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Observations of near‐surface NO2 show a diurnal pattern with midday minima and daily maxima in the morning and evening. These surface cycles are dependent on chemical processing, transport, and emissions. We evaluate these cycles with the United States Environmental Protection Agency (EPA) community multiscale air quality (CMAQ) model data from the EPA Air Quality Time Series (EQUATES) project, compared with ground‐based measurements from the EPA air quality system, two Pandora measurement sites, and satellite data from TROPOMI. We find that the morning vertical column density (VCD) lags surface concentrations by 1 hr on average, where this lag varies with location and day. The peak VCD can also lead the surface maximum concentration, especially in the evening, responding to transport and afternoon compression of the boundary layer. Modeled NO2 VCD is sensitive to column calculation technique. With hourly daytime satellite‐based NO2 observations newly available from the TEMPO instrument, the timing and magnitude of cycles in near‐surface NO2 versus column NO2 will help inform the utilization of hourly satellite data. This work will help inform the timing of surface‐column connections to better interpret new hourly satellite observations for health and air quality applications, including emissions characterization. Plain Language Summary: Nitrogen dioxide (NO2) is a major air pollutant that may be measured from the ground or from satellites in space. Over the U.S., hourly satellite data are now available from a new instrument called TEMPO. We use a computer model to understand the expected relationship between the satellite column and ground‐level measurements. We find that the connection between the surface and column varies across the day: TEMPO observations are more likely to see morning peak and daytime minimum near‐surface amounts; TEMPO observations may miss evening peak near‐surface amounts. By helping to connect space‐based data with on the ground air quality, this work will help inform the timing of surface‐column connections to better interpret new hourly satellite observations for health and air quality applications. Key Points: Satellite observations of nitrogen dioxide (NO2) show column values, which show different hourly patterns than surface monitoring of NO2Column NO2 values lag surface NO2 in the morning but lead surface NO2 in the eveningModel simulations of column NO2 are sensitive to calculation approach [ABSTRACT FROM AUTHOR]

In this study, we investigate the changing relationship between the boreal winter Arctic Oscillation (AO) and eastern African precipitation. The results show that the negative correlation has significantly weakened since the late 1970s. And the Arabian anticyclonic circulation changed concurrently with the AO. In the mid‐high troposphere, the anomalous anticyclone centred over the Arabian Peninsula in 1980–2020 moved northeastward and weakened compared to 1950–1979. Correspondingly, the anomalous downward motion over eastern Africa significantly weakened compared with that in 1950–1979, corresponding to insignificant precipitation. A further analysis suggests that AO‐related Rossby wave trains in the upper troposphere may modulate the Arabian anticyclone. From 1950 to 1979, the wave activity preferred propagating eastward from the Mediterranean to Russia along a high‐latitude path. In contrast, during 1980–2020, it tended to emanate southeastward towards Saudi Arabia, with a notably stronger and more eastward extension than in the earlier period. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP), a crucial area influencing global climatic patterns and water resources, is experiencing a unique climatic paradox, particularly evident in the Three Rivers Source Region (TRSR). A striking summer asymmetry in the increases of near-surface temperature and precipitation is observed from 1989 to 2018: the rate of daily minimum temperature (0.64°C decade−1) surpasses the daily maximum temperature (0.52°C decade−1), while the daytime precipitation intensity (0.40 mm day−1 decade−1) increases at a faster rate compared to nighttime (0.30 mm day−1 decade−1). Despite these trends, the summer mean nighttime precipitation intensity consistently remains higher than the daytime average. Notably, this pattern is accompanied by an increasing trend of moisture transport during both daytime and nighttime in TRSR. This paper deciphers the thermodynamic and dynamic processes behind this trend. The daytime warmth not only alters the stability of atmosphere but also modulates convective inhibition (CIN), thereby reshaping precipitation mechanics and potentially dampening or delaying daytime convection. Thermodynamically, a shift from unstable to stable anomalies in the summer troposphere suppresses precipitation development. The combination of increased CIN during those period leads to fewer but more intense rainy days. Dynamically, the shift from a consistent downward motion anomaly throughout troposphere to an upward motion anomaly becomes dominant during nighttime, exhibiting a similar transition but only below 500 hPa during daytime. These findings reveal the complex interplay between thermodynamics, dynamics, and precipitation, highlighting the need for refined climatic models that can accurately simulate these summer diurnal processes. [ABSTRACT FROM AUTHOR]

Although evidence of recovery in Antarctic stratospheric ozone has been found, evidence of recovery in Arctic ozone is still elusive, even though 25 years have passed since the peak in ozone depleting substances. Here we have used a Dynamic Linear Model to derive time-varying trends over 20-year periods in the Arctic ozone time series, measured in-situ by ozonesondes from 6 stations, from 1994 to 2022. The model accounts for seasonality, external forcing and 1st-order correlation in the residuals. As proxies for the external forcing, we have used tropopause pressure (replaced with Arctic Oscillation in the troposphere), eddy heat flux, the volume of polar stratospheric clouds multiplied by effective equivalent stratospheric chlorine, and solar radio flux at 10.7 cm for the 11-year solar cycle. Our results indicate that the ozone recovery in the lower Arctic stratosphere is not detectable. Though significant positive trends have been detected prior to 2017 at some stations, there are no statistically significant positive trends after 2017. Moreover, at a number of stations the trends after 2019 are rather negative and significant, varying between − 0.30 ± 0.25 and − 1.00 ± 0.85% per decade. Furthermore, the Arctic troposphere exhibited only statistically significant negative trends over 20-year periods ending in 2017 or later, varying between − 0.31 ± 0.27 and − 1.76 ± 0.41% per decade. These results highlight the importance of continued monitoring of the Arctic ozone. [ABSTRACT FROM AUTHOR]

The interdecadal change in the relationship between the Antarctic Oscillation (AAO) and autumn rainfall in the Yunnan–Guizhou Plateau of Southwest China is investigated by using the observed autumn rainfall data at 119 stations and the National Centers for Environment Prediction/National Center for Atmospheric Research reanalysis data for the period 1961–2021. Results show the AAO correlates well with the autumn rainfall in the Yunnan–Guizhou Plateau for the second period (2002–2021) because the AAO becomes stronger. The possible influencing mechanism of AAO on autumn rainfall in the Yunnan–Guizhou Plateau during 2002–2021 is related to the meridional teleconnection pattern and associated convection over the Philippine Sea. The positive AAO can trigger a meridional teleconnection pattern in the upper troposphere to propagate from the southern Pacific to northern Pacific and cause anomalous westerly over the tropical west Pacific, which inhibits the convection over the Philippine Sea. On the one hand, the weakened convection over the Philippine Sea causes the anomalous ascending motion over the Yunnan–Guizhou Plateau; on the other hand, it results in an anomalous anticyclone over the tropical Northwest Pacific and strengthens the transport of water vapor from the tropical Pacific to the Yunnan–Guizhou Plateau. [ABSTRACT FROM AUTHOR]

In December and January, the pressure contrast between the subtropical plains of South America (CHA) and northwestern Africa (WEM) reaches its maximum. This happens because of simultaneous and opposite surface pressure variations in these two regions. The low tropospheric winds flow from the tropical Atlantic into Amazonia and then divert southwards into the Gran Chaco region, suggesting a possible strengthening of this circulation associated with the CHA-WEM gradient. This article analyses the effect of a cross-equatorial CHA-WEM gradient intensification on the monsoon system of South America. The reinforcement of the gradient is associated with increased easterlies in the eastern equatorial Atlantic, a stronger low-level jet east of the Andes, increased moisture transport towards the CHA region, and reduced precipitation over much of Brazil. The large quasi-stationary wave in the upper troposphere, consisting of the Bolivian High and the Nordeste Low, weakens in amplitude, altering the return flow to the North Atlantic. Extratropical dynamics, such as Rossby wave trains that reach southern South America and the North Atlantic Oscillation, affect the variability in the CHA and WEM regions and modulate the CHA-WEM gradient intensity. [ABSTRACT FROM AUTHOR]

Ice nucleating particles (INPs) affect the cloud radiative budget in the rapidly warming Arctic by changing the cloud liquid/ice phase balance. Dust emitted in the Arctic (Arctic dust) has been suggested to be a major contributor to INPs in the Arctic lower troposphere. However, how Arctic dust and its impacts on ice nucleation change with Arctic warming has not been explored. Here we find that the simulated dust emission flux in the Arctic (>60°N) in global model simulations increases by 20% from 1981–1990 to 2011–2020. This increase weakens the sensitivity of ice nucleation in Arctic lower tropospheric clouds to warming by 40% compared to the case without considering Arctic dust emission increases. Our results demonstrate a better understanding of the counterbalancing feedbacks of Arctic dust (i.e., increasing emissions and decreasing ice nucleation efficiency) is needed for more accurate estimates of changes in ice nucleation in the rapidly changing Arctic climate. [ABSTRACT FROM AUTHOR]

