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• UV-microbursts were measured by a highly sensitive photometer in auroral zone. • UV-microbursts are observed as series of pulses with complex time structure. • The relativistic electron microbursts origin of UV emission is considered. In this paper we present data on the UV-microburst (300–400 nm flashes with a duration less than 1 s) measurements in the auroral zone. Measurements were performed during the period 09.2021–04.2022 by the highly sensitive imaging photometer installed at the Verkhnetulomsky observatory of the Polar Geophysical Institute. It is shown that microbursts are grouped in a series with a duration from 10 s to 10 min. They were observed in relatively quiet geomagnetic conditions (K P < 3) at the southern boundary of the auroral oval in the evening magnetic local time (MLT) sector. UV-microbursts are observed in different observational conditions (clouds, transparent clouds and clear sky) and spatially represent various patterns: uniform diffuse illumination, local spots. Joint analyses of the optical measurements and satellite data on charged particle fluxes demonstrates that an auroral oval, characterized by a plasma, is placed to the north of the observatory. At the same time increased flux of electrons with energy more than 100 keV is observed at the same L-shell and MLT sector. The possible origin of the UV-microbursts is a precipitation of energetic electrons from a poleward boundary of the outer radiation belt in a form of relativistic electron microbursts is discussed. [ABSTRACT FROM AUTHOR]

Downbursts belong to sudden, local, and strong convection weather, which present significant destruction for structures. At any given time, there are approximately 2000 thunderstorms occurring on the Earth. Many studies have investigated the effects of downbursts on different structures. However, the extensive range of varying wind field parameters and the diverse representations of wind speeds render the study of structural wind effects complex and challenging under downbursts. This study firstly reviews the research of wind field properties of downbursts according to four common approaches, and the major findings, advantages, and disadvantages of which are concluded. Then, failure analysis of transmission line systems under stationary and moving downbursts is explored. The article also reviews the wind pressure on the roof of different kinds of low-rise buildings, and some dominant parameters, namely roof slope, distance of building from downburst center, wind direction angle, and so on, are discussed. Moreover, the wind effects caused by downbursts on high-rise buildings and some specialized structures are also considered because more and more wind hazards are related to downbursts. Finally, the limitations of the current study are pointed out, and recommendations for further research are given for the accurate assessment of the effects of wind on buildings, with a view to providing safer and more economical wind-resistant design solutions for structures. [ABSTRACT FROM AUTHOR]

We report an Arase‐all sky imager (ASI) conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus bursts. Wavelet analysis displayed three peaks at ∼0.3 Hz, 4 Hz, and >10 Hz, corresponding to the main pulsation, internal modulation, and fast modulation, respectively. These correspond to the old terms of ∼5–15 s pulsations, chorus risers/elements and subelements/subpackets, respectively. Electron "microbursts" correspond to the 4‐Hz peak. The internal and fast modulations are further verified by the analysis based on fast Fourier transform analyses. Moreover, the spatial distributions of the Fourier spectral amplitude show that the internal and fast modulations are well‐structured within auroral patches. The above results indicate a paradigm shift away from quasilinear theory which implicitly assumes diffuse wave generation. The three time‐scale modulations are consistent with coherent chorus which has been theoretically argued to lead to pitch angle transport three orders of magnitude faster. Plain Language Summary: Pulsating aurora exhibit irregular patches of brightness with quasiperiodic on‐off transitions (∼2–20 s). More rapid modulations, such as internal modulation (∼3–4 Hz) or fast modulation (>10 Hz), have been detected within the pulsation "on" time. However, due to the measurement limitations, the simultaneous observation of three time‐scale modulations has never been reported. In this study, we analyze the conjugate observations of the pulsating aurora (PsA) and chorus recorded by the ground‐based Arase‐all sky imager and the Arase satellite, which demonstrates the coexistence of three time‐scale modulations in the PsA. The spatial distributions of Fourier spectral amplitude show that the internal and fast modulations are well‐structured within the aurora patches. The three time scales of chorus modulation have been previously called chorus 5–15 s pulsations, risers/elements and subelements/subpackets corresponding to the three aurora peaks. This study verifies the existence of internal and fast modulations in PsA, implying an extremely rapid electron loss mechanism. Quasilinear theory cannot explain any of the three time‐scale modulations. The discovery that chorus is coherent and will lead to pitch angle transport 1,000 times faster than diffuse waves is consistent with the fast modulations shown in this paper. A new theory of chorus generation is needed to update that of quasilinear theory. Key Points: We report a conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus burstsThe frequency spectra of auroral intensities obtained by wavelet analysis and fast Fourier transform (FFT) show the coexistence of ∼0.3 Hz, 4 Hz, and >10 Hz modulationsSpatial distributions show that the internal and fast modulations are well‐structured within aurora patches [ABSTRACT FROM AUTHOR]

In the autumn of 2021, a multichannel imaging photometer of the Pulsating Aurora Imaging Photometer System was installed at the Verkhnetulomsky Observatory. During the first season of operation (2021/2022), measurements were made over the course of 163 nights in three modes of temporal resolution: 2.5 μs, 320 μs, and 41 ms. The high temporal resolution makes it possible to investigate the fine temporal structure of the emission, which are short (less than 1 s) bursts of UV radiation, so-called "microbursts" that can be single or follow in series. The long-term series of microbursts registered on November 27–29, 2021, were analyzed. It is shown that the series of bursts have a complex temporal structure, individual bursts have several peaks with intervals of 100–400 ms, the intervals between bursts are of the order of 1 s, and they appear in packs lasting from several seconds to minutes. The series appear both in quiet geomagnetic conditions and during substorms; the frequency and amplitude of bursts in the second case are significantly larger. [ABSTRACT FROM AUTHOR]

Downburst is a hazardous strong wind commonly encountered in thunderstorm weather. Influenced by ambient wind, the downdraft often accompanies horizontal movement while descending. Based on the impinging jet model, this study employs the large eddy simulation method to conduct computational fluid dynamics numerical simulations of stationary and moving downbursts. The evolution process, time-varying wind speed, and wind profile characteristics of the two types of wind fields are compared. This study also discusses the influence of η—which represents the ratio of the storm traveling speed (Vtr) to the jet velocity (Vjet)—on the wind field structure and verifies the applicability of the vector superposition principle in the simulation of the moving downburst. The results show the following: The wind speed and direction time histories obtained from the numerical simulation of moving downbursts have good consistency with the actual wind field records. The movement of the storm causes the near-surface wind field to lose its central symmetry, resulting in a bow-shaped distribution of extreme wind speeds at the storm's leading edge. As η increases, the non-central symmetry of the wind field becomes more pronounced, the maximum horizontal wind speed at the storm's leading edge gradually moves closer to the storm's center, peak wind speeds gradually increase, and the wind speed distribution at the storm's trailing edge shows the opposite trend. The vector superposition principle is not entirely applicable in the simplified analysis of moving downburst, causing significant errors when η is larger than 0.1. [ABSTRACT FROM AUTHOR]

