL model in understanding the evolution of convective storms and making more trustable nowcasts. Plain Language Summary: Severe convective precipitation is a major cause of many hazards. However, very short‐term forecasting, i.e., nowcasting, of convective precipitation is rather challenging. Current nowcasting methods suffer from insufficiency of physics information of input data and ineffectiveness of model architecture. As an advanced observing tool, polarimetric weather radar can provide crucial microphysics and dynamic structure information of convective precipitation systems. To incorporate polarimetric radar variables into the nowcasting task, this study proposes a novel model architecture termed FURENet based on deep learning. FURENet uses U‐Net as a flexible backbone, and is specially designed to facilitate exploiting information from multiple input variables. By training the model with polarimetric radar variables (KDP and ZDR) as input, significant improvement of forecasting the initiation, development and evolution of convective storms is achieved. The results also show the effectiveness of model architecture. Key Points: A deep‐learning approach termed FURENet is proposed for convective precipitation nowcasting with multiple input variablesPolarimetric radar variables are used to provide microphysics and dynamic structure information of convective storms in the modelExperiments with FURENet show significant improvement on nowcasting performance [ABSTRACT FROM AUTHOR]

Lunar paleoregolith was formed by repeated asteroid impact and space weathering and then buried by later lava flows, serving as important records for early solar system history. However, direct observational evidence for the paleoregolith layer is rather limited. We present the evidence for the existence of the paleoregolith layer by processing 60 MHz lunar penetrating radar data acquired by the Chang'E‐3 Yutu rover. We find successive reflections with reversed polarities due to a low permittivity (paleoregolith) layer sandwiched in two high permittivity (lava) layers. From modeling and migration imaging of radar reflections, we determine an ultra‐thick paleoregolith layer (∼5–9 m) beneath the Eratosthenian unit and on the top of the Imbrian unit, suggesting a high regolith production rate of 5.8–10.5 m/Ga between late Imbrian and early Eratosthenian periods compared to the previous estimation ∼2 m/Ga, implying fast regolith formation and possible high meteoric flux during these periods. Plain Language Summary: The surface of the Moon experienced multiple volcano eruptions since its birth and the large basin was filled with dark lava flows. Within this period due to repeated asteroid impact and space weathering between lava flows, the paleoregolith layers will be likely formed and buried by younger lava flows. The paleoregolith layers have been undisturbed since their formation thus are critical for determining early impacting and volcano history of the Moon and are regarded as important records for uncovering early Moon's history and early solar system history. We present the evidence for the existence of the paleoregolith layer in the northern Mare Imbrium basin by processing 60 MHz lunar penetrating radar data acquired by the Chang'E‐3 Yutu rover. From modeling and migration imaging of radar reflections, we determine an ultra‐thick paleoregolith layer (∼5–9 m) beneath the Eratosthenian unit and on the top of the Imbrian unit, implying fast regolith formation and possible high meteoric flux during these periods. Key Points: Successive reflections with reversed polarities due to a low permittivity layer sandwiched in two high permittivity layersAn ultra‐thick paleoregolith layer (∼5–9 m) beneath Eratosthenian unit and on the top of Imbrian unitA high regolith production rate of 5.8–10.5 m/Ga between late Imbrian and early Eratosthenian periods compared to previous estimate ∼2 m/Ga [ABSTRACT FROM AUTHOR]