The western North Pacific Subtropical High (WNPSH) significantly influences East Asian weather. In the Northwest Pacific where sea‐salt aerosols (SSAs) are abundant and the large‐scale environment is dominated by the dry subsidence of the WNPSH during summer, inhomogeneous SSAs form as a product of the environment. However, the extent to which inhomogeneous SSAs affect the WNPSH remains unclear. This study investigates the radiative effects of SSAs through numerical simulations, revealing a novel mechanism for the strengthening of the WNPSH. The results demonstrate that inhomogeneous SSAs enhance the WNPSH by generating diabatic cooling in the upper troposphere and associated unstable subsidence motion. Further considering the radiative hysteresis effects of inhomogeneous SSAs, the WNPSH further strengthens under the combined dynamic and thermodynamic influences associated with upper‐level radiative cooling. Inhomogeneous SSAs not only enhance the WNPSH but also influence the location where the central area of high pressure intensifies. Plain Language Summary: Variations in the western North Pacific Subtropical High (WNPSH), including changes in its intensity and position, significantly impact weather patterns in East Asia, particularly during the summer months. At high relative humidity (RH) levels, sea‐salt aerosols (SSAs) appear as homogeneous droplets; however, in moderate RH environments (45%–70%), SSAs exist in inhomogeneous states characterized by water coatings on their core particles. The dry subsiding airflow of the WNPSH creates extensive areas of moderate RH over the western Pacific Ocean. Despite this, the potential effects of SSAs on the WNPSH through radiative processes have not been previously considered. This study demonstrates that inhomogeneous SSAs induce cooling in the upper troposphere, which contributes to an overall strengthening of the WNPSH. Additionally, these aerosols influence the position where the primary area of high pressure intensifies. This newly identified mechanism by which inhomogeneous SSAs strengthen the WNPSH offers a unique perspective for understanding the evolution of the WNPSH. Key Points: Inhomogeneous sea‐salt aerosols (SSAs) contribute to strengthening the western North Pacific Subtropical High (WNPSH)Inhomogeneous SSAs also influence the position where the main body of the high pressure intensifiesDirect radiative effects of inhomogeneous SSAs offer a distinctive perspective for exploring the dynamics of the WNPSH [ABSTRACT FROM AUTHOR]

The impact of model vertical resolutions on simulation of the Madden–Julian Oscillation (MJO) was investigated using five AMIP simulations by the Grid‐point Atmospheric Model of IAP LASG, version 3 (GAMIL3) with different vertical layers. Results showed that higher vertical resolutions produce a stronger and superior eastward propagation, coupled circulation–convection relationship, and MJO strength, as well as other convectively coupled equatorial waves when compared to the lowest vertical resolution. The improvements may be related to a better description of the tropical circulation in the higher vertical resolutions and model top, albeit without the significant improvement of MJO convection and stratospheric quasi‐biennial oscillation in all simulations. Among the four tested high resolutions, the simulations with higher vertical resolutions from the surface to about 850 hPa produced better eastward propagation and larger total explained variance of the MJO, indicating the importance of the lower troposphere in simulating the MJO. [ABSTRACT FROM AUTHOR]

Despite the growing number of GNSS sites, GNSS meteorology still suffers from insufficient observational data. This deficiency is particularly evident in GNSS tomography processing in the lowermost part of the troposphere, where the highest variability of weather conditions is observed. To address this challenge, we propose an integrated tomography solution that uses both ground-based GNSS observations and radio occultation (RO) excess phase in the same tomography model. Our integrated approach benefits from improved geometry and increased number of observations. In order to verify presented methodology, we use 41 GNSS stations and 78 RO profiles gathered during four different high precipitation events over the region of southern Poland. The results are validated against radiosonde observations and WRF reanalysis. Our study indicates an enhancement in the representation of wet refractivity on the order of 20 to 30% RMSE decrease when RO are integrated into the tomography solution with the mean error of final integrated product equal 3.5 ppm. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, the characteristics of tropical cyclones (TCs) over the Bay of Bengal (BoB) that affect snowfall on the Tibetan Plateau (TP) and spatiotemporal distribution of snowfall related to BoB TCs are statistically analysed by using multi‐sources data from 1981 to 2020, with partitioning TC‐influenced snowfall by tracking cloud clusters. The results show that 141 TCs formed during the 40‐year period of 1981–2020, of which about 35% (50 TCs) impacted snowfall at 83% of meteorological stations on the TP during their northward or westward movement, and the average distance between the TC centre and the snowfall stations is 1277 km. The proportion of snowfall‐related TC frequency shows a significantly decreasing trend with a predominant cycle of 10a. The TC‐influenced snowfall frequency (SF), precipitation amount (PA) on a snowfall day and snow depth (SD) during 1981–2020 all show a non‐significant weak decreasing trend, while TC‐influenced snowfall is significantly increased in the eastern and southern edges of Xizang, western Sichuan and the southern margin of Qinghai. PA and SD in December account for more than 75% and 55% of the monthly total, respectively. The spatial pattern of PA could be objectively categorised into west‐type (24%) and southeast‐type (76%). The moisture transported by the BoB TC and a southerly jet stream formed between the trough and the western Pacific subtropical high (WPSH), the convergence of cold air and warm–moist airstream over the TP and the change in position of the south Asian high in the upper troposphere are significant factors causing the different spatial distribution. The results can provide reference for TC‐related snowfall, SD prediction and disaster assessment on the TP. [ABSTRACT FROM AUTHOR]

Hazards fromconvective weather pose a serious threat to the contiguous United States (CONUS) every year. Previous studies have examined howfuture projected changes in climate might impact the frequency and intensity of convective weather using simulations with both convection-permitting regionalmodels and coarser-grid climate and Earth systemmodels.We build on this existing literature by utilizing a large-ensemble of historical and future Earth system model simulations to investigate the time evolution of the forced responses in large- scale convective environments and how those responses might be modulated by the rich spectrum of internal climate variability. Specifically, daily data from an ensemble of 50 simulations with the most recent version of the Community Earth SystemModel was used to examine changes in the convective environment over the eastern CONUS during March-June from1870 to 2100. Results indicate that anthropogenically forced changes include increases in convective available potential energy and atmospheric stability (convective inhibition) throughout this century, while tropospheric vertical wind shear is projected to decrease across much of the CONUS. Internal climate variability on decadal and longer time scales can either significantly enhance or suppress these forced changes. The time evolution of two-dimensional histograms of convective indices suggests that future springtime convective environments over the eastern CONUS may, on average, be supportive of relatively less frequent and shorter-lived, but deeper and more intense convection. [ABSTRACT FROM AUTHOR]

Formic acid (HCOOH) is one of the most abundant volatile organic compounds (VOCs) in the Earth's atmosphere and an important source of atmospheric acidity. Here we present the first retrieval of HCOOH from a geostationary orbit using observations from the Geostationary Interferometric Infrared Sounder (GIIRS) on board FengYun‐4B. The results from July 2022 to June 2023 show the monthly variation of the HCOOH distribution in Asia. In addition, we show that FY‐4B/GIIRS effectively tracks HCOOH enhancements from wildfires both during the day and at night. Finally, inter‐comparison with IASI HCOOH data shows good agreement, indicating that FY‐4B/GIIRS observations have comparable sensitivity to IASI. This study is an important first step toward monitoring VOCs from geostationary infrared sounders, including the existing and future GIIRS on board the FY‐4 series and the forthcoming European geostationary infrared sounder (IRS) on board Meteosat Third Generation (MTG). Plain Language Summary: Satellite observations play an indispensable role in improving our understanding of the global sources and sinks of HCOOH, an important volatile organic compound in the atmosphere. However, existing polar‐orbiting satellites that are sensitive to tropospheric HCOOH typically provide up to two overpasses per day over the same spot, one during the day and one during the night. The diurnal variations of tropospheric HCOOH are therefore under‐constrained to track its evolution in the lower troposphere and transport in the free troposphere. The Geostationary Interferometric Infrared Sounder (GIIRS) onboard China's FengYun‐4 satellite series is the world's first geostationary hyperspectral infrared sounder. Using observations from FY‐4B/GIIRS, this study presents the first retrieval of HCOOH from a geostationary satellite instrument that captures the HCOOH variation in Asia driven by biomass burning emissions and possible biogenic sources, providing key observations for understanding the complex processes of HCOOH production, evolution, and loss in the atmosphere. This study is an important first step toward monitoring VOCs from geostationary infrared sounders. Key Points: First retrieval of atmospheric HCOOH from a geostationary sounder is demonstratedHCOOH in fire plumes can be continuously tracked with FY‐4B/GIIRSFY‐4B/GIIRS HCOOH observations have comparable sensitivity to IASI [ABSTRACT FROM AUTHOR]

Reanalysis and satellite data indicate a decreasing precipitation area in recent decades, affecting local water resources and precipitation intensities. We have used CMIP6 simulations to test the hypothesis of a shrinking precipitation area in a warming climate. Our analyses reveal that SSP5-8.5 projections show a robust decrease in the precipitation area between 50 °S and 50 °N, and globally in 75% of the simulations. The new findings support the observed relationship, although to a lesser extent than earlier found in reanalysis and satellite data. We find a poleward shift of precipitation, increasing the daily precipitation area in the Arctic from 18% to 28%. At lower latitudes the precipitation area is reduced due to a decreasing occurrence of precipitation. These changes are related to the expansion of low relative humidity zones in the lower-to-mid troposphere, specifically at the poleward edges of the subtropics. [ABSTRACT FROM AUTHOR]