Short-lived and poorly organized convective cells, often called weakly forced thunderstorms (WFTs), are a common phenomenon during the warm season across the eastern and southeastern United States. While typically benign, wet downbursts emanating from such convection can have substantial societal impacts, including tree, power line, and property damage from strong outflow winds. Observational studies have documented the occurrence of severe (25.7 m s−1 or higher) wind speeds from wet downbursts, but the frequency of severe downbursts, including the spatial extent and temporal duration of severe winds, remains unclear. The ability for modern observing networks to reliably observe such events is also unknown; however, answering these questions is important for improving forecast skill and verifying convective warnings accurately. This study attempts to answer these questions by drawing statistical inferences from 97 high-resolution idealized simulations of single-cell downburst events. It was found that while 35% of the simulations featured severe winds, the spatial and temporal extent of such winds is limited—O(10) km2 or less and persisting for around 5 min on average. Furthermore, through a series of simulated network experiments, it is postulated that the probability that a modern mesonet observes a severe wind gust given a severe downburst is around 1%. From these results, a statistical argument is made that most tree impacts associated with pulse convection are likely caused by subsevere winds. Several implications for forecasting, warning, and verifying WFT events fall out from these discussions. [ABSTRACT FROM AUTHOR]

Thunderstorm downbursts can impose critical wind loads on long-span transmission line structural system. Existing research has predominantly focused on stationary downbursts, with limited attention given to the analysis of travelling downbursts influenced by environmental wind and parent cloud motion. This paper utilizes the impinging jet method to simulate a travelling downburst at a small Reynolds number, employing the Delayed Detached Eddy Simulation turbulence model for solving the wall-bounded turbulent flow. The radial wind speed distribution of the travelling downburst is subsequently validated against previous experimental, numerical, and observational data. A comparative study is then conducted to discern the differences in the radial wind speed distribution between stationary and travelling downbursts. Finally, a parametric study investigates the influence of jet inlet speed, environmental wind and parent cloud motion on the maximum radial wind speed distribution of the travelling downburst. The findings reveal a significant disparity in the radial wind speed distribution between stationary and travelling downbursts. Notably, parent cloud motion, environmental wind, or their combined effects can alter the height providing the maximum radial wind speed. This variation has implications for the wind load distribution of travelling downburst across the conductor span of the transmission line structural system. [ABSTRACT FROM AUTHOR]

Microbursts are short duration intensifications in precipitating electron flux that are believed to be a significant contributor to electron losses in the magnetosphere. Microbursts have been observed in the form of bouncing electron packets, which offer a unique opportunity to study their properties and importance as a loss process. We present a collection of bouncing microbursts observed by the HILT instrument on SAMPEX from 1994 to 2004. We analyze the locations of the bouncing microbursts in L and MLT and find they align well with the properties of relativistic microbursts as a whole. We find that the majority of bouncing microbursts observed by SAMPEX have scale sizes of ∼30 km at the point of observation, or ∼300 km when mapped to the magnetic equator. The time separation between the peaks of these bouncing microbursts is usually either half a bounce period or a whole bounce period. Key Points: A large collection of bouncing microbursts from SAMPEX are foundA distribution of scale sizes for bouncing microbursts shows that most are a few tens of kilometers in sizeThere are roughly equal numbers of one way and two way bouncing microbursts observed in this study [ABSTRACT FROM AUTHOR]

We made observations of magnetic field variations in association with pulsating auroras with the magneto‐impedance sensor magnetometer (MIM) carried by the Loss through Auroral Microburst Pulsations (LAMP) sounding rocket that was launched at 11:27:30 UT on 5 March 2022 from Poker Flat Research Range, Alaska. At an altitude of 200–250 km, MIM detected clear enhancements of the magnetic field by 15–25 nT in both the northward and westward components. From simultaneous observations with the ground all‐sky camera, we found that the footprint of LAMP at the 100 km altitude was located near the center of a pulsating auroral patch. The auroral patch had a dimension of ∼90 km in latitude and ∼25 km in longitude, and its major axis was inclined toward northwest. These observations were compared with results of a simple model calculation, in which local electron precipitation into the thin‐layer ionosphere causes an elliptical auroral patch. The conductivity within the patch is enhanced in the background electric field and as a result, the magnetic field variations are induced around the auroral patch. The model calculation results can explain the MIM observations if the electric field points toward southeast and one of the model parameters is adjusted. We conclude that the pulsating auroral patch in this event was associated with a one‐pair field‐aligned current that consists of downward (upward) currents at the poleward (equatorward) edge of the patch. This current structure is maintained even if the auroral patch is latitudinally elongated. Key Points: Magneto‐impedance (MI) sensor was carried by a sounding rocket and first applied to magnetic field measurement in spaceMagnetic field variations were observed when the footprint of the payload was located near the center of an auroral patchThe magnetic field variations are thought to be caused by field‐aligned currents flowing at the edges of the pulsating auroral patch [ABSTRACT FROM AUTHOR]

Electron microburst precipitation has been shown to have significant potential for depletion of the outer radiation belt. We present observations from the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) of six (five unique and one dual‐balloon observation) microburst events, each containing minutes to hours of persistent microbursts. We find that each event included a slowly‐varying smooth precipitation component underlying the bursty component. The smooth component has not yet been fully characterized in the literature; we have written a program to identify microburst events and quantify the relative contributions of each component in the BARREL data. In all six events analyzed, the smooth component contributed more to the total X‐ray counts measured, indicating that the smooth component could contribute significantly to radiation belt loss. Plain Language Summary: Earth's radiation belts have been of interest since their discovery in 1958. Understanding and predicting the particle dynamics of the radiation belts can help us to better understand the space environment around Earth, improving our ability to protect the satellites on which our technological society depends. The microburst events studied here are known to have a significant impact on the radiation belts, with the potential to completely deplete them in as little as a few hours. We report here previously unclassified precipitation that occurs concurrently with microbursts. Unlike the short microbursts (each lasting less than a second) the additional component is less intense but is sustained for much longer periods (minutes to several hours). The analysis presented indicates that this smooth component comprises a significant fraction of the depletion potential, implying that the smooth component should be included in radiation belt models and loss rate estimates. Key Points: Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) microburst events comprise at least two precipitation components: a smooth component (minutes–hours) and a bursty component (∼100 ms)An automated algorithm was created to identify microburst events in BARREL data and separate the smooth and bursty precipitation componentsFor the events studied, the smooth component contributed more to the measured X‐ray count rate than the bursty component [ABSTRACT FROM AUTHOR]

A downburst is a strong downdraft that causes radiating, catastrophic winds at or near the ground. Relevant research conducted in the past decades shows that downbursts not only bring damage to the built environment but also have surprisingly destructive effects on aircraft and other flying objects. However, due to the complexity of atmospheric phenomena and limited measurement datasets, many uncertainties remain to be clarified in the understanding and modelling of the downburst wind field. To further explore the evolution characteristics of thunderstorms, this study uses data recorded during thunderstorms by a Doppler profiler of the Hong Kong Observatory and a near-ground anemometer installed at the Hong Kong International Airport to conduct a detailed feature analysis of the vertical profile and turbulence characteristics of downbursts within the atmospheric boundary layer. Based on long-term field measurements during thunderstorm events, the occurrence of a thunderstorm should be determined by a combination of simultaneous variations in multiple meteorological factors, including wind speed and direction, vertical wind speed, and signal-to-noise ratio (SNR). During thunderstorms, extreme wind speeds occurred in the lower atmosphere and the height of the local peaks seems to be stable in the thunderstorm wind profiles. In addition, both methods, which consider a constant mean component (stationary) and a slowly varying mean component (non-stationary), are used to decompose the measured wind speed and are then compared to investigate the turbulence characteristics of downbursts. Although the gust factor and turbulence intensity show some deviation due to the influence of atmospheric stratification instability, this effect does not appear to be reflected in the turbulence integral scale. [ABSTRACT FROM AUTHOR]