The observable lunar surface is represented by a ubiquitous layer of fine‐grained materials produced by billions of years of hypervelocity pounding of its crustal layer. The data from the Lunar Penetrating Radar onboard Chang'e‐4 (CE‐4) rover (Yutu‐2), which is exploring the Von Kármán Crater on the lunar farside, are helping to peel back the upper layer of finely comminuted materials, interpreted as a thick layer of ejecta from the neighboring Finsen crater, to reveal a complex paleo‐surface morphology. During the rover's 560 m journey, from depths of 7–20 m, distinct variations in the returned signal characteristics reveal a possible 270 ± 10 m buried crater with an estimated age of less than 100 m.y. A smooth surface depression that lies to the southwest of the Yutu‐2 rover's travel path might not be the remnant of a degraded crater but a related surface expression of the hidden structure. Plain Language Summary: The absence of an atmosphere and substantial resurfacing events in the last couple of billion years means that the Moon's surface has journeyed through space exposed to anything that travels through it and, unlike the Earth, most of these "encounters" have left a mark. Thus, the good news for scientists is that the lunar surface represents a record of astrophysical phenomena within the Solar System that is no longer available on its larger companion: our planet. However, the larger impacts have indeed contributed to modify its surface hiding some of its old terrains. The Lunar Penetrating Radar (LPR) onboard Chang'e‐4 (CE‐4) rover (Yutu‐2) can see below the surface as it makes its way across the lunar ground, revealing the ancient lunar surface (paleo‐surface) at the depth of ∼12 m. It has mapped its elevation profile, which now we can see that it has been substantially modified following its formation some 3.1 billion years ago. Notably, the LPR has discovered a 270 m sized buried crater corresponding to a large ground depression seen on the surface, a first achievement for lunar radar sounding. Key Points: The CE‐4 LPR reveals the paleo‐surface of the lunar farside at a depth between 7 and 20 mThe LPR has mapped a 270 m sized buried crater probably related to a larger circular depression on the lunar surfaceThe geomorphology, the subsurface structure, the impact melt deposits, and the exposure time of the buried crater, are analyzed [ABSTRACT FROM AUTHOR]

Upper‐tropospheric clouds in the outflow layer of typhoons can affect the track of typhoons (tropical cyclones) through radiation effects. In this study, the microstructure of the outflow‐layer clouds of several typhoons was examined. Cloud radar observations of three typhoons around Japan revealed numerous protuberances in echoes along the base of the upper‐level clouds, which are referred to as mammatus‐like echoes. The horizontal and vertical scales of these mammatus‐like echoes were 0.5–3.0 and 0.3–1.5 km, respectively. Vertical observations revealed downward (upward) Doppler velocities in (between) the hanging echo regions. Upward and downward velocity maxima were estimated at 3 m s−1 around the mammatus‐like echoes. Neutral stratification developed in the dry layer beneath the cloud base in which the mammatus‐like echoes formed. These mammatus‐like structures may promote mixing along the cloud base that contributes to dissipation of the outflow‐layer clouds. Plain Language Summary: Heating and cooling due to radiative effects associated with upper‐level clouds spreading from a typhoon have been reported to have an impact on the actual track of the typhoon. Therefore, it is important to clarify the processes of formation and dissipation of the upper‐level clouds of typhoons. However, observations of such clouds, which are formed from relatively small ice particles, are scarce and the process of their dissipation is not well understood. In this study, we used cloud radar, which can detect clouds formed of small ice particles, to observe the upper‐level clouds of three typhoons that approached Japan. At the base of the upper‐level clouds, we found many protuberance structures with 0.5–3.0 km width and 0.3–1.5 km depth. These protuberance structures were accompanied by relatively large vertical motions of approximately 3 m s−1. The temperature and water vapor distributions near the base of the upper‐level clouds of the studied typhoons were conducive to these vertical motions. The small‐scale protuberance structures might promote dissipation of upper‐level clouds along their base. Key Points: Protuberances observed by cloud radar along the base of outflow‐layer clouds of typhoons are termed mammatus‐like echo structuresThe mammatus‐like echo structures are associated with vertical motions with velocity of several meters per secondMammatus‐like echoes occur in a layer with high potential for turbulence caused by sublimation of ice particles falling into a drier layer [ABSTRACT FROM AUTHOR]

The polar summer mesosphere is the Earth's coldest region, allowing the formation of mesospheric ice clouds. These ice clouds produce strong polar mesospheric summer echoes (PMSE) that are used as tracers of mesospheric dynamics. Here, we report the first observations of extreme vertical drafts (±50 ms−1) in the mesosphere obtained from PMSE, characterized by velocities more than five standard deviations larger than the observed vertical wind variability. Using aperture synthesis radar imaging, the observed PMSE morphology resembles a solitary wave in a varicose mode, narrow along propagation (3–4 km) and elongated (>10 km) transverse to propagation direction, with a relatively large vertical extent (∼13 km). These spatial features are similar to previously observed mesospheric bores, but we observe only one crest with much larger vertical extent and higher vertical velocities. Plain Language Summary: Extreme events are ubiquitous in geophysical flows. Examples of these events are tornadoes and rogue waves in the lower atmosphere and oceans, respectively. In the mesosphere, the boundary of Earth's atmosphere and outer space, extreme events can also occur, although this region is poorly observed. Here, we present the first observations of extreme vertical velocities (±50 ms−1) in the mesosphere, that are more than five times their expected standard deviation. These observations are possible by tracking and imaging strong mesospheric radar echoes that occur in the summer at polar latitudes, with a radar used in a radio camera mode. The morphology of our observations resembles previously observed instabilities called bores or wave walls, but with much larger vertical velocities and vertical extents. Key Points: First observations of extreme vertical velocities in the polar summer mesosphereThe observed solitary wave in a varicose mode resembles a mesospheric bore, with large vertical extent and vertical velocitiesSuch extreme events might have been missed or ignored in previous observations of vertical velocities or other mesospheric observations [ABSTRACT FROM AUTHOR]