Long-term trends of the three-dimensional Plumb wave activity flux are studied using data from the JRA-55 global atmospheric reanalysis. The vertical component of the Plumb flux characterizes the propagation of atmospheric planetary waves generated in the troposphere to the upper atmosphere and is used for the analysis of the stratosphere–troposphere dynamic interaction. The study of the wave activity flux covers three latitudinal sectors of the Northern Hemisphere from December to March for a 64-year period since 1958. It is shown that in January and March over the Russian Far East there is a statistically significant trend for an increase in the wave activity flux from the troposphere to the stratosphere, which can contribute to an increase in the frequency of cold wave formation in the mid-latitude troposphere. The study of the stratosphere–troposphere dynamic interaction in general and wave activity fluxes in particular is necessary for solving problems related to both global and regional climatic changes and mixing of long-lived atmospheric components. [ABSTRACT FROM AUTHOR]

The Normalized Differential Spectral Attenuation (NDSA) technique was proposed years ago as an active method for measuring integrated water vapor (IWV) along a Ku/K-band radio link immersed (totally or partially) in the troposphere. The approach is of the active kind, as it relies on the transmission of a couple of sinusoidal signals, whose power is measured at the receiver, thus providing the differential attenuation measurements from which IWV estimates can be in turn derived. In 2018, a prototype instrument providing such differential attenuation measurements was completed and set up for a first measurement campaign aimed at demonstrating the NDSA method. By the end of June 2022, the instrument was profoundly modified and upgraded so that a second measurement campaign could be carried out from 1 August to 30 November 2022. The transmitter was placed on the top of Monte Gomito (44.1277°lat, 10.6434°lon, 1892 m a.s.l.) and the receiver on the roof of the Department of Information Engineering of the University of Florence (43.7985°lat, 11.2528°lon, 50 m a.s.l.). The resulting radio link length was 61.15 km. Four ground weather stations of the regional weather service were selected among those available. In this paper, we describe the upgraded instrument and present the outcomes of the new measurement campaign, whose purpose was mainly to compare the IWV estimates provided by the instrument with the ground sensor measurements of air temperature, air humidity, barometric pressure, and rainfall. In particular, we show that the temporal trends of the two IWV estimates are qualitatively consistent, and that the instrument is able to provide IWV estimates also in the presence of fog and rainfall. Conversely, a quantitative evaluation through comparison with IWV data from point weather station measurements appears challenging due to the significant spatial variability in temperature and relative humidity, even between couples of stations that are quite close to each other. [ABSTRACT FROM AUTHOR]

Western Himalayas (WH) receives precipitation through eastward propagating synoptic weather systems, Western Disturbances (WDs), embedded in large scale subtropical westerly jet (SWJ) during Northern Hemispheric boreal winter (December, January and February; DJF). In the recent decade, WDs have undergone certain changes in their characteristic's viz., decreased associated precipitation, no significant changes in the WDs frequency and intensity, etc. However, Non-WDs days and associated precipitation gained its importance as it contributes ~ 20% of winter precipitation over WH. In this study, structure, dynamics and moisture availability associated with changing WDs and Non-WDs precipitating days are analyzed. Robust statistical methods and Theil-Sen slope on ERA5 data during 1987–2020 is considered to comprehend why active WDs' (Non-WDs) precipitation frequency is declining (notably increasing). Interestingly, it is as well found that all the WDs do not precipitate and there are Non-WDs days when precipitation occur. It shows declining trend in WD precipitation and the increasing importance of Non-WDs. The findings show formation of 'Ω shape' flow - where high pressure is locked between two low pressure systems - over Indian subcontinent during Non-WDs days. Such dynamical wind col formation ('Ω shape') from lower (850Hpa) to upper (200 hPa) troposphere provides conducive situation for enhanced moisture incursion from Bay of Bengal (BoB) towards and over the WH during Non-WDs days leading to precipitation. This new finding needs to be looked from sustaining and replenishment of glaciers; agriculture; socioeconomic benefits during winters and needs to be addressed. [ABSTRACT FROM AUTHOR]

The tropopause is an important transition layer and can be a diagnostic of upper-tropospheric and lower-stratospheric structures, exhibiting unique atmospheric thermal and dynamic characteristics. A comprehensive understanding of the evolution of fine tropopause structures is necessary and primary for the further study of complex multi-scale atmospheric physical–chemical coupling processes in the upper troposphere and lower stratosphere. A novel method utilizing the bi-Gaussian function is capable of identifying the characteristic parameters of vertical tropopause structures and providing information on double-tropopause (DT) structures. The new method improves the definition of the cold-point tropopause and detects one (or two) of the most significant local cold points by fitting the temperature profiles to the bi-Gaussian function, which defines the point(s) as the tropopause height(s). The bi-Gaussian function exhibits excellent potential for explicating the variation trends of temperature profiles. The results of the bi-Gaussian method and lapse rate tropopause, as defined by the World Meteorological Organization, are compared in detail for different cases. Results indicate that the bi-Gaussian method is able to more stably and obviously identify the spatial and temporal distribution characteristics of the thermal tropopauses, even in the presence of multiple temperature inversion layers at higher elevations. Moreover, 5 years of historical radiosonde data from China (from 2012 to 2016) revealed that the occurrence frequency and thickness of the DT, as well as the single-tropopause height and the first and second DT heights, displayed significant meridional monotonic variations. The occurrence frequency (thickness) of the DT increased from 1.07 % (1.96 km) to 47.19 % (5.42 km) in the latitude range of 16–50° N. The meridional gradients of tropopause height were relatively large in the latitude range of 30–40° N, essentially corresponding to the climatological locations of the subtropical jet and the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

GNSS (Global Navigation Satellite Systems) tropospheric delay, specifically zenith wet delay (ZWD), shows clear spatial–temporal variations and is usually modeled as RWPN (random walk process noise). However, because RWPN does not take the geographical position of GNSS stations and local weather conditions into account for precise point positioning (PPP), it may lead to biased ZWD estimates. To address the scientific problem and improve ZWD estimates, we adopt the Expectation–Maximization algorithm (EM algorithm) to validate the feasibility of estimating RWPN using only GNSS measurements. Numerical experiments reveal that using only GNSS observations is capable of determining the RWPN parameter, although it could take several days to reach a stable solution if the initial guess deviates far away from the truth. It is also shown that estimating RWPN can almost always effectively improve ZWD estimates by several millimeters in contrast with traditional PPP results. If the ambiguities are fixed to their integer values correctly, the accuracy of RWPN estimates for ZWD can be greatly reduced by 2 mm / hour . [ABSTRACT FROM AUTHOR]

This study proposes a data-driven troposphere zenith total delay (ZTD) modeling method that takes into account the distance of GNSS continuously operating reference station (CORS) to the coast. Using ZTD data from 106 CORS stations, the strong correlation between the CORS-to-coast distance and ZTD is first identified. Then, this CORS-to-coast distance is incorporated as a new input data, along with the traditional time epoch and location input data, to develop a deep learning-based neural network model for ZTD prediction. This model is trained using 96 CORS stations spaced an average of 92 km apart. Ten testing CORS stations are divided into five inner stations and five outer stations from the CORS network to evaluate the ZTD prediction accuracy. Results from the study show that the proposed method improves the accuracy of ZTD prediction over traditional methods for a four-month period in 2018. At inner testing stations, the average ZTD prediction Root-mean-square errors (RMSEs) of the proposed method is 29.5 mm, which is smaller than the 34.2 mm of the traditional method. For outer testing stations, the average ZTD prediction RMSEs are 31.0 mm and 41.3 mm for the proposed method and traditional method respectively, resulting in a 5/17% ZTD prediction accuracy improvement. To sum up, the proposed method, which considers the CORS-to-coast distance for ZTD modeling, is demonstrated to enhance ZTD prediction accuracy over the traditional method. [ABSTRACT FROM AUTHOR]

Based on TIMED/SABER temperature observations and the Real‐Time Multivariate Madden‐Julian Oscillation (MJO) Index, for the first time, we discussed the daily resolution non‐migrating DE3 temperature tide response to the MJO phenomenon during MJO phases between ±45° latitude from 2003 to 2012 in the Mesosphere and Lower Thermosphere (MLT) region. We found: (a) The DE3 tide significantly depends on different MJO phases in the four seasons above 100 km altitude. The DE3 tidal phase exhibits varying responses to the different MJO phases. (b) The DE3 tide response to the MJO phenomenon is stronger at the solstices than at the equinoxes, and the response at the June solstices is stronger. (c) The symmetric component of the DE3 response to the MJO phenomenon is stronger than the asymmetric component. In summary, the daily resolved DE3 tide response to the MJO phenomenon, especially the DE3 tidal phase dominates certain seasonal characteristics and symmetry in the MLT region. Plain Language Summary: The MJO phenomenon is one of the dominant modes of intra‐seasonal variability in the troposphere in the tropical regions, and some seasonal atmospheric characteristics of the MLT region thought to be generated from the tropospheric MJO phenomenon. Based on TIMED/SABER temperature observations and the RMM Index of the troposphere, we quantified the daily resolution MLT tide response to the MJO phenomenon for the first time. We discuss the 30‐90‐day period DE3 tide response to the MJO phenomenon during phases 1–8 in different seasons between ±45° latitudes from 2003 to 2012 at the altitude of 70–108 km. We found: (a) The DE3 tide, including its amplitude and phase, shows a significant dependence on the MJO phase in the four seasons above 100 km altitude. (b) The DE3 tide affected by the MJO phenomenon are stronger at the solstices than at the equinoxes, and the June solstices exhibits the strongest MJO response. Moreover, there are some differences between the northern and southern hemispheres. (c) The symmetric component of the DE3 tide response to the MJO phenomenon is stronger than the asymmetric component in the four seasons. In summary, the daily resolved DE3 tide response to the MJO phenomenon, especially during the different MJO phase, dominates certain seasonal characteristics and symmetry in the MLT region. Key Points: The daily resolved DE3 temperature tidal component response to the Madden‐Julian Oscillation (MJO) phenomenon is presented in the Mesosphere and Lower Thermosphere regionThe DE3 tidal component response to the MJO phenomenon during different MJO phases dominates certain seasonal characteristicsThe symmetric component of the DE3 tide response to the MJO phenomenon is stronger than the asymmetric component [ABSTRACT FROM AUTHOR]