The full spatiotemporal distribution of chorus wave‐induced relativistic electron microburst is modeled for chorus waves originated from different L shells and MLTs, based on the newly developed numerical precipitation model (Kang et al., 2022, https://doi.org/10.1029/2022gl100841). The wave‐particle interaction process that induces each microburst is analyzed in detail, and its relation to the chorus wave propagation effects is explained. The global distribution of maximum precipitation fluxes and scale sizes of relativistic microbursts is then obtained by modeling chorus waves at different L‐shells and local times. The characteristics of dawn and midnight sector microbursts have little difference, but the noon sector has much larger maximum flux and much smaller full width at half maximum, which may be due to dayside's low electron flux in the Landau resonance range. This suggests the controlling effect of keV electrons on the MeV electron precipitation intensity and properties and the overall relativistic electron loss in the outer radiation belt. Key Points: Based on statistical chorus and electron distributions, the global distribution of relativistic microburst scale size is modeledDawn and night sectors are fairly similar in precipitation scale size, and dayside has larger maximum precipitation flux and smaller widthThe keV electron populations on dayside play a critical role on the MeV electron precipitation intensity and the microburst scale size [ABSTRACT FROM AUTHOR]

The significant role of nonlinear wave–particle interactions in the macrodynamics and microdynamics of the Earth's outer radiation belt has long been recognised. Electron dropouts during magnetic storms, microbursts in atmospheric electron precipitation, and pulsating auroras are all associated with the rapid scattering of energetic electrons by the whistler-mode chorus, a structured electromagnetic emission known to reach amplitudes of about 1 % of the ambient magnetic field. Despite the decades of experimental and theoretical investigations of chorus and the recent progress achieved through numerical simulations, there is no definitive theory of the chorus formation mechanism, not even in the simple case of parallel (one-dimensional) propagation. Here we follow the evolution of these theories from their beginnings in the 1960s to the current state, including newly emerging self-consistent excitation models. A critical review of the unique features of each approach is provided, taking into account the most recent spacecraft observations of the fine structure of chorus. Conflicting interpretations of the role of resonant electron current and magnetic field inhomogeneity are discussed. We also discuss the interplay between nonlinear growth and microscale propagation effects and identify future theoretical and observational challenges stemming from the two-dimensional aspects of chorus propagation. [ABSTRACT FROM AUTHOR]

Resonant interactions with whistler-mode waves are a crucial mechanism that drives the precipitation of energetic electrons. Using test particle simulations, we investigated the impact of nonlinear interactions of whistler-mode waves on electron precipitation across a broad energy range (10 keV-1 MeV). Specifically, we focused on the combined effects of conventional phase bunching and anomalous scattering, which includes anomalous trapping and positive bunching. It is shown that anomalous scattering transports electrons away from the loss cone and the only process directly causing precipitation in the nonlinear regime is the phase bunching. We further show that their combined effects result in a precipitation-to-trapped flux ratio lower than the quasilinear expectations in a quasi-equilibrium state. Additionally, we calculated the diffusion and advection coefficients associated with the nonlinear trapping and bunching processes, which are vital for understanding the underlying mechanisms of the precipitation. Based on these coefficients, we characterized the phase bunching boundary, representing the innermost pitch angle boundary where phase bunching can occur. A further analysis revealed that electrons just outside this boundary, rather than near the loss cone, are directly precipitated, while electrons within the boundary are prevented from precipitation due to anomalous scattering. Moreover, we demonstrated that the regime of dominant nonlinear precipitation is determined by the combination of the phase bunching boundary and the inhomogeneity ratio. This comprehensive analysis provides insights into the nonlinear effects of whistler-mode waves on electron precipitation, which are essential for understanding physical processes related to precipitation, such as microbursts, characterized by intense and bursty electron precipitation. [ABSTRACT FROM AUTHOR]

Abstract Electron microburst precipitation has been shown to have significant potential for depletion of the outer radiation belt. We present observations from the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) of six (five unique and one dual‐balloon observation) microburst events, each containing minutes to hours of persistent microbursts. We find that each event included a slowly‐varying smooth precipitation component underlying the bursty component. The smooth component has not yet been fully characterized in the literature; we have written a program to identify microburst events and quantify the relative contributions of each component in the BARREL data. In all six events analyzed, the smooth component contributed more to the total X‐ray counts measured, indicating that the smooth component could contribute significantly to radiation belt loss.

We have identified for the first time an energy‐time dispersion of precipitating electron flux in a pulsating aurora patch, ranging from 6.7 to 580 keV, through simultaneous in‐situ observations of sub‐relativistic electrons of microburst precipitations and lower‐energy electrons using the Loss through Auroral Microburst Pulsation sounding rocket launched from the Poker Flat Research Range in Alaska. Our observations reveal that precipitating electrons with energies of 180–320 keV were observed first, followed by 250–580 keV electrons 0–30 ms later, and finally, after 500–1,000 ms, 6.7–14.6 keV electrons were observed. The identified energy‐time dispersion is consistent with the theoretical estimation that the relativistic electron microbursts are a high‐energy tail of pulsating aurora electrons, which are caused by chorus waves propagating along the field line. Plain Language Summary: Microbursts, which are bursts of high energy electrons, and pulsating auroras, which periodically blink and caused by the precipitation of low energy electrons, are observed in the Earth's polar ionosphere. The detection time differences of the electrons associated with microbursts and pulsating auroras were detected by a sounding rocket. A possible mechanism for the generation of these precipitations is the interaction of electrons with a particular type of wave, known as "chorus," which propagates along geomagnetic lines. The observed energy‐time dispersion of the precipitating electrons is quantitatively consistent with theories of electron precipitation based on this interaction. Key Points: A sounding rocket observed simultaneously precipitating sub‐relativistic electron microbursts and pulsating auroral electrons250–580 keV electron precipitations were detected 0–30 ms after 180–320 keV electron precipitations in a single auroral patchThe energy dispersion of observed electrons is consistent with the theory that they are due to chorus waves propagating to higher latitudes [ABSTRACT FROM AUTHOR]

In this research, a wide-body aircraft was analyzed with critical monitoring of its states, a function of several control inputs (wind gust, turbulence, and microburst). The aerodynamic and stability coefficients of a Boeing 747-200 were obtained from previously published works and 6- DOF equations were formulated. Simulations were conducted for various control inputs to determine the aircraft's free response, as well as the forced response. In order to understand the nature of the atmosphere, three different models were incorporated, including (i) the Dryden Model, (ii) wind gust, and (iii) microburst. The aircraft was found to be stable in the longitudinal and lateral flight modes, with trim conditions agreeing with published data. For a vertical wind gust of −10 ft/s, the AoA and pitch rate were observed to oscillate sinusoidally and became stable with new trim conditions. These states were found to regain trim conditions once the gust was removed. In the case of 3D gust, it was found that the longitudinal modes achieved a new trim condition through Phugoid oscillations, whereas the lateral modes underwent short-period oscillations. For the case of turbulence, random fluctuations were observed for trim conditions with no unstable behavior. When considering the microburst case, it was found that the aircraft initially gained altitude in the region of the headwind; this was followed by a sharp descent under the influence of a vertical velocity component. [ABSTRACT FROM AUTHOR]