The current paper investigates the shallow layers of the lunar regolith at the Chang'E‐4 landing site. Four layers between 0 and 10 m were identified using lunar penetrating radar. Based on these outputs, a revised stratigraphic model is suggested for the post‐Imbrian ejecta at the Von Kármán crater. The layers were previously unseen due to the smooth boundaries between them. The revised model was inferred using an advanced hyperbola‐fitting scheme. Applying conventional hyperbola‐fitting to non‐homogeneous media results in errors and inaccuracies that are often wrongly assumed to be negligible. We propose a novel hyperbola‐fitting scheme that is, not constrained to homogeneous media and can be applied subject to any arbitrary one‐dimensional permittivity distribution. Via this approach, we can estimate the permittivity profile of an investigated area and detect layered structures that were previously transparent to electromagnetic waves due to the gradational dielectric properties at their interfaces. Plain Language Summary: The landing site of Chang'E‐4 is at the Von Kármán (VK) crater at the South Pole‐Aitken (SPA) basin. The SPA basin is the oldest and biggest basin on the Moon created at the early stages of its evolution by an impact that is, believed to have penetrated the lunar crust and uplifted materials from the top mantle. Understanding the geology and stratigraphy of the SPA basin can help us understand cratering processes and shed light on the evolution of the Moon. To that extent, ground penetrating radar (GPR) has been added to the payload of Yutu‐2 to investigate the stratigraphy of the Chang'E‐4 landing site. One drawback of GPR is that conventional processing methods fail to detect layers with smooth boundaries between them. Consequently, GPR data might give the false impression that an area is relatively homogenous, while in fact it might consist of numerous gradational layers representing a much more complex geological history. In the current paper, a novel interpretation tool is described, capable of detecting smooth transitions between layers. The proposed methodology revealed a previously unseen layered structure for the first ∼10 m of the VK crater. Key Points: We suggest a novel hyperbola‐fitting technique that assumes an arbitrary permittivity distribution with respect to depthThe proposed method is used to map the lunar regolith at the Chang'E‐4 landing siteA layered structure is revealed at the first 10 meters. A new stratigraphic model is suggested for the Von Kármán crater [ABSTRACT FROM AUTHOR]

Bright radar reflections observed beneath the south polar layered deposits (SPLD) by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument were interpreted to represent liquid water, but the required amounts of salt and heat to form and maintain liquids in this location are implausible given what is known about Mars. Here, we present another hypothesis that accounts for the bright reflections: hydrated and cold clay‐rich deposits at the base of the SPLD create the observed radar response. To support this hypothesis, we present experimental measurements and wave propagation modeling that show that smectites, cooled to 230 K, have real and imaginary parts of the dielectric permittivity large enough to cause the bright reflections, even when mixed with other materials. Further, we find that absorptions attributable to these minerals are present in south polar orbital visible‐near infrared reflectance spectra. Because these minerals are present at the south pole and can cause the reflections, we believe this to be a more viable scenario than the liquid water interpretation. Plain Language Summary: In 2018, radar observations beneath the south polar cap of Mars were interpreted to indicate the presence of bodies of liquid water. Liquid water is important for considering the current state of the planet and for potential habitats; however this interpretation is controversial because the amount of dissolved salt and heat required to maintain liquid water at this location is difficult to reconcile with what we know about Mars. To date, no alternative explanations have been presented. Here, we demonstrate that clay minerals known to exist in the south polar region of Mars are sufficient to fully explain the radar observations without invoking enhanced heat or salt. Key Points: Bright radar reflectors at the south pole of Mars have been attributed to liquid water, requiring implausible amounts of heat and saltOur experimental and modeling work shows that smectite minerals can explain the observed radar reflections without liquidsWe find spectral evidence of smectites at the south pole, demonstrating that smectites are present and the most plausible interpretation [ABSTRACT FROM AUTHOR]