Using 11 years of hourly merged rainfall records and ERA reanalysis data, this paper reveals two major circulation modes leading to two types of short‐term heavy rainfall (STHR) along the Taihang Mountains' eastern foothills, and further explains their mechanisms. One circulation mode has a distinct warm anomaly at 300 hPa covering most areas of North China, together with the boundary‐layer westerly anomaly occurring in North China and its southern region (UTWA‐BLWA). UTWA‐BLWA effectively contributes to the reinforcement of the upper‐level divergence and the low‐level moisture convergence by promoting the strengthening of upper anticyclonic and low‐level southwesterly anomalies. The combined effects of low‐level jet (LLJ) and topographic uplift form the central‐northern STHR pattern. The other circulation structure has a 300‐hPa warm anomaly located to the southeastern Russia and a prominent 300‐hPa cold anomaly covering the south, together with the boundary‐layer easterly anomaly occurring over the whole region of eastern China (UTCA‐BLEA). The southern STHR pattern is attributed to the exit of the boundary‐layer jet (BLJ) over lower elevations due to moist transport and dynamic uplift associated with the easterly anomaly. The results indicate that the locations of STHR are related to the direction, intensity, and height of the LLJ. The findings highlight that the upper‐tropospheric temperature anomalies (UTTA) and boundary‐layer easterly flow jointly modulate heavy rainfall. Analysis of the coupled upper troposphere and boundary layer could help understand and forecast heavy rainfall. Plain Language Summary: Short‐term heavy rainfall (STHR) frequently occurs along the eastern foothills of the Taihang Mountains, which is difficult to forecast accurately and causes significant damage. Operational weather analysis and previous studies have suggested that the atmospheric conditions at the middle‐ and lower troposphere are the most important for STHR. Our findings highlight that the upper‐tropospheric temperature anomalies (UTTA) could represent the evolution of atmospheric structures favorable for heavy rainfall, and the boundary‐layer flow can refine the intensity and locations of orographic rain. The coupled UTTA and boundary‐layer easterly flow can jointly represent the major patterns of STHR over the eastern slope of Mt. Taihang. The results may provide helpful information for weather forecasts and warning issuing and give a better understanding of heavy rainfall in North China. Key Points: Upper‐tropospheric temperature anomalies (UTTA) can contribute to the favorable atmospheric conditions for short‐term heavy rainfall (STHR)The locations of STHR correspond to the direction, intensity, and height of the low‐level jetCoupling UTTA and boundary‐layer jet provides the two major patterns of STHR [ABSTRACT FROM AUTHOR]

The era of modern smartphones, running on Android version 7.0 and higher, facilitates nowadays acquisition of raw dual-frequency multi-constellation GNSS observations. This paves the way for GNSS community data to be potentially exploited for precise positioning, GNSS reflectometry or geoscience applications at large. The continuously expanding global GNSS infrastructure along with the enormous volume of prospective GNSS community data bring, however, major challenges related to data acquisition, its storage, and subsequent processing for deriving various parameters of interest. In addition, such large datasets cannot be managed manually anymore, leading thus to the need for fully automated and sophisticated data processing pipelines. Application of Machine Learning Technology for GNSS IoT data fusion (CAMALIOT) was an ESA NAVISP Element 1 project (NAVISP-EL1-038.2) with activities aiming to address the aforementioned points related to GNSS community data and their exploitation for scientific applications with the use of Machine Learning (ML). This contribution provides an overview of the CAMALIOT project with information on the designed and implemented cloud-native software for GNSS processing and ML at scale, developed Android application for retrieving GNSS observations from the modern generation of smartphones through dedicated crowdsourcing campaigns, related data ingestion and processing, and GNSS analysis concerning both conventional and smartphone GNSS observations. With the use of the developed GNSS engine employing an Extended Kalman Filter, example processing results related to the Zenith Total Delay (ZTD) and Slant Total Electron Content (STEC) are provided based on the analysis of observations collected with geodetic-grade GNSS receivers and from local measurement sessions involving Xiaomi Mi 8 that collected GNSS observations using the developed Android application. For smartphone observations, ZTD is derived in a differential manner based on a single-frequency double-difference approach employing GPS and Galileo observations, whereas satellite-specific STEC time series are obtained through carrier-to-code leveling based on the geometry-free linear combination of observations from both GPS and Galileo constellations. Although the ZTD and STEC time series from smartphones were derived on a demonstration basis, a rather good level of consistency of such estimates with respect to the reference time series was found. For the considered periods, the RMS of differences between the derived smartphone-based time series of differential zenith wet delay and reference values were below 3.1 mm. In terms of satellite-specific STEC time series expressed with respect to the reference STEC time series, RMS of the offset-reduced differences below 1.2 TECU was found. Smartphone-based observations require special attention including additional processing steps and a dedicated parameterization in order to be able to acquire reliable atmospheric estimates. Although with lower measurement quality compared to traditional sources of GNSS data, an augmentation of ground-based networks of fixed high-end GNSS receivers with GNSS-capable smartphones would however, form an interesting source of complementary information for various studies relying on GNSS observations. [ABSTRACT FROM AUTHOR]

Ethane is the most abundant non-methane hydrocarbon in the troposphere, where it impacts ozone and reactive nitrogen and is a key tracer used for partitioning emitted methane between anthropogenic and natural sources. However, quantification has been challenged by sparse observations. Here, we present a satellite-based measurement of tropospheric ethane and demonstrate its utility for fossil-fuel source quantification. An ethane spectral signal is detectable from space in Cross-track Infrared Sounder (CrIS) radiances, revealing ethane signatures associated with fires and fossil fuel production. We use machine-learning to convert these signals to ethane abundances and validate the results against surface observations (R2 = 0.66, mean CrIS/surface ratio: 0.65). The CrIS data show that the Permian Basin in Texas and New Mexico exhibits the largest persistent ethane enhancements on the planet, with regional emissions underestimated by seven-fold. Correcting this underestimate reveals Permian ethane emissions that represent at least 4-7% of the global fossil-fuel ethane source. The authors have developed a satellite-based method that detects tropospheric ethane, revealing major sources like fires and fossil fuels. It agrees with ground-based observations and shows Permian Basin emissions of ethane are higher than estimated. [ABSTRACT FROM AUTHOR]

This study is an additional investigation of stratosphere–troposphere coupling based on the recent stratospheric winter descriptions in five distinct modes: January, February, Double, Dynamical, and Radiative. These modes, established in a previous study, categorize the main stratospheric winter typologies modulated by the timing of important sudden stratospheric warmings (SSWs) and final stratospheric warmings (FSWs). The novelty of this research is to investigate the Northern Annular Mode, mean sea level pressure (MSLP) anomalies in the Ural and Aleutian regions, and the decomposition of Eliassen–Palm flux into wavenumbers 1 and 2 within each mode. The results show that the January and Double modes exhibit similar pre-warming surface signals, characterized by Ural blocking and Aleutian trough events preceding weak polar vortex events. The January mode displays a positive MSLP anomaly of +395 hPa (−191 hPa) in the Ural (Aleutian) region in December, while the Double mode shows +311 hPa (−89 hPa) in November. These modes are primarily wave-1 driven, generating tropospheric responses via negative Arctic Oscillation patterns. Conversely, the February and Dynamical modes show opposite signals, with Aleutian blocking and Ural trough events preceding strong polar vortex events. In December, the February mode exhibits MSLP anomalies of +119 hPa (Aleutian) and −180 hPa (Ural), while the Dynamical mode shows +77 hPa and −184 hPa, respectively. These modes, along with important SSWs in February and dynamical FSWs, are driven by both wave-1 and wave-2 and do not significantly impact the troposphere. The Radiative mode's occurrence is strongly related to the Aleutian blocking presence. These findings confirm that SSW timing is influenced by specific dynamical forcing related to surface precursors and underscore its importance in subsequent tropospheric responses. This study establishes a connection between early winter tropospheric conditions and upcoming stratospheric states, potentially improving seasonal forecasts in the northern hemisphere. [ABSTRACT FROM AUTHOR]