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A leading cause of accidents during the landing phase of a flight lies in a considerable altitude loss by an aircraft as a result of the impact of a microburst of wind. One of the significant factors focuses primarily on the need to simultaneously satisfy various requirements regarding conditions of environmental disturbances and a wide range of systemic changes. The paper presents an algorithm for synthesizing an optimal controller that solves the mixed H2/H∞ control problem for the stabilization of aircraft in glidepath landing mode in the presence of uncertainty. Firstly, the principles of multi-criteria optimization are presented, and the mixed H2/H∞ problem is interpreted as the synthesis of a system with optimal quadratic performance, subject to its readiness to operate with the worst disturbance. Then, the ensuing section expounds upon the mathematical depiction of the vertical trajectory of aircraft, duly considering the perturbations imposed by wind phenomena. Subsequently, the effectiveness of mixed H2/H∞ control is confirmed compared to autonomous H2 or H∞ regulators through simulation outcomes acquired from the created system. Optimization based on a hybrid (mixed) criterion allowed combining the strengths of locally optimal systems based only on H2 or H∞ theory. [ABSTRACT FROM AUTHOR]

Here we examine properties of MeV electron microbursts to better understand their generation mechanisms. Using 15 years of data from Solar, Anomalous, Magnetospheric Particles Explorer/Heavy Ion Large Telescope, >1 MeV microburst repetition periods (time spacing between bursts) are examined and clear dependencies on Auroral Electrojet (AE), L shell, and magnetic local time (MLT) are discovered. Microburst repetition periods are shortest around 0–6 hr MLT and 4–5 L shell, and grow longer toward the day and afternoon sectors and larger L shells. Shorter repetition periods (<1 s) are also found to be more common during higher AE, while longer periods (>10 s) more common during quiet times. The microburst repetition period distributions are compared directly to those of rising tone chorus wave elements and found to be similar in the night, dawn and day MLT sectors, suggesting chorus wave repetition periods are likely directly controlling those of microburst precipitation. However, dusk‐side distributions differ, indicating that the dusk‐side microbursts properties may be controlled by other processes. Plain Language Summary: Looking at energetic electrons in Earth's magnetosphere from the Heavy Ion Large Telescope instrument on board the Solar, Anomalous, Magnetospheric Particles Explorer satellite helps us better understand the feature called microbursts. Microbursts are very rapid bursts of enhanced high‐energy electrons entering our atmosphere. In this study, we characterize the properties of the microbursts to understand when, and where they happen, what causes them, and what impact they might have. We do so by looking at factors such as their magnetic local time as well as their time separation. This study also compares these microbursts' results to previous chorus wave studies. Chorus waves have been thought to be related to microbursts and could be a cause for some of their properties. We discuss more about this correlation in the result section of this study. Key Points: The repetition period of MeV electron microbursts is studied for the first time using 15 years of Solar, Anomalous, Magnetospheric Particles Explorer/Heavy Ion Large Telescope dataMicroburst repetition periods are most often <1 s, and drop off as a power law moving to longer periodsRepetition periods show strong agreement with chorus wave element periodicities in all magnetic local time sectors except for the dusk sector [ABSTRACT FROM AUTHOR]

Analyses of cloud-top temperature and lightning characteristics of 48 Weather Research and Forecasting (WRF) Model–simulated ocean-based wind events, with 1-min temporal and 0.5-km horizontal resolution, revealed signatures similar to the corresponding 13 observed events detected by buoys and Coastal-Marine Automated Network (C-MAN) stations as shown in prior research on ocean-based wind events by the first author. These events occurred in the eastern Gulf of Mexico and in the Atlantic Ocean from Florida northward through South Carolina. The coldest WRF cloud-top temperature (WCTT) and peak WRF-estimated lightning flash rate values of the model-simulated events, where each event was required to have a negative vertical velocity of at least 10 m s−1 in the lowest 2 km associated with a convective storm, occurred at an average of 4.2 and 1.1 min prior to the events, respectively. With 36 of the events, the peak estimated flash rate occurred within 5 min of the coldest WCTT. Cloud depth typically increased as the WCTT decreased, and the maximum depth occurred at an average of 2.9 min prior to the events. Thermal cooling and precipitation loading provided negative buoyancy needed to help drive the wind events. Environmental characteristics of the model-simulated ocean-based wind events also resembled those associated with land-based wet downbursts, including moist air near the surface, lapse rates near moist adiabatic, and low cloud bases. [ABSTRACT FROM AUTHOR]

Wind turbines are among the fastest-growing technologies for producing sustainable energy. Although numerous studies have been carried out to investigate the dynamic behavior of wind turbines exposed to synoptic winds, the dynamic behavior of such structures under downbursts is not comprehensively studied and not yet well-understood. As such, the main objective of the current study is to develop a numerical model that can be used to investigate the dynamic response of wind turbines subjected to downbursts taking into account the fluid-structure interaction (FSI) effect. To achieve this task, two modules are developed in this study and are integrated with the open-source code, fatigue, aerodynamics, structures, and turbulence (FAST). These modules introduce a three-dimensional (3D) time history of the downburst wind field into FAST inflow-wind module and simulate the interaction between the downburst wind field and the wind turbine structure. The downburst wind field consists of a moving mean component with a superimposed turbulence. The mean component is generated using a previously conducted computational fluid dynamics (CFD) simulation. The turbulence is simulated using the consistent discrete random flow generation (CDRFG) method as a stochastic process based on the turbulence power spectral density and the coherence functions pertaining to downbursts. The developed numerical model is validated using the results of a previously performed experiment on a wind turbine model sited in microburst-liked winds. A parametric study is then conducted on a wind turbine model to investigate the dynamic responses of the tower and blades with and without FSI effect under various downburst configurations. A comparison between the wind turbine quasistatic and dynamic responses is conducted, and the dynamic amplification factor (DAF) is then calculated. The aerodynamic damping of the blades is also estimated and compared with a previously developed closed form solution. [ABSTRACT FROM AUTHOR]

Dry microbursts in weak weather backgrounds, due to their small scale and general lack of precipitation, are often difficult to observe using weather radar. On the night of 26 April 2023, a dry microburst occurred at the Yinchuan Airport. Based on conventional meteorological observations, Automated Weather Observing System (AWOS) data, and the Doppler Wind Lidar data, an analysis was conducted on the causes and wind field structure of this microburst. It was found that (1) the sounding data indicated a DCAPE value of 880 J·kg−1, which is important for forecasting the potential for dry microburst events; (2) the foehn from the Helan Mountains contributed to the occurrence of microburst weather at the Yinchuan Airport; (3) the Doppler Wind Lidar wind data showed distinct characteristics of the wind field during this microburst event, including a symmetric horizontal wind field structure, significant vertical downdraft velocities (reaching −5.76 m·s−1), and low-level wind shear over the airport runway and its vicinity; and (4) effective monitoring of such microburst weather events with the Doppler Wind Lidar wind measurements is crucial for ensuring aviation safety. [ABSTRACT FROM AUTHOR]

Goals include determining the origins of cosmic rays and electron microbursts. [ABSTRACT FROM AUTHOR]

Abstract Here we examine properties of MeV electron microbursts to better understand their generation mechanisms. Using 15 years of data from Solar, Anomalous, Magnetospheric Particles Explorer/Heavy Ion Large Telescope, >1 MeV microburst repetition periods (time spacing between bursts) are examined and clear dependencies on Auroral Electrojet (AE), L shell, and magnetic local time (MLT) are discovered. Microburst repetition periods are shortest around 0–6 hr MLT and 4–5 L shell, and grow longer toward the day and afternoon sectors and larger L shells. Shorter repetition periods (10 s) more common during quiet times. The microburst repetition period distributions are compared directly to those of rising tone chorus wave elements and found to be similar in the night, dawn and day MLT sectors, suggesting chorus wave repetition periods are likely directly controlling those of microburst precipitation. However, dusk‐side distributions differ, indicating that the dusk‐side microbursts properties may be controlled by other processes.