Recent results from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument have been interpreted as evidence of subsurface brine pooled beneath 1.3 km‐thick South Polar Layered Deposit (SPLD). This interpretation is based on the assumption that the regionally high strength of MARSIS radar reflections from the base of the ice cap is due to a strong contrast in dielectric permittivity across the basal interface. Here, we demonstrate that the high‐power reflections could instead be the result of a contrast in electric conductivity. While not explicitly excluding a liquid brine, our results open new potential explanations for the observed strong radar reflections, some of which do not require liquid brine beneath SPLD. Potential basal materials with suitably high conductivity include clays, metal‐bearing minerals, or saline ice. Plain Language Summary: Previous work reported a regionally strong radar reflection under Mars' south polar ice sheet. Due to its brightness, this radar reflection was interpreted as liquid water (likely with high concentration of dissolved salts). A radar reflection can be bright due to a large contrast in either dielectric permittivity or electric conductivity. Previous work only considered contrasts in dielectric permittivity. We find that contrasts in electric conductivity between materials could also explain the brightness of the reflection. We suggest that this difference could be due to clays, metal‐bearing minerals, or saline ice under the polar ice sheet. Key Points: Radar reflections can be caused by contrasts in either dielectric permittivity or electric conductivityReflections observed by Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) at Mars' south polar region can be explained by contrasts in electric conductivity and do not require liquid waterWe propose that a polar‐ice substrate containing clays, other conductive minerals, or saline ice are the most plausible candidate materials [ABSTRACT FROM AUTHOR]

We expand on previous studies of the South Polar Layered Deposits' (SPLD) basal interface using data acquired by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) to obtain detailed maps of elevation, topography, and reflected radar power. Using these maps, we derive the thickness (ranging from 0 to 3.7 km) and volume of the SPLD (∼1.60 × 106 km3). While most basal interfaces reflect less power than the average SPLD surface, areas with basal echo power exceeding that of the surface are widespread throughout the SPLD, including at the location of potential subglacial water bodies in Ultimi Scopuli. The occurrence of these high basal echo power signatures appears to be largely frequency independent in MARSIS data. While the cause of the relatively high basal echo power values is uncertain, our observations suggest that this behavior is widespread, and not unique to Ultimi Scopuli. Plain Language Summary: Using radio waves, we investigated the properties of materials lying below the surface of Mars' south polar regions (South Polar Layered Deposits; "SPLD"). We made maps of the buried surface below the SPLD ("basal interface") and measured the radar energy reflected by this interface. From these maps, we measured the SPLD volume as 1.6 × 106 km3. Additionally, we found that there are multiple areas throughout the south polar region where the energy reflected from the basal interface is unexpectedly higher than that of the surface. Previous analyses of one such region suggested that these stronger reflections could be caused by the presence of an underground lake. Key Points: We present new, detailed (∼44,000 points) maps of South Polar Layered Deposits' basal elevation and topography using Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) dataWe report the reflected basal echo power from three MARSIS frequencies, showing large spatial variability but minimal dependence on frequencyAreas with high basal echo power are widespread, and not limited to the location of potential subglacial water bodies in Ultimi Scopuli [ABSTRACT FROM AUTHOR]

During three winter seasons from November 2016 to March 2019, 11 gamma‐ray glows were detected at a single observation site of our ground‐based gamma‐ray monitoring network in Kanazawa, Japan. These events are analyzed with observations of an X‐band radar network, a ceilometer, a disdrometer, and a weather monitor. All the detected glows were connected to convective high‐reflectivity regions of more than 35 dBZ, developed up to an altitude of >2 km. They were also accompanied by heavy precipitation of graupels. Therefore, graupels in the lower layer of thunderclouds that correspond to high‐reflectivity regions can form strong electric fields producing gamma‐ray glows. Also, these events are compared with a limited sample of nondetection cases, but no significant differences in meteorological conditions were found between detection and nondetection cases in the present study. Plain Language Summary: Strong electric fields inside thunderclouds can accelerate electrons to relativistic energies, and gamma‐ray photons from the accelerated electrons can be observed as minute‐lasting "gamma‐ray glows." During winter thunderstorms in Japan, we detected eleven gamma‐ray glows at sea level. The observations were combined with meteorological measurements such as atmospheric temperature, cloud bases, types of precipitation particles, and radar‐echo measurements. All the detected gamma‐ray glows were taking place during a passage of tall and well‐developed radar‐echo regions, and accompanied by heavy precipitation of graupel/hail pellets. The graupel/hail particles in the lower layer of thunderclouds could have contributed to gamma‐ray glows production. Key Points: Eleven gamma‐ray glows were observed during three winter seasons at a single observation site in JapanAll the detected gamma‐ray glows were associated with tall and well‐developed radar‐echo structures of thunderstormsGraupel pallets in the lower layer of thunderclouds could be related to downward electron acceleration for the gamma‐ray glows [ABSTRACT FROM AUTHOR]