The spatiotemporal and cloud features of the extreme rainfall under the warm and cold upper-level anomalies over the central Yungui Plateau (YGP) were investigated using the hourly rain gauge records, ERA5 reanalysis data, TRMM, and Fengyun satellite data, aiming to refine the understanding of different types of extreme rainfall. Extreme rainfall under an upper-level negative temperature anomaly (cold events) presents stronger convective cloud features when compared with the positive temperature anomaly (warm events). The maximum rainfall intensity and duration in cold events is much larger than that of warm events, while the brightness temperature of the cloud top is lower, and the ratio of convective rainfall is higher. In cold events, the middle-to-upper troposphere is dominated by a cold anomaly, and an unstable configuration with upper (lower) cold (warm) anomalies is observed around the central YGP. Although the upper-level temperature anomaly is positive, the anomalous divergence and convergence of southerly and northerly winds, as well as the strong moisture center and upward motions, are also found over the central YGP in warm events. The stronger atmospheric instability and higher convective energy under the upper-level cold anomalous circulation are closely associated with the rainfall features over the central YGP. The results indicate that the upper tropospheric temperature has significant influences on extreme rainfall, and thus more attention should be paid to the upper tropospheric temperature in future analyses. [ABSTRACT FROM AUTHOR]

This study investigates the influence of synoptic-scale activities on extreme precipitation during March–April–May (MAM) over Southern East Asia (SEA) using observational data and compares the results with the outputs from 20 Coupled Model Intercomparison Project phase 6 (CMIP6) historical runs. Observations show that SEA intense daily precipitation in MAM is linked to enhanced upper-level synoptic-scale waves; these disturbances are associated with significant anomalous temperature advection as well as moisture flux convergence, creating favorable conditions for extreme rainfall. Furthermore, it is found that a temperature advection index (TAI) can be utilized to characterize such synoptic-scale activities. Inspection of CMIP6 historical runs reveals that, among 20 models, 13 models perform well in accurately capturing the observed SEA rainfall pattern; such extreme events are also closely linked to TAI in the model environment. Overall, observed (simulated) results show that 78% (75%) of extreme events in the Yangtze River Basin–South Korea–south of Japan region can be attributed to positive TAI. Additionally, the related circulation anomalies such as the upper-level synoptic-scale wave feature, temperature advection, and moisture anomalies from these models closely resemble those observed during extreme precipitation days in SEA. Our findings suggest that TAI can effectively indicate both the frequency and intensity of extreme rainfall events in SEA, along with the associated synoptic-scale activities. Further study reveals a close lead-lag correlation between TAI and rainfall patterns over SEA. This correlation is characterized by eastward-propagating wave trains across the entire troposphere. Consequently, TAI not only acts as a benchmark for quantifying synoptic-scale extreme rainfall in SEA but also shows potential in predicting SEA rainfall linked to synoptic-scale disturbances. [ABSTRACT FROM AUTHOR]

This study evaluates the prediction of MJO teleconnections in two versions of the NOAA Unified Forecast System (UFS): prototype 5 (UFS5) and prototype 6 (UFS6). The differences between the two prototypes in the number of vertical layers (64 in UFS5 vs. 127 in UFS6) and the model top (54 km in UFS5 vs. 80 km in UFS6) can potentially impact the prediction of MJO teleconnections. With respect to ERA-Interim, the global teleconnections of the MJO to the Northern Hemisphere show similar biases in 500 hPa geopotential height over the North Atlantic and European sectors in both prototypes, whereas UFS6 has slightly smaller biases over the North Pacific region. Both prototypes capture the extratropical cyclone activity occurring in weeks 3–4 over the North Atlantic after the MJO phases 6–7 and over the North Pacific and North America after MJO phases 4–5. Both prototypes successfully forecast the sign and approximate locations of 2-m temperature anomalies over the mid-to-high latitude continents occurring in weeks 3–4 after MJO phase 3 but fail to capture the sign reversal of anomalies over North America between weeks 3 and 4 after MJO phase 7. Overall, the two prototypes show similar performance in simulating the tropospheric basic state as well as prediction skill of the MJO and MJO teleconnections, suggesting that the increase in model vertical resolution and model top does not strongly improve the prediction of MJO teleconnections in the troposphere in UFS. [ABSTRACT FROM AUTHOR]

While influences of lower and upper troposphere large-scale circulations on tropical cyclone (TC) genesis in the western North Pacific (WNP) is generally well understood, their synergic influences on the WNP TC genesis and their relative contribution remain unclear. Here two leading modes of lower and upper troposphere large-scale circulation over the WNP are identified using multivariate empirical orthogonal function analysis on the low and upper-level large-scale parameters. The first mode features an anticyclonic circulation over the WNP, while the second mode is characterized by a cyclonic circulation in the tropical eastern WNP but an anticyclonic circulation to the north WNP. Both two modes have profound impacts on the WNP TC genesis, which can be reasonably interpreted by the collective influences of lower and upper-level large-scale parameters. Further diagnostic suggests that the lower troposphere large-scale circulations play a crucial role, while the upper troposphere large-scale circulations play a secondary but important role in TC genesis over the WNP. Moreover, it is found that the extremely favorable lower and upper large-scale conditions associated with the extremely negative phases of two leading modes led to the unprecedented WNP TC genesis frequency in 1994 since the geo-satellite era. The result indicates the important role of synergic influences of the lower and upper troposphere large-scale circulations in the interannual variability of TC genesis over the WNP. [ABSTRACT FROM AUTHOR]

Global Navigation Satellite System (GNSS) radio occultation (RO) provides valuable 3-D atmospheric profiles with all-weather, all the time and high accuracy. However, GNSS RO mission data are still limited for global coverage. Currently, more commercial GNSS radio occultation missions are being launched, e.g., PlanetiQ. In this study, we examine the commercial GNSS RO PlanetiQ mission performance in comparison to KOMPSAT-5 and PAZ, including the coverage, SNR, and penetration depth. Additionally, the quality of PlanetiQ RO refractivity profiles is assessed by comparing with the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5) data in October 2023. Our results ensure that the capability of PlanetiQ to track signals from any GNSS satellite is larger than the ability of KOMPSAT-5 and PAZ. The mean L1 SNR for PlanetiQ is significantly larger than that of KOMPSAT-5 and PAZ. Thus, PlanetiQ performs better in sounding the deeper troposphere. Furthermore, PlanetiQ's average penetration height ranges from 0.16 to 0.49 km in all latitudinal bands over water. Generally, the refractivity profiles from all three missions exhibit a small bias when compared to ERA5-derived refractivity and typically remain below 1% above 800 hPa. [ABSTRACT FROM AUTHOR]

Tropical cyclone intensification is simulated with a cloud-resolving model under idealized conditions of constant SST and unidirectional environmental vertical wind shear maximized in the middle troposphere. The intensification process commonly involves a sharp transition to relatively fast spinup before the surface vortex achieves hurricane-force winds in the azimuthal mean. The vast majority of transitions fall into one of two categories labeled S and A. Type S transitions initiate quasi-symmetric modes of fast spinup. They occur in tropical cyclones after a major reduction of tilt and substantial azimuthal spreading of inner-core convection. The lead-up also entails gradual contractions of the radii of maximum wind speed rm and maximum precipitation. Type A transitions begin before an asymmetric tropical cyclone becomes vertically aligned. Instead of enabling the transition, alignment is an essential part of the initially asymmetric mode of fast spinup that follows. On average, type S transitions occur well after and type A transitions occur once the cyclonically rotating tilt vector becomes perpendicular to the shear vector. Prominent temporal peaks of lower-tropospheric CAPE and low-to-midlevel relative humidity averaged over the entire inner core of the low-level vortex characteristically coincide with type S but not with type A transitions. Prominent temporal peaks of precipitation and midlevel vertical mass flux in the meso-β-scale vicinity of the convergence center characteristically coincide with type A but not with type S transitions. Despite such differences, in both cases, the transitions tend not to begin before the distance between the low-level convergence and vortex centers divided by rm reduces to unity. [ABSTRACT FROM AUTHOR]

Tropical tropospheric ozone (TTO) is important for the global radiation budget because the longwave radiative effect of tropospheric ozone is higher in the tropics than midlatitudes. In recent decades the TTO burden has increased, partly due to the ongoing shift of ozone precursor emissions from midlatitude regions toward the Equator. In this study, we assess the distribution and trends of TTO using ozone profiles measured by high-quality in situ instruments from the IAGOS (In-Service Aircraft for a Global Observing System) commercial aircraft, the SHADOZ (Southern Hemisphere ADditional OZonesondes) network, and the ATom (Atmospheric Tomographic Mission) aircraft campaign, as well as six satellite records reporting tropical tropospheric column ozone (TTCO): TROPOspheric Monitoring Instrument (TROPOMI), Ozone Monitoring Instrument (OMI), OMI/Microwave Limb Sounder (MLS), Ozone Mapping Profiler Suite (OMPS)/Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), Cross-track Infrared Sounder (CrIS), and Infrared Atmospheric Sounding Interferometer (IASI)/Global Ozone Monitoring Experiment 2 (GOME2). With greater availability of ozone profiles across the tropics we can now demonstrate that tropical India is among the most polluted regions (e.g., western Africa, tropical South Atlantic, Southeast Asia, Malaysia and Indonesia), with present-day 95th percentile ozone values reaching 80 nmol mol -1 in the lower free troposphere, comparable to midlatitude regions such as northeastern China and Korea. In situ observations show that TTO increased between 1994 and 2019, with the largest mid- and upper-tropospheric increases above India, Southeast Asia, and Malaysia and Indonesia (from 3.4 ± 0.8 to 6.8 ± 1.8 nmol mol -1 decade -1), reaching 11 ± 2.4 and 8 ± 0.8 nmol mol -1 decade -1 close to the surface (India and Malaysia–Indonesia, respectively). The longest continuous satellite records only span 2004–2019 but also show increasing ozone across the tropics when their full sampling is considered, with maximum trends over Southeast Asia of 2.31 ± 1.34 nmol mol -1 decade -1 (OMI) and 1.69 ± 0.89 nmol mol -1 decade -1 (OMI/MLS). In general, the sparsely sampled aircraft and ozonesonde records do not detect the 2004–2019 ozone increase, which could be due to the genuine trends on this timescale being masked by the additional uncertainty resulting from sparse sampling. The fact that the sign of the trends detected with satellite records changes above three IAGOS regions, when their sampling frequency is limited to that of the in situ observations, demonstrates the limitations of sparse in situ sampling strategies. This study exposes the need to maintain and develop high-frequency continuous observations (in situ and remote sensing) above the tropical Pacific Ocean, the Indian Ocean, western Africa, and South Asia in order to estimate accurate and precise ozone trends for these regions. In contrast, Southeast Asia and Malaysia–Indonesia are regions with such strong increases in ozone that the current in situ sampling frequency is adequate to detect the trends on a relatively short 15-year timescale. [ABSTRACT FROM AUTHOR]