This study aims to investigate the impact of microbursts, which are a common meteorological phenomenon, on aircraft performance. A multiphase computational fluid dynamics model has been developed to simulate the microburst-generated downdraft environment over an airfoil. The simulations were conducted at a 12° angle of attack for takeoff conditions with a Reynolds number of 2 × 10 5 . The lift and drag coefficients were compared to experimental results and exhibited a strong correlation. Additionally, this study examined the volume of fluid and the coefficient of pressure distribution. It also examined the boundary-layer velocity profiles at five different chord positions, the location of rivulet formation on the upper surface of the airfoil, and water film height. The results revealed a maximum variation of 72.18% and 16.53% in lift and drag coefficients, respectively, between the numerical and experimental results. These findings provide valuable insight into the effects of microbursts on aircraft performance and have implications for the aviation industry. [ABSTRACT FROM AUTHOR]

Three windshear/microburst detection algorithms are used operationally at the Hong Kong International Airport. Their performance is studied in the present article by assuming the availability of the complete set of meteorological data from numerical weather prediction model output. Only selected tropical cyclone cases are considered. The performance is evaluated using pilot reports of windshear. It turns out that the glide path scan windshear detection algorithm has the best overall performance by considering the area under curve using the relative operating characteristic curves. This may be related to the better geometry in covering the glide paths of the airport for this particular algorithm. [ABSTRACT FROM AUTHOR]

Lower‐band chorus waves are known to play dual roles in radiation belt dynamics (electron acceleration and precipitation), and understanding their properties and excitation is very important. A systematic study of chorus waves properties in terms of background plasma parameters (electron perpendicular beta β⊥ and the ratio of plasma frequency to electron cyclotron frequency fpe/fce) has not been performed previously. We use burst‐mode data from Van Allen Probe A from 2012 to 2019 and develop an algorithm to extract individual lower band rising tone chorus elements. We statistically analyze four properties of rising tone chorus elements: time duration τ, frequency width Δf, chirping rate Γ, and maximum amplitude Ia. Statistical results show typical properties of chorus waves: Ia ∼ 0.09 nT, Δf ∼ 0.08fce, Γ ∼ 2 × 103 Hz/s, τ ∼ 0.015–0.25 s. On the nightside and dawnside, Γ shows wider spread and is more distributed over larger values, which is associated with the wider distribution of β⊥. Chorus waves at duskside and dayside have larger values of τ, associated with smaller β⊥ and larger fpe/fce. The dependence of chorus wave properties on background plasma parameters is also examined. We show that normalized Δf has positive correlation with β⊥ and fpe/fce. Normalized τ shows negative correlation with β⊥ and positive correlation with fpe/fce. Normalized Γ and Ia show positive correlations with β⊥ and negative correlations with fpe/fce. These results help us better understand chorus waves excitation and their relation to the microburst. Key Points: We present statistical distribution of lower‐band chorus waves' properties: time duration, chirping rate, frequency width, and amplitudeThe spatial dependence of chorus waves properties is strongly associated with that of background plasma parametersNormalized time duration and chirping rate of chorus waves show significant correlations with β⊥ and fpe/fce [ABSTRACT FROM AUTHOR]

As the localized microbursts or flooding occur frequently due to the unusual weather phenomena, it is the very urgent and important problem to detect these weather hazards using the short range weather radar networks. For a short range weather radar of this purpose, the appropriate method is essential to verify the algorithm performance of this system. Therefore, in this paper, the efficient method to generate a short range weather radar signal was proposed for simulation of various weather phenomena by changing Doppler spectrum parameters. Using the proposed spectrum modeling method, both the strong clutter of a ground surface and man-made structures and the moving clutter such as birds can be easily simulated which often occur in a short range weather radar operation environment. In this paper, various weather signals with typical clutters were simulated using the proposed method and the results were analyzed. [ABSTRACT FROM AUTHOR]

Downbursts are rapidly evolving meteorological phenomena with numerous vertically oriented precursor signatures, and the temporal resolution and vertical sampling of the current NEXRAD system are too coarse to observe their evolution and precursor signatures properly. A future all-digital polarimetric phased-array weather radar (PAR) should be able to improve both temporal resolution and spatial sampling of the atmosphere to provide better observations of rapidly evolving hazards such as downbursts. Previous work has been focused on understanding the trade-offs associated with using various scanning techniques on stationary PARs; however, a rotating, polarimetric PAR (RPAR) is a more feasible and cost-effective candidate. Thus, understanding the trade-offs associated with using various scanning techniques on an RPAR is vital in learning how to best observe downbursts with such a system. This work develops a framework for analyzing the trade-offs associated with different scanning strategies in the observation of downbursts and their precursor signatures. A proof-of-concept analysis--which uses a CloudModel 1 (CM1)-simulated downburst-producing thunderstorm--is also performed with both conventional and imaging scanning strategies in an adaptive scanning framework to show the potential value and feasibility of the framework. Preliminary results from the proof-of-concept analysis indicate that there is indeed a limit to the benefits of imaging as an update time speedup method. As imaging is used to achieve larger speedup factors, corresponding data degradation begins to hinder the observations of various precursor signatures. [ABSTRACT FROM AUTHOR]

As the region with the highest amount of solar radiation, Qinghai–Tibet Plateau has the highest frequency of thunderstorms in China. Downburst generated in thunderstorm can cause high‐intensive wind near the ground, which is a critical threat to heliostats. Herein, the impact of the wind loads on the heliostat response under downburst is investigated using a proven finite‐element model and the influence is also compared with that under atmospheric boundary layer (ABL) wind. The results demonstrate that the downburst development process, the movement of the storm center, and the heliostat's elevation angle are highly associated with the wind‐induced response characteristics of the heliostat under downburst. The maximum displacement and stress of the heliostat with different elevation angles under the downburst are significantly larger than those under ABL wind. Additionally, when the elevation angle of the heliostat is 30°, the maximum value of the mirror's first principal stress under downburst is 1.88 times greater than it is under ABL wind. Therefore, the impact of wind‐induced vibration caused by downburst is necessarily considered on the safety of heliostat in Qinghai‐Tibet Plateau with abundant solar energy resources and high incidence of downbursts. [ABSTRACT FROM AUTHOR]