On January 12, 2020, Taal volcano, Philippines, erupted after 43 years of repose, affecting more than 500,000 people. Using interferometric synthetic aperture radar (InSAR) data, we present the pre‐ to post‐eruption analyses of the deformation of Taal. We find that: (1) prior to eruption, the volcano experienced long‐term deflation followed by short‐term inflation, reflecting the depressurization‐pressurization of its ∼5 km depth magma reservoir; (2) during the eruption, the magma reservoir lost a volume of 0.531 ± 0.004 km3 while a 0.643 ± 0.001 km3 lateral dike was emplaced; and (3) post‐eruption analyses reveal that the magma reservoir started recovery approximately 3 weeks after the main eruptive phase. We propose a conceptual analysis explaining the eruption and address why, despite the large volume of magma emplaced, the dike remained at depth. We also report the unique and significant contribution of InSAR data during the peak of the crisis. Plain Language Summary: Taal volcano in the Philippines erupted on January 12, 2020. Here, we present the pre‐, co‐, and post‐eruption data, model, and analyses using interferometric synthetic aperture radar (InSAR) data acquired by various satellite systems. We find that: (1) prior to the eruption, the volcano experiences a sequence of long‐term (>1 year) deflation followed by short‐term (≤1 year) inflation as a result of the depressurization‐pressurization of its ∼5 km depth magma reservoir; (2) during the eruption, the magma reservoir lost a volume of 0.531 ± 0.004 km3 while a 0.643 ± 0.001 km3 lateral dike was emplaced; and (3) post‐eruption analyses reveal that the magma reservoir is in recovery starting ∼3 weeks after the main eruptive phase. We propose a conceptual analysis to explain the 2020 Taal eruption and the dike emplacement. We also report the unique and significant contribution of remote sensing data, particularly InSAR during the peak of the crisis. Key Points: We present a comprehensive interferometric synthetic aperture radar (InSAR)‐based data, analyses, and models of Taal's pre‐ to post‐eruptive stateDuring the eruptive crisis, Taal's magma reservoir lost 0.531 ± 0.004 km3 of volume while a 0.643 ± 0.001 km3 lateral dike was emplacedLow‐latency InSAR‐derived products provided crucial and significant information to PHIVOLCS during the January 2020 eruptive event [ABSTRACT FROM AUTHOR]

A phase‐sensitive radar (ApRES) was deployed on Totten Ice Shelf to provide the first in situ basal melt estimate at this dynamic East Antarctic ice shelf. Observations of internal ice dynamics at tidal time scales showed that early arrivals from off‐nadir reflectors obscure the true depth of the ice shelf base. Using the observed tidal deformation, the true base was found to lie at 1,910–1,950‐m depth, at 350–400 m greater range than the first reflection from an ice‐ocean interface. The robustness of the basal melt rate estimate was increased by using multiple basal reflections over the radar footprint, yielding a melt rate of 22 ± 2.1 m a−1. The ApRES estimate is over 40% lower than the three existing satellite estimates covering Totten Ice Shelf. This difference in basal melt is dynamically significant and highlights the need for independent melt rate estimates using complementary instrumentation and techniques that rely on different sets of assumptions. Plain Language Summary: Observations of the rate of melting at the base of ice shelves are needed to model accurately ice sheet evolution. Local measurements are scarce, yet necessary for validation of satellite products and ocean models. We deployed a phase‐sensitive radar in the proximity of grounding zone of Totten Ice Shelf in East Antarctica, to measure basal melt in this dynamic region where uncertainties on melt rate estimates are high. We developed a method that accounts for basal geometry complexities and derived a melt rate estimate of ∼22 m per year, which is lower than previous estimates, but it confirms that the basal melt rate Totten Ice Shelf experiences is unusually high for East Antarctica. Key Points: First in situ radar‐derived basal melt estimate on Totten Ice Shelf yields 22 ± 2.1 m a−1, at least 40% lower than existing satellite estimatesRadar‐derived observation of tidal ice dynamics constrains estimate of ice thickness for a complex baseThe use of multiple basal reflections in melt derivation increases robustness of the estimate when early off‐nadir returns are present [ABSTRACT FROM AUTHOR]