Abstract Although evidence of recovery in Antarctic stratospheric ozone has been found, evidence of recovery in Arctic ozone is still elusive, even though 25 years have passed since the peak in ozone depleting substances. Here we have used a Dynamic Linear Model to derive time-varying trends over 20-year periods in the Arctic ozone time series, measured in-situ by ozonesondes from 6 stations, from 1994 to 2022. The model accounts for seasonality, external forcing and 1st-order correlation in the residuals. As proxies for the external forcing, we have used tropopause pressure (replaced with Arctic Oscillation in the troposphere), eddy heat flux, the volume of polar stratospheric clouds multiplied by effective equivalent stratospheric chlorine, and solar radio flux at 10.7 cm for the 11-year solar cycle. Our results indicate that the ozone recovery in the lower Arctic stratosphere is not detectable. Though significant positive trends have been detected prior to 2017 at some stations, there are no statistically significant positive trends after 2017. Moreover, at a number of stations the trends after 2019 are rather negative and significant, varying between − 0.30 ± 0.25 and − 1.00 ± 0.85% per decade. Furthermore, the Arctic troposphere exhibited only statistically significant negative trends over 20-year periods ending in 2017 or later, varying between − 0.31 ± 0.27 and − 1.76 ± 0.41% per decade. These results highlight the importance of continued monitoring of the Arctic ozone.

The Earth can be divided by main layers, including the atmosphere, geosphere (solid Earth), and biosphere, depending on its predominant component. In this work, the layer of the Earth which constantly contains a high concentration of muons (~8 × 1012 muons) and its upper border are respectively defined as the muosphere and muopause. The altitude of the muosphere spans from the lower stratosphere to the upper crust of the Earth. In order to study its dynamics, the muopause height was spatiotemporally studied with a new kind of technique called the distance of flight (DoF) which utilizes variations in the muon's decay length. In this work, (A) numerical modeling was performed, and it was clarified that seasonal variations in the cosmic muon flux are predominantly ruled by muopause dynamics, (B) the muon data were compared with the balloon-based measurement results, and it was confirmed that muopause dynamics is closely related with lower-stratospheric height variations. Since the muopause is the region spanning between the upper troposphere and the lower stratosphere, the potential of the current DoF approach needs to be further investigated by cross-comparing related case studies and other atmospheric climate datasets. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a global atmospheric pollutant. In its oxidized form (HgII), it can readily deposit to ecosystems, where it may bioaccumulate and cause severe health effects. High HgII concentrations are reported in the free troposphere, but spatiotemporal data coverage is limited. Underestimation of HgII by commercially available measurement systems hinders quantification of Hg cycling and fate. During spring–summer 2021 and 2022, we measured elemental (Hg0) and oxidized Hg using a calibrated dual-channel system alongside trace gases, aerosol properties, and meteorology at the high-elevation Storm Peak Laboratory (SPL) above Steamboat Springs, Colorado. Oxidized Hg concentrations displayed diel and episodic behavior similar to previous work at SPL but were approximately 3 times higher in magnitude due to improved measurement accuracy. We identified 18 multi-day events of elevated HgII (mean enhancement of 36 pgm-3) that occurred in dry air (mean ± SD of relative humidity = 32 ± 16 %). Lagrangian particle dispersion model (HYSPLIT–STILT, Hybrid Single-Particle Lagrangian Integrated Trajectory–Stochastic Time-Inverted Lagrangian Transport) 10 d back trajectories showed that the majority of transport prior to events occurred in the low to middle free troposphere. Oxidized Hg was anticorrelated with Hg0 during events, with an average (± SD) slope of - 0.39 ± 0.14. We posit that event HgII resulted from upwind oxidation followed by deposition or cloud uptake during transport. Meanwhile, sulfur dioxide measurements verified that three upwind coal-fired power plants did not influence ambient Hg at SPL. Principal component analysis showed HgII consistently inversely related to Hg0 and generally not associated with combustion tracers, confirming oxidation in the clean, dry free troposphere as its primary origin. [ABSTRACT FROM AUTHOR]

Ozone is a potent air pollutant in the lower troposphere and an important short-lived climate forcer (SLCF) in the upper troposphere. Studies investigating long-term trends in the tropospheric column ozone (TCO 3) have shown large-scale spatio-temporal inconsistencies. Here, we investigate the long-term trends in lower-tropospheric column ozone (LTCO 3 , surface–450 hPa sub-column) by exploiting a synergy of satellite and ozonesonde data sets and an Earth system model (UK's Earth System Model, UKESM) over North America, Europe, and East Asia for the decade 2008–2017. Overall, we typically find small LTCO 3 linear trends with large uncertainty ranges using the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), while model simulations indicate a stable LTCO 3 tendency. The satellite a priori data sets show negligible trends, indicating that any year-to-year changes in the spatio-temporal sampling of these satellite data sets over the period concerned have not artificially influenced their LTCO 3 temporal evolution. The application of the satellite averaging kernels (AKs) to the UKESM simulated ozone profiles, accounting for the satellite vertical sensitivity and allowing for like-for-like comparisons, has a limited impact on the modelled LTCO 3 tendency in most cases. While, in relative terms, this is more substantial (e.g. on the order of 100 %), the absolute magnitudes of the model trends show negligible change. However, as the model has a near-zero tendency, artificial trends were imposed on the model time series (i.e. LTCO 3 values rearranged from smallest to largest) to test the influence of the AKs, but simulated LTCO 3 trends remained small. Therefore, the LTCO 3 tendencies between 2008 and 2017 in northern-hemispheric regions are likely to be small, with large uncertainties, and it is difficult to detect any small underlying linear trends due to interannual variability or other factors which require further investigation (e.g. the radiative transfer scheme (RTS) used and/or the inputs (e.g. meteorological fields) used in the RTS). [ABSTRACT FROM AUTHOR]

Synoptic forcings have traditionally played a pivotal role in extreme rainfall over North China. However, there are still large unexplained gaps in understanding the formation of extreme rainfalls over this region. The heavy rainfall event, lasting from 29 July to 2 August 2023 (referred to as "23·7" event), is characterized by long duration, widespread coverage, and high accumulated rainfall over North China. Overall, the persistent extreme rainfall is closely associated with the remnant vortex originating from typhoon Doksuri(2305), tropical storm Khanun(2306), and the unusual westward extended western Pacific subtropical high (WPSH), as well as quasi-stationary cold dry air masses surrounding North China on the west and north sides. Based on wind profiles and rainfall characteristics, the life history of the "23·7" event is divided into two stages. In the first stage, the western boundary of the western Pacific subtropical high (WPSH) was destroyed by the tropical storm Doksuri, appearing that the WPSH retreated eastward with decreasing height. As a result, an inclined vertical distribution on the western boundary was established below 500 hPa. Therefore, convections were limited by the tilted WPSH with warm-dry cover embedded in the low-to-middle troposphere. Meanwhile, the orography in the west of North China was controlled by cold air masses above nearly 3.0 km. Combining the orographic and cold air blockings, only a shallow southeasterly layer (between 1.3 and 3.0 km) can overpass mountains. Although the warm and moist southeasterly flows were lifted by orography, no convections were triggered because of the local capped cold and dry air masses overhead. Under this framework, equivalent potential temperature (θe) gradients were established between warm humid and dry cold air masses, similar to a warm front, causing warm air to lift and generate widespread rainfall but low intensity. However, the lifting was too weak to allow convection to be highly organized. In the second stage, the WPSH was further destroyed by enhanced Khanun, and thus the embedded warm-dry cover associated with the tilted WPSH was significantly thinned. Consequently, convections triggered by orographic blocking can move upward and consequently further develop, forming deep convections. Comparatively speaking, the convections in the second stage are much deeper than those in the first stage. The results gained herein may shed new light on better understanding and forecasting of long-lasting extreme rainfall. [ABSTRACT FROM AUTHOR]