Long-span bridges located in thunderstorm-prone areas can potentially be struck by downburst transient winds. In this study, the downburst time-varying mean wind was simulated by an impinging jet model based on computational fluid dynamics (CFD). To make the simulation results fit well with the measurements, a parameter optimization method was developed. The objective function was established based on the errors between the simulated characteristic points and the target values from the measurement data. To increase the effectiveness, a Kriging surrogate model that was trained using data from numerical simulations was used. The parameter optimization method and the Kriging model were verified using five groups of test samples. The optimization efficiency was significantly increased by replacing the numerical model with a surrogate model during the optimization iteration. The simulation accuracy was clearly improved by the numerical modeling of a downburst based on optimized parameters. Subsequently, the nonstationary turbulent downburst wind was obtained by the combination of the Hilbert-based nonstationary fluctuations and the CFD-based time-varying trend. Finally, the dynamic response of a long-span bridge subjected to the moving downburst was presented. The results based on the simulation validate the optimized downburst wind field and highlight the significant influence on the bridge's aerodynamics and buffeting response. [ABSTRACT FROM AUTHOR]

At midlatitudes (40 ° - 65 ° S), stratospheric ozone loss has been linked to a variety of physico-chemical effects. In this study, however, we examine how ozone responded to a fortuitous magnetic cloud (MC)-triggered storm on 20 November 2003 using a synthesis of multiple platform approach. We employed this approach to predict how energetic particles interact with the atmosphere by determining realistic NOx production rates without having to run a full ion chemistry model. Using simplified numerical approximations and synthetic satellite observations, we found that this method of ion pair production might be more economical in terms of numerical cost. Following three days of MC, the tendency for ozone reduction became apparent. According to this study, relativistic electron microbursts may promote particle precipitation, which in turn induced the occurrence of NO x around 55 km midlatitude. This corroborates the hypothesis that existing electron precipitation proxies, which do not yet take relativistic microburst energies into account, are likely overlooking a significant source of precipitation that affects atmospheric ozone levels. [ABSTRACT FROM AUTHOR]

With the rapid development of computational fluid dynamics (CFD) technology, it has been widely used to study the wind field characteristics of downbursts in mountainous areas. However, there is little guidance on the selection of different turbulence models for simulating downburst wind fields over hills using CFD, and few comparative studies have been conducted. This paper used nine turbulence models to simulate the wind field of a downburst over a 3D quadratic ideal hill. The simulated values of average and transient winds were compared with wind tunnel test data, and the flow characteristics at different moments under a downburst were analyzed. The flow characteristics in the wake region of the downburst over the hill are also quantitatively analyzed using the proper orthogonal decomposition (POD) method. The results show that approximately 85% of the results from the LES and REA models fall within a 30% error range, so the large eddy simulation (LES) model and the realizable k-ε model (REA) are more accurate in simulating the mean wind field, and the transient wind field simulated by the LES model is also in good agreement with the experimental data. In addition, this paper reveals the evolution mechanism of the transient wind field structure over a hill model under a downburst and finds that the first-order mode obtained by POD may be related to the acceleration effect on the hilltop. [ABSTRACT FROM AUTHOR]

This study develops a generalised multi‐hazard analysis framework for evaluating the impact of secondary hazard events on structures that have previously been damaged by major hazardous events. More specifically, the present work investigates the effects of thunderstorm downbursts on earthquake‐damaged tall steel buildings giving an emphasis in assessing the revised accumulated earthquake‐wind ductility demands. The novel approach is validated on a 20‐storey steel building. The structure is initially subjected to a ductility‐controlled pushover analysis to simulate the earthquake‐induced damage, and then, a parametric dynamic analysis is performed using damaged structure models under synthetic downburst time‐histories developed for various wind profiles, wind velocities and terrain types. The method accurately controls the damage level induced by the primary hazard and separately assesses secondary hazard effects which enables a direct quantification of the new multi‐hazard design requirements. This can provide effective guidance in the stage of preliminary structural design or post‐hazard assessment of structures identifying new serviceability limits, as well as support repair procedures and decision‐making frameworks for existing structures to prevent collapse. The results demonstrate a significant increase up to three to four times in ductility demands of the structure after the occurrence of the strong downburst winds, compared with the ductility demands that were initially imposed by the varying severity degrees of the earthquake. [ABSTRACT FROM AUTHOR]

So far, wind loads on structures are almost exclusively evaluated by Extended Pressure Systems (EPS) cyclone models according to design codes. However, there is evidence that downburst wind fields, originating from thunderstorms (TSs) events, may have caused or contributed to the collapse of transmission line towers around the world. In this context, this work proposes a complete methodology to simulate a wind field from combined EPS (synoptic) and TS (nonsynoptic) storms. The proposed combined wind field model is applied to a real transmission line segment and the results are compared with those of the system subjected to the equivalent static loading method proposed in the IEC 60826/2003 code. The finite central differences explicit method is adopted for dynamic integration, taking into account nonlinearities. The assessment confirmed the localized and transient nature of the TS phenomenon. The results indicated that, for most of the elements analyzed, the equivalent static method for synoptic winds of IEC 60826/2003 resulted in a lower response than that obtained through the proposed combined model, which considers a TS carried by an EPS background wind. Thus, the proposed method to combine synoptic and nonsynoptic wind fields might be a useful tool to assist in the design of structures, such as transmission lines, which are susceptible to the effects of combined EPS and TS wind fields. [ABSTRACT FROM AUTHOR]

On 30 June 2013, 19 Granite Mountain Hotshots firefighters were killed fighting a wildfire near Yarnell in the mountains of Central Arizona. They succumbed when the wildfire, driven by erratic winds, blocked their escape route and overran their location. A previous study is extended to simulate and analyze the downscale organization of convective circulations that redirected the wildfire, which started from the scale of the Rossby Wave Breaking over North America to a convective gust front that redirected the wildfire, trapping the firefighters. Five stages are found: Stage I, the initial deep prolonged gust front; Stage II, a front-to-rear jet and its ascending motions that organized high-based convection; Stage III, high-based dry microburst-induced downdrafts organized initially by ascending flow in Stage II that transported mass and entropy to the surface; Stage IV; multiple meso-γ-scale high centers and confluence zones formed that encompassed the firefighters' location, which established a favorable environment leading to Stage V, canyon-scale circulations formed surrounding the fire. The atmosphere thus transitioned from supporting a deep and long-lived convective density current to elevated dry microbursts with mass and wind outflow into a canyon, redirecting the ongoing wildfire. [ABSTRACT FROM AUTHOR]

The generation of wind fields is of interest in the study of the structural performance of wind turbines in critical events, such as thunderstorm downbursts. Various methods ranging from the use of empirical data to employing computational simulations are typically adopted to study the response of wind turbines in downburst flow fields. While the former approach is limited in the ability to account for accurate and spatially resolved details of the flow field, the latter is expensive and, therefore, has limitations in its use. As an alternative, in this work, we propose a paused downburst model in which a snapshot of a time-dependent computational fluid dynamics (CFD) simulation is used to generate "mean" wind fields during thunderstorm downbursts. The developed model for the mean wind field is validated against recorded downburst data in the literature. The turbulent component of the wind field is generated using computationally inexpensive techniques based on Fourier-based power spectral density functions and coherence functions. In an illustrative example, the combined mean and turbulence wind fields are generated and applied on a utility-scale wind turbine to study structural load characteristics during a downburst event. [ABSTRACT FROM AUTHOR]