Typhoon Lekima (2019) possessed a double‐eyewall structure before making landfall in eastern China, with its outer eyewall showing quasi‐periodic convective intensification. Several physical mechanisms that may cause eyewall convection asymmetries were examined. The upshear occurrence of the strongest convection could not be explained by either the typhoon's motion or the effect of descending inflow from outer rainbands. Radar reflectivity analysis showed that phase locking occurred between the wavenumber‐2 vortex Rossby waves (VRWs) propagating radially outward from the inner eyewall and the azimuthally propagating wavenumber‐1 VRWs on the inner edge of the outer eyewall. Additional phase locking further arose between the aforementioned wavenumber‐1 VRWs and the azimuthally propagating wavenumber‐2 VRWs on the outer edge of the outer eyewall. These two phase‐locking processes led to the pronounced quasi‐periodic intensification of the convective asymmetry in the western semicircle of Lekima's outer eyewall. Plain Language Summary: Tropical cyclone eyewalls are commonly accompanied by torrential rainfall that tends to cause flooding and devastation. In particular, precipitation prediction becomes more challenging when the eyewall convective activity exhibits a markedly asymmetric structure. This work focuses on the repeated occurrence of strong convection in the outer eyewall of Typhoon Lekima (2019) before it made landfall in China. The coastal ground‐based radar observations show that phase locking between waves associated with the double eyewalls serves as a plausible mechanism for the presence of the repeated strong precipitation in Lekima's outer eyewall. The findings in this study highlight the merit of coastal ground‐based radar observations in examining physical processes of tropical cyclone structure evolution, particularly in regions without aircraft reconnaissance. Key Points: Radar observations revealed quasi‐periodic intensification of convective asymmetries in the outer eyewall of Typhoon LekimaThe strongest convection in the outer eyewall occurred upshearPhase locking between vortex Rossby waves in the concentric eyewalls led to the occurrence of quasi‐periodic asymmetric convection [ABSTRACT FROM AUTHOR]

Convective vertical velocity (wc) and convective mass flux (Mc) lie at the heart of global climate model cumulus parameterizations, but few observations of these critical parameters are available. This paper develops and evaluates a novel, satellite‐based method for estimating profiles of wc and Mc. Comparisons with collocated ground‐based radar wind profiler (RWP) observations show that satellite‐estimated median wc is slightly greater than the RWP estimates, but they show solid agreement when compared at the 95th percentiles (intense updrafts). RWP‐derived and satellite‐estimated Mc are broadly comparable in the lower and middle troposphere, with some differences in the upper troposphere due to differences in convective core sampling. A k‐means cluster analysis of multiple years of wc data shows that convective characteristics are distinctly different among extratropical convection, tropical land convection, and tropical oceanic convection. Tropical land convection is significantly more intense and more variable than the oceanic counterpart. Plain Language Summary: Properly representing and parameterizing cumulus convection has long been a central task of the global climate models (GCMs), as GCM simulations are highly sensitive to the assumptions made in the cumulus parameterization schemes, resulting in large uncertainties in predicting the future climate change. At the heart of many of today's GCM convection parameterizations are convective vertical velocity (wc) and mass flux (Mc). Yet, few global observations of these critical parameters are available at this time globally. To improve this situation, the authors developed a novel, satellite‐based method for estimating profiles of wc and Mc. We first evaluate the satellite estimates against collocated ground‐based radar wind profiler (RWP) retrievals. RWP‐derived and satellite‐estimated wc and Mc are found to be broadly comparable, and their agreement improves when differences in sampling and definitions of convective clouds are considered. A k‐means cluster analysis of multiple years of satellite‐estimated wc data is performed to identify salient patterns related to the structure and global distributions of convective vertical velocity. Results show that convective characteristics are distinctly different among extratropical convection, tropical land convection, and tropical oceanic convection. Our ongoing work seeks to use this newly developed wc and Mc data set to evaluate GCM cumulus parameterizations. Key Points: A novel, satellite‐based method was developed to retrieve profiles of convective vertical velocity (wc) and convective mass flux (Mc)Comparisons with collocated ground‐based radar wind profiler observations show broad agreementsA k‐means cluster analysis shows that convective vertical velocity is distinctly different between tropical land and oceanic convection [ABSTRACT FROM AUTHOR]