In this study, the Atmospheric Infrared Sounder (AIRS) Observations for Model Intercomparison Projects (Obs4MIPs) V2.1 tropospheric air temperature, specific humidity, and relative humidity data are utilized to evaluate the global tropospheric temperature and humidity simulations in the fully coupled global climate models from the Coupled Model Intercomparison Project phases 3, 5, and 6 (CMIP3, CMIP5, and CMIP6), and possible simulation improvement in CMIP6 models in comparison to CMIP3 and CMIP5 models. Our analyses indicate that all three phases of CMIP models share similar tropospheric air temperature, specific humidity, and relative humidity biases in their multi‐model ensemble means relative to AIRS. Cold biases up to 4 K and positive relative humidity biases up to 20% are found in the free troposphere almost globally with maxima over the mid‐latitude storm tracks. Warm biases up to 2 K are seen over the Southern Ocean in the lower troposphere. Positive specific and relative humidity biases exist over the off‐equatorial oceans while negative specific and relative humidity biases are seen near the equator in the tropical free troposphere, which are related to the double‐intertropical convergence zone bias in the models. Both the air temperature and specific humidity biases are important to the relative humidity biases except in the tropical free troposphere where the specific humidity biases dominate. The tropospheric air temperature, specific humidity, and relative humidity biases are reduced from CMIP3 to CMIP5 and to CMIP6 at almost all pressure levels except at 300 hPa for specific humidity and in the boundary layer for relative humidity. Plain Language Summary: In this study, we use the Atmospheric Infrared Sounder (AIRS) data to assess how well the fully coupled global climate models simulate temperature and humidity in the troposphere. We focus on the climate models from the recent three phases of Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6) and investigate if there is any bias reduction in the recent CMIP6 models compared to the earlier CMIP3 and CMIP5 models. To conduct our evaluation, we compare the CMIP model ensemble averages with the AIRS data and find that all three phases of CMIP models exhibit similar biases in tropospheric temperature and humidity. For example, cold biases up to 4 K and positive relative humidity biases up to 20% are seen in the free troposphere over most regions, with the maximum biases over the midlatitudes. Positive specific and relative humidity biases are found over the off‐equatorial oceans while negative specific and relative humidity biases are seen near the equator in the tropical free troposphere. We also note a possible temperature and humidity bias reduction from the CMIP3 models to the CMIP5 models and then to the CMIP6 models at most pressure levels. Key Points: CMIP Phase 3, 5 and 6 climate models share similar tropospheric air temperature, specific and relative humidity biases relative to Atmospheric Infrared SounderCold biases and positive relative humidity biases are prevalent in free troposphere almost globally with maxima over the mid‐latitude oceansDouble‐intertropical convergence zone bias‐related specific and relative humidity biases are evident in tropical free troposphere: positive (negative) off (near) the equator [ABSTRACT FROM AUTHOR]

This study investigates dynamic and thermodynamic components of moisture flux convergence in Southwest China (SW-MFC) and their underlying physical mechanisms during early and late summer. Using precipitation observation and CRA-40 reanalysis datasets from 1979 to 2023, the results show that both dynamic and thermodynamic processes modulate the SW-MFC in early summer (May-June), with dynamics playing a pivotal role. In contrast, the precipitation anomaly in late summer (July-August) is predominantly driven by the dynamic factors. Meanwhile, the large-scale circulation over the northern Indian Peninsula significantly modulates the SW-MFC. In early summer, anomalous convection around the Maritime Continent with the tripole sea surface temperature (SST) mode in the tropical Indo-Pacific can trigger the formation of "double ring" vertical zonal circulation cells. A large-scale westerly anomaly at the lower troposphere over the northern Arabian Sea foster cyclone strengthening over the northern Indian Peninsula, enhancing southerly moisture transport and increasing precipitation over Southwest China. During the late summer, large-scale dipole SST pattern between the subtropical central-eastern Pacific and the Indo-Pacific warm pool generates significant easterly anomalies towards the Maritime Continent. The SST gradient stimulates an extensive anticyclonic shear zone over the western equatorial Pacific, with an intensified low-pressure zone to its north. This atmospheric pattern over Southwest China and Indian Peninsula can form a vertical circulation circle that largely intensifies widespread precipitation. Numerical model experiments can reproduce the mechanisms of tropical Indo-Pacific joint effects on the Southwest precipitation in both early and late summer, providing a theoretical basis for understanding and forecasting summer precipitation over Southwest China. [ABSTRACT FROM AUTHOR]

The interannual variability of South China spring rainfall (SCSR) is examined in relation to the North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO) based on longer-term reanalysis datasets and simulation outputs from Coupled Model Intercomparison Project Phase 6 (CMIP6) for the period 1901–2014. The interannual fluctuations of SCSR exhibit a significantly positive (negative) correlation with those of the spring NPO (NAO), thus the above-normal and below-normal SCSR tend to occur during the positive NPO (NPO+) and negative NPO (NPO−) phases, whereas the negative NAO (NAO−) and positive NAO (NAO+) phases favor the above-normal and below-normal SCSR, respectively. However, the variations in spring NPO and NAO are independent of each other, suggesting that anomalous SCSR depends on circulation anomalies induced jointly by various combinations of the NPO–NAO phases. Composite analyses show that the synergetic impacts on SCSR of the out-of-phase NPO+/NAO– combination are through the enhanced dipole circulation patterns, with both the subtropical negative and mid-latitude positive sea level pressure anomalies intensifying as compared to those for the individual NPO+ phase in the lower troposphere. The intensified low-level moisture convergence associated with the southwestward-extending subtropical anticyclone is in conjunction with the upper-level divergence over South China due to the northward-extending westerly-jet, resulting in more SCSR. Inversely, another out-of-phase NPO–/NAO+ combination favors less SCSR. In contrast, the preference for normal SCSR appears in the in-phase NPO+/NAO+ and NPO−/NAO− combinations, because the circulation anomalies in the NPO+ (NPO−) phase tend to be partly offset by those in the NAO+ (NAO−) phase. Importantly, the above respectively and jointly influencing mechanisms of the spring NPO and NAO on SCSR are validated by historical simulations from some CMIP6 models. [ABSTRACT FROM AUTHOR]

A Stratospheric Northern Annular Mode (SNAM) phase-based composite analysis reveals that continental Cold Air Outbreaks (CAOs) can occur during both positive and negative SNAM events. CAOs tend to occur over Asia, characterized by a meridional-dipole surface temperature anomaly pattern (cold midlatitudes and warm high-latitudes) when the SNAM index is decreasing or the stratospheric polar vortex is weakening, but over North America and Europe with a meridionally-homogeneous pattern when the SNAM index is increasing or the stratospheric polar vortex is strengthening. While the decreasing SNAM is dominated by a stronger stratospheric poleward warm branch (WB-ST) of the isentropic meridional mass circulation and vice versa, the CAOs always follow a stronger tropospheric poleward warm branch (WB-TR) and an equatorward cold branch (CB) of the isentropic meridional mass circulation. The correspondence between the stronger/weaker WB-ST and stronger/weaker WB-TR&CB during majority of SNAM phases (referred to as stratosphere-troposphere coupling regimes) is responsible for the CAOs in Asia. During the remaining phases (stratosphere-troposphere decoupling regimes), in accompany with a weaker/stronger WB-ST, the WB-TR&CB are stronger/weaker and relates to the CAOs occurred in North America and Europe. The coupling regimes when the stratospheric polar vortex is weakening/strengthening are mainly attributed to the E-P flux convergence/divergence from the middle troposphere to the lower stratosphere, the larger wave amplitude throughout the column, and anomalous tropospheric wave flux mainly in the Asia in subpolar latitudes. The decoupling regimes, however, are mainly related to the anomalous westward-tilting of waves and the wave flux reflection toward the North America or Europe. [ABSTRACT FROM AUTHOR]

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Upward propagating waves of lower atmospheric origin play an important role in coupling terrestrial weather with space weather on daily to inter‐annual timescales. Quantifying their short‐term (<30 days) variability is a difficult challenge because simultaneous observations at multiple local times are needed to sample diurnal cycles. This study demonstrates and validates a short‐term estimation method of the DE3 non‐migrating tide at the equator and then applies the technique to three independent data sets: MIGHTI, SABER, and TIDI. We find that daily DE3 estimates from SABER, MIGHTI, and TIDI at equator agree well with correlation coefficients ranging between 0.76 and 0.85. The daily DE3 amplitude variability is typically ∼7 m/s in zonal winds and ∼3 K in temperature. We also find that daily MLT variations and F‐region ionospheric DE3 from COSMIC‐2 Global Ionospheric Specification (GIS) show a correlation of 0.55–0.65, suggesting that not all ionospheric variability can be attributed to the E‐region dynamo; however, increasing correlation with increasing time‐scale suggests that lower atmospheric variability has pronounced impact on the ionosphere on intra‐seasonal scales. We find that the MLT and the F‐region ionosphere exhibit strong coherent intra‐seasonal oscillations (residual amplitudes upto 50%–60%); their coherency with the MJO in 2020 suggests a possible modulation of the upward propagating DE3 tide related to this major tropical tropospheric weather pattern. In addition, we find stratospheric QBO signatures in the MLT DE3 on inter‐annual scales. This study offers fresh observational insights into the pivotal role of tropospheric weather in shaping variability in the coupled thermosphere‐ionosphere system. Plain Language Summary: A growing body of research unequivocally demonstrates the important role upward propagating waves from the lower atmosphere play in shaping the meteorology of the mesosphere, thermosphere and ionosphere from daily to inter‐annual time scales. Understanding these variations is crucial for space weather studies and applications, including radio wave propagation, satellite communication, and space orbital debris. Among these oscillations, the diurnal eastward propagating tide with zonal wave number 3 (DE3) holds particular importance. This oscillation, driven by expansive tropospheric weather systems, can attain substantial amplitudes in the upper atmosphere. The primary objective of this study is to demonstrate and validate a methodology facilitating the short‐term estimation of DE3 to understand the tidal weather of the upper atmosphere. Our approach starts by using a physics‐based model of the upper atmosphere to test and validate this new daily DE3 retrieval method. Next, we apply our tidal estimation method to extensive satellite‐based measurements of winds, temperatures, and ionospheric electron density. Lastly, we analyze the results to provide fresh observational insights into the pivotal role of tropospheric weather on Earth's near space environment from daily to inter‐annual time scales. Key Points: Demonstrated and validated a short‐term DE3 tidal estimation technique at the equatorDaily DE3 tidal amplitude variability at the equator is typically ∼7 m/s in zonal winds and ∼3 K in temperatureStrong intra‐seasonal variations in MLT region DE3 and F‐region ionosphere electron densities possibly due to the MJO are observed [ABSTRACT FROM AUTHOR]