Downbursts occur frequently in mountainous regions, such as the southwest of China, and causing extensive damage to engineering structures. While some researchers have developed semiempirical models for the speedup effect, most are based on the wind field in the boundary layer over the hill, and there is a lack of semiempirical models for the downburst wind field over the hill. This study employs three RANS (Reynolds Average Navier-Stokes) turbulence models to numerically simulate the downburst wind field over a quadratic curved hill. The realizable k-ε model is selected as the optimal model for the subsequent numerical simulations based on comparison with wind tunnel test results. Then, a semiempirical model of the speedup effect of the downburst wind field over the hill is constructed by numerically simulating the downburst wind field over the hill with different radial locations and different slopes. Finally, the constructed semiempirical model is validated and demonstrates good accuracy. [ABSTRACT FROM AUTHOR]

Microburst (MB) wind shear is one of the most important meteorological dangers threatening the aircraft (AC) safety and the life of passengers. Though there are some ground-based 3D Lidar systems to detect low-level MB wind shears to alert the pilots, there have been fewer scientific attempts to identify model-based MB parameters via AC onboard air and position data. The latter refers to the development and identification of an acceptable MB model upon which an automatic flight control (AFC) system can be designed to control the AC through wind shear microburst. In essence, accurate knowledge of MB model is an essential prerequisite for design and analysis of AFC systems that can safely fly the AC against microbursts, especially in crucial phases of flight such as takeoff and landing. The present study focuses on online estimation of MB parameters whose results pave the way for effective MB autopilot designs for safe flights through MB. The proposed task is accomplished via a model-based approach using the AC six degrees of freedom (6 DoF) equations of motion (EOM) integrated with the latest verified model of the MB utilizing the extended Kalman filter (EKF). In addition to a sensitivity analysis to determine the key MB model parameters, the performance of the estimation process is enhanced via a hybridization of the genetic algorithm (GA) with the EKF. The results are promising and indicate that the proposed scheme can identify the MB model parameters with sufficient accuracy needed for online applications with AFC design. [ABSTRACT FROM AUTHOR]

One important component of precipitating convection is the formation of convective downdrafts. They can terminate the initial updraft, affect the mean properties of the boundary layer, and cause strong winds at the surface. While the basic forcing mechanisms for downdrafts are well understood, it is difficult to formulate general relationships between updrafts, environmental conditions, and downdrafts. To better understand what controls different downdraft properties, we analyze downdrafts over tropical oceans in a global storm resolving simulation. Using a global model allows us to examine a large number of downdrafts under naturally varying environmental conditions. We analyze the various factors affecting downdrafts using three alternative methods. First, hierarchical clustering is used to examine the correlation between different downdraft, updraft, and environmental variables. Then, either random forests or multiple linear regression are used to estimate the relationships between downdraft properties and the updraft and environmental predictors. We find that these approaches yield similar results. Around 75% of the variability in downdraft mass flux and 37% of the variability in downdraft velocity are predictable. Analyzing the relative importance of our various predictors, we find that downdrafts are coupled to updrafts via the precipitation generation argument. In particular, updraft properties determine rain amount and rate, which then largely control the downdraft mass flux and, albeit to a lesser extent, the downdraft velocity. Among the environmental variables considered, only lapse rate is a valuable predictor: a more unstable environment favors a higher downdraft mass flux and a higher downdraft velocity. Plain Language Summary: Once a cloud begins to rain, the air inside or below the cloud can gain negative buoyancy and sink to the ground. This downward movement of air is called a downdraft. Downdrafts can end the life cycle of a cloud and also result in strong, sometimes destructive, wind gusts at the surface. The basic driving forces for downdrafts are well understood. For example, we know that evaporation of rain and the associated latent cooling of air is usually critical in causing the air to become negatively buoyant. Even though the basic driving forces are known, many interrelated processes contribute simultaneously to the strength of the downdraft, making it difficult to predict the strength of a downdraft under specific conditions. In this study, we use an atmospheric simulation whose model domain spans the globe and can explicitly resolve rain clouds. Compared to previous studies, the use of a global domain allows us to study a very large number of rain clouds, and their associated downdrafts, which form under very different, naturally varying environmental conditions. Machine learning techniques and traditional statistical methods agree on the result that the strength of the downdraft can be well predicted if we know the strength of the updraft that caused the downdraft or, even better, if we know the amount of rain that an updraft produced. Surprisingly, we have found that downdrafts can be predicted only slightly better if we also know other environmental conditions of the air surrounding the downdraft, such as the temperature and/or humidity profiles. Key Points: The best predictors of downdraft mass flux and velocity are rain amount and rate, respectivelyUpdraft properties impact downdraft properties through their control on rain formationFor a given rain amount and rate, environmental conditions add little skill to downdraft prediction [ABSTRACT FROM AUTHOR]

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Severe winds associated with thunderstorms and convection are a hazard affecting key aspects of society, including emergency management and infrastructure design. Several studies around the world have shown that severe convective winds (SCWs) can occur due to several different processes, in a range of atmospheric environments, with significant regional and temporal variations. However, in eastern Australia, the types of SCWs and their variability have not been assessed outside of individual case studies. Here, a combination of reanalysis, lightning, radar, and station data are used to characterize a set of 36 SCW events in four locations in eastern Australia. These events are objectively chosen based on the strongest measured wind gusts from station data (greater than 25 m s−1) over a 14-yr period, with 6-hourly lightning data and a 30-dBZ radar reflectivity threshold used to infer moist convective processes. Radar data analysis suggests that these SCW events are produced by several different types of parent thunderstorms, with station observations suggesting a range of temporal characteristics for these different event types. A clustering algorithm applied to environmental data is used to suggest three dominant types of events, based on low-level moisture, low-level temperature lapse rate, and deep-layer mean wind speed and vertical shear. Based on the distribution of synoptic conditions and thunderstorm properties for each environmental cluster, it is suggested that these three event types correspond to the following: 1) shallow vertical transport of strong synoptic-scale winds to the surface, 2) downbursts driven by subcloud evaporation, and 3) intense thunderstorms including supercells. Significance Statement: The purpose of this study is to better understand the different types of severe wind events in eastern Australia that are produced by convective storms. We looked at 36 historical cases in four locations and find that severe winds can be produced by very different classes of convective storms. We also suggest that there are three key types of atmospheric environment that are associated with events in this region. These environments vary in terms of the vertical structure of temperature, moisture, and wind speed above the surface. Understanding the different types of environments that lead to severe convective winds can help to reduce uncertainties in future climate projections for this region based on environmental changes. [ABSTRACT FROM AUTHOR]