The Dorsa Argentea Formation (DAF) is an extensive Hesperian plains unit in the south polar region of Mars that was once ice rich and may still contain a substantial fraction of water ice. Given its 3.5‐Ga age, the possibility for massive ice in the DAF has significant implications for the preservation of volatiles during large obliquity changes. Here Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and Shallow Radar (SHARAD) sounder data are analyzed to determine whether massive water ice is present at depth by utilizing subsurface DAF reflectors to infer the loss tangent of overlying material. SHARAD (0.005 ± 0.003) and especially MARSIS (0.009 ± 0.006) loss tangent values are closer to those of dry sediment than to equatorial glaciers or the polar caps, but a fraction of remnant ice at depth may occur. Water ice is not the predominant component of the DAF, so much of the ancient ice has been lost during large obliquity changes over the past 3.5 Ga. Plain Language Summary: The Dorsa Argentea Formation (DAF) is a 3.5 billion‐year‐old plains deposit that surrounds the south polar ice cap on Mars. The DAF was once ice rich and may still contain a large fraction of water ice below the surface. The possibility that ancient ice is preserved in the subsurface of the DAF has implications for the climate history of Mars, especially how warm the polar regions were in the past. Here we analyze Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and Shallow Radar (SHARAD) sounder data to determine whether water ice is present. Radar sounder data provide detailed information about materials beneath a planetary surface. The MARSIS and SHARAD data reveal a greater similarity of the DAF to dry sediments than to pure water ice. Much of the ancient ice has been lost over the history of the DAF due to changes in the polar tilt of Mars. Key Points: Loss tangent values are consistent with the DAF being largely composed of volatile‐poor materialMARSIS reflections provide evidence that the DAF continues underneath the SPLDMost water ice deposits have not persisted in the DAF through large changes in obliquity [ABSTRACT FROM AUTHOR]

Efficient quantification of conditional errors in radar rainfall (RR) products requires a realistic model of random error distribution. Nonparametric estimate of the probability density function (pdf) of standardized RR errors is obtained using a large data sample. The standardization is based on a second‐order separation of systematic and random effects. The estimated empirical distribution is skewed and has exponential shapes of its tails with two different scales. It cannot be modeled with the Gaussian law that was used in several previous studies. A good representation of the tails of the empirical distribution of RR errors is obtained with a three‐parameter modified Laplace model with a shift and unequal slopes of its two sides. Plain Language Summary: Reliable measurement of rainfall is important for hydrology, water management, agriculture, and other users. It is a challenging task because rainfall is highly variable and networks of rain gauges are too sparse to capture this variability. Thus, rainfall estimates based on weather radars are becoming indispensable because they provide continuous coverage of the area up to about 100 km around the radars. However, they are ridden by many uncertainties that must be known in order to avoid dangerous mistakes in all applications of the estimates. In this paper, we contribute an important new piece to this knowledge—the empirically based information about the frequency of occurrence of different magnitudes of the errors that is called the error probability distribution. It allows to evaluate their impact on different applications of radar rainfall. It also enables more reliable comparisons of different rainfall estimation methods in order to monitor progress in this area. The new information was obtained by applying a new mathematical modeling method to a large sample of radar rainfall estimates combined with rain gauge measurements of true rainfall. Many previous applications assumed that radar rainfall errors were distributed according to the commonly used bell curve called normal distribution. Our results show that the actual error distribution differs considerably from normality and that using the normal model leads to misrepresentation of the uncertainties. We now have the strongly supported mathematical tool to treat them in a much more realistic way. Key Points: Empirical distribution of conditional random errors in radar rainfall is estimated using second‐order separation of systematic and random effectsThe error distribution cannot be described with Gaussian model used beforeAsymmetric shifted Laplace model fits well the tails of the empirical distribution [ABSTRACT FROM AUTHOR]