The tropospheric production of O3 is complex, depending on nitrogen oxides (NOx = NO + NO2), volatile organic compounds (VOCs), and solar radiation. We present a case study showing that the O3 concentration is higher in a rural area, 14 km downwind from a coastal town in Central Italy, compared with the urban environment. The hypothesis is that the O3 measured inland results from the photochemical processes occuring in air masses originating at the urban site, which is richer in NOx emissions, during their transport inland.To demonstrate this hypothesis, a feed forward neural network (FFNN) is used to model the O3 measured at the rural site, comparing the modeled O3 and the measured O3 in different scenarios, which include both input parameters related to local O3 production by photochemistry and input parameters associated with regional transport of O3 precursors. The simulation results show that the local NOx concentration is not a good input to model the observed O3 (R = 0.17); on the contrary including the wind speed and direction as input of the FFNN model, the modelled O3 is well correlated with that measured O3 (R = 0.82). [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O3) is an important greenhouse gas that is also hazardous to human health. The formation of O3 is sensitive to the levels of its precursors NOx (≡NO + NO2) and peroxy radicals, for example, generated by the oxidation of volatile organic compounds (VOCs). A better understanding of this sensitivity will show how changes in the levels of these trace gases could affect O3 levels today and in the future, and thus air quality and climate. In this study, we investigate O3 sensitivity in the tropical troposphere based on in situ observations of NO, HO2 and O3 from four research aircraft campaigns between 2015 and 2023. These are OMO (Oxidation Mechanism Observations), ATom (Atmospheric Tomography Mission), CAFE Africa (Chemistry of the Atmosphere Field Experiment in Africa) and CAFE Brazil, in combination with simulations using the EMAC atmospheric chemistry—climate model. We use the metric α(CH3O2) together with NO to investigate the O3 formation sensitivity. We show that O3 formation is generally NOx‐sensitive in the lower and middle tropical troposphere and is in a transition regime in the upper troposphere. By distinguishing observations impacted by lightning or not we show that NO from lightning is the most important driver of O3 sensitivity in the tropics. NOx‐sensitive chemistry predominates in regions without lightning impact, with α(CH3O2) ranging between 0.56 and 0.82 and observed average O3 levels between 35 and 55 ppbv. Areas affected by lightning exhibit strongly VOC‐sensitive O3 chemistry with α(CH3O2) of about 1 and average O3 levels between 55 and 80 ppbv. Plain Language Summary: Ozone (O3) in the troposphere is both an air pollutant and a greenhouse gas. It is formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs). The formation can be sensitive to either of these precursors depending on their abundance. Considering the high relevance of O3 in regard to human health and global warming, it is important to understand this sensitivity of O3 formation, which allows to predict future changes in O3. Here, we investigate O3 formation sensitivity toward NOx and VOCs in the tropical troposphere based on aircraft measurements during four research campaigns between 2015 and 2023, and a global model. We include observations of NO, HO2 (hydroperoxyl radicals) and O3 over South America, the Middle East and the Pacific, Atlantic and Indian Ocean. We find that O3 formation is sensitive to NOx in the lower tropical troposphere. In the upper tropical troposphere, lightning events control O3 chemistry and promote strong VOC‐sensitive O3 formation. Key Points: α(CH3O2) correlated with NO is a powerful metric for indicating O3 sensitivity and is valid throughout the troposphereO3 chemistry in the remote tropical lower troposphere is found to be NOx‐sensitiveNO emissions from lightning drive O3 sensitivity in the tropical upper troposphere and induce highly VOC‐sensitive chemistry [ABSTRACT FROM AUTHOR]

A historic rainstorm occurred over Henan, China, in July 2021 ("7.20" extreme precipitation event), resulting in significant human casualties and socioeconomic losses. A global variable-resolution model (MPAS-Atmosphere v7.3) was employed to simulate this extreme precipitation event. A series of simulations have been done at both quasi-uniform (60 and 15 km) and variable-resolution (60–15 and 60–3 km) meshes from hydrostatic to nonhydrostatic scale with two parameterization scheme suites. For the 48 h peak precipitation duration (20–22 July), the 60–3 km variable-resolution simulation coupled with the scale-aware convection-permitting parameterization scheme suite stands out predominantly among other simulation experiments as it reproduces this extreme precipitation event most accurately. At 15 km resolution, the 60–15 km variable-resolution simulation achieves comparable forecasting skills to the 15 km quasi-uniform simulation but at a much reduced computing cost. In addition, we found that the default mesoscale suite generally outperforms the convection-permitting suite at 15 km resolution as simulations coupled with the convection-permitting suite missed the third peak of this extreme precipitation event, while the mesoscale suite did not. Furthermore, it is found that the large-scale circulation plays a critical role in the peak precipitation simulations at 15 km resolution, via influencing the simulated low-level wind. During the second peak precipitation period, simulations with the convection-permitting parameterization scheme suite at 15 km resolution generate a prominent low-level easterly wind component bias, which is largely attributed to the excessively evaporative cooling in the lower troposphere. This study further reveals that at 15 km resolution the diabatic heating from the grid-scale precipitation accounts more for the low-level wind bias than the convective-scale precipitation. Given that two different cloud microphysics schemes, namely Thompson and WSM6 schemes, are used in the convection-permitting and default mesoscale parameterization scheme suites, respectively, these microphysics schemes are found to be the primary contributor to the low-level wind simulation bias. [ABSTRACT FROM AUTHOR]

New particle formation in the free troposphere is a major source of cloud condensation nuclei globally. The prevailing view is that in the free troposphere, new particles are formed predominantly in convective cloud outflows. We present another mechanism using global observations. We find that during stratospheric air intrusion events, the mixing of descending ozone-rich stratospheric air with more moist free tropospheric background results in elevated hydroxyl radical (OH) concentrations. Such mixing is most prevalent near the tropopause where the sulfur dioxide (SO2) mixing ratios are high. The combination of elevated SO2 and OH levels leads to enhanced sulfuric acid concentrations, promoting particle formation. Such new particle formation occurs frequently and over large geographic regions, representing an important particle source in the midlatitude free troposphere. [ABSTRACT FROM AUTHOR]

Ozone pollution shows a trend of increasing, but the potential contribution of volatile organic compounds (VOCs) to ozone formation in the vertical direction is still unclear. The vertical distribution characteristics of VOCs and its impact on ozone formation were studied based the enhanced sounding data in the lower troposphere (0-1000 m) from October 17 to November 15, 2020 observed in the northern suburbs of Nanjing city, Jiangsu province. The results show that the volume fraction of VOCs ((73.4±26.1)×10-9 (50.4±20.3)×10-9) decreases with the increase of height, and alkanes account for the largest proportion (64.3%-71.6%) at each height, and the proportion of olefin (3.4%-9.9%) gradually decrease with the increase of height; there is no significant change in the proportion of aromatics, halohydrocarbons and acetylene; the VOCs accumulate in the lower layer in the morning and evening with a great gradient, and the proportion of alkane increases due to the traffic emissions and lower boundary layer; the VOCs profiles are uniform in the afternoon. Affected by atmospheric junction, the alkanes and olefin dominate the photochemical reactions in the mixing layer, with the loss rate of OH radicals (LOH) of 3.3 and 2.7s-1, accounting for 42.8% and 35.1%, respectively; alkanes and aromatics are the dominant species in the stable boundary layer and residual layer. The LOH and ozone formation potential (OFP) of VOCs in the afternoon mixing layer are positively correlated with the ozone volume fraction, which represent the relationship between local VOCs and ozone; however, there is no significant correlation between LOH, OFP and ozone in the stable boundary layer and residual layer in the morning; the contributions of VOCs components with high reaction activity to OFP are occurred in the low layer, and ethene and propene are the representative components; alkane makes greater contribution to the photochemical generation of ozone in the upper atmosphere, and the analysis result of X/E ratio reflects higher aging and the characteristics of regional atmospheric. Limiting chemical plant emissions, and improving vehicle emission standards and fuel quality, are the key to improve ozone pollution in Nanjing. [ABSTRACT FROM AUTHOR]