The increasing complexity of West African mesoscale convective systems (MCSs) has signified the need for consistent studies of their evolution. Currently, most studies have focused on wet‐season MCS dynamics, but studies of dry‐season storms are equally important to enhance their predictability. Here, two severe thunderstorm case studies in the dry (February 13, 2018) and wet (June 18, 2018) seasons were probed to understand their synoptics, cloud properties, thermodynamic anomalies, and kinematic environments. Storm cloud properties were analysed using the cloud‐top temperature (CTT), cloud‐top height (CTH), cloud water path (CWP), and cloud optical thickness (COT) from the Advanced Very High Resolution Radiometer. Storm kinematics was analysed through the low‐ and mid‐level wind shear and corresponding microburst activities. Finally, the Weather Research and Forecasting model was used to simulate the vertical motion during the storms. The results showed that both storms initiated in anomalous conditions of convective available potential energy and convection inhibition, mean sea‐level pressure, and sea‐surface temperature. The development and merging of the Malian and Niger lows were pivotal in attracting moisture to feed and sustain the June storm. In addition, the formation of a surface–mid‐level moist cyclonic vortex stagnated the June storm, allowing for continual moisture uplift for storm intensification and longevity. Enhanced microburst activities on June 17 were a possible trigger of night‐time convective storm initiation, whereas strong surface to mid‐level wind shear increased storm longevity. Both storms showed high CWP, COT, CTH, and colder CTT during evolution. The synoptic mechanism for the February 13 storm differed from the midlatitudinal trough interactions with the Inter‐Tropical Discontinuity, due to an anomalous extension of the Azores High over North Africa. The dynamics of these storms enhance understanding of MCS evolution over West Africa and have implications for the region's weather forecasting. [ABSTRACT FROM AUTHOR]

Downbursts are negatively buoyant downdrafts that emerge from a storm and spread outward upon hitting the surface. The produced outflow, however, is not spreading through a calm environment, but rather through an atmosphere characterized by larger-scale atmospheric boundary layer (ABL) winds. This interaction between ABL winds and downbursts forms an outflow that is more complex than an outflow created by an isolated downdraft. Here, we propose an analytical solution of the interaction between the ABL winds and an isolated downburst outflow. The model is applicable when the ratio of centerline downdraft velocity to the horizontal ABL velocity at the cloud base is larger than the nondimensional group (H/D)(r/H)1.1, where H is the cloud-base height, D is the diameter of the downdraft, and r is the distance from the centerline of isolated downdraft. Also, the solution is derived for a specific direction in the outflow when the ABL winds and the isolated downburst outflow are aligned and the vertical profiles of radial velocity are self-similar. The model is based on the use of impinging-jet dynamics, their spreading rates, and a universal renormalization group that describes numerous laboratory measurements of velocity profiles of impinging jets issuing into both quiescent and crossflowing backgrounds. The model assumes a "known" base state corresponding to an isolated downburst, and then derives its interaction with ABL winds by way of perturbation analysis. The radial and vertical profiles of horizontal velocity from our analytical model are compared against field observations of actual downbursts and other analytical models and physical simulations of downburst-like outflows. Significance Statement: Downbursts are intense downdrafts that emerge from a thunderstorm and spread outward upon hitting the surface. Near-surface wind gusts in a downburst can be similar to those observed in an EF3-rated tornado (∼75 m s−1). A downburst outflow is not spreading through a calm environment, but rather through an atmosphere characterized by larger-scale atmospheric boundary layer (ABL) winds. This interaction between ABL winds and downbursts forms an outflow that is more complex than an outflow created by an isolated downdraft. However, most of the current analytical models of downbursts do not account for this interaction. Using experimental measurements of downburst-like impinging jets and mathematical rigor, our study derives an equation that quantifies this interaction between downburst and ABL winds. [ABSTRACT FROM AUTHOR]

Observations of relativistic electron precipitation events from Earth's inner magnetosphere to the upper atmosphere provide essential information about the source/loss processes and dynamics of Earth's radiation belts. Equatorial electron pitch angle isotropy can be an indicator of electron precipitation into the atmosphere, with isotropic populations resulting in a full bounce loss cone (BLC) and anisotropic populations resulting in an emptier BLC. Here we study electron microbursts, which are ˜100 millisecond duration streams of relativistic electrons thought to be scattered into the BLC by particle‐wave scattering. By measuring the ratios of >1 MeV electron flux across different detector rows in the Heavy Ion Large Telescope (HILT) instrument onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, we quantify the pitch angle isotropy of an electron population. Applying this calculation to microburst events, we have determined the dependence of microburst pitch angle distributions on burst magnitude, magnetic local time (MLT), L‐Shell, and storm phase. We found that high flux magnitude microbursts are typically highly isotropic, while lower flux magnitude microbursts have varying degrees of anisotropy. Furthermore, microbursts are most isotropic as compared to the background population in the afternoon/evening sector as well as initial and main phases of geomagnetic storms. These results have important implications for microburst scattering mechanisms as well as quantification of precipitation driven by this process. Plain Language Summary: Understanding the fundamental mechanisms that drive electron loss from Earth's radiation belts is essential for the success of various human operations. During geomagnetically active periods, increased electron fluxes around Earth can damage orbiting satellites, cause GPS signal errors, and black out ground‐based power grids. When electrons resonate with plasma waves in the magnetosphere, sub‐second bursts of electrons can be generated. These loss mechanisms, called microbursts, often cause electrons previously trapped in the radiation belts to stream into the atmosphere. Although past studies have quantified total microburst‐induced electron loss for given time periods, assumptions were made regarding how directly electrons were scattered into the atmosphere. This study uses differential electron count rates across the detector rows of the HILT instrument onboard the SAMPEX satellite to develop a qualitative measurement of the distribution of electrons within microbursts relative to the local magnetic field lines. By applying this measurement, called the isotropy index, to past calculations of electron loss, a more accurate electron loss rate associated with microbursts could be determined. Furthermore, the spatial and storm‐time dependence of the microburst isotropy index is detailed in the results section. Key Points: The pitch angle distribution of electrons within MeV microbursts is explored using 1993 data from SAMPEX/HILTLow flux magnitude microbursts have varying levels of anisotropy and high flux magnitude microbursts are consistently close to full isotropyMicroburst isotropy also shows trends with respect to local time and storm phase [ABSTRACT FROM AUTHOR]

Downbursts, as a strong localized wind event, have caused significant damage to engineering structures throughout the world. However, given the spatial and temporal randomness of such strong winds, on-site measurements are often difficult to obtain a sufficient amount of valid wind field information in a short period of time. To refine the resolution of the wind field, this study proposes a physics-informed neural network network-based (PINN-based) approach to reconstruct the downburst from limited observed data. The Navier-Stokes (N-S) equations are embedded into the fully connected neural network as a physical constraint to construct the PINN. The PINN model is then validated by the reconstruction of numerical downburst generated by large eddy simulations. The reconstruction of the sparse downburst wind field by PINN performs well in both interpolation and extrapolation prediction. The optimal construction of the PINN has been evaluated through parameter analysis of the influence of training data and network parameters. Finally, the optimal PINN construction is used to reconstruct the wind field of the experimental data with a relative error of 5% for the horizontal wind velocity. [ABSTRACT FROM AUTHOR]

This study investigates the wind-induced vibration response of TSSs under thunderstorm downbursts and introduces self-centering braces to analyze the vibration control. The study reveals that the vertical peak displacement of the upper chord nodes exhibits a " V " shaped distribution along the span direction, while the vertical residual displacement follows an "S" shaped distribution. The wind-induced vibration response at the cantilevered end is much stronger, and after the introduction of self-centering braces, the wind-induced vibration response significantly decreases, indicating that the self-centering braces play a noticeable role in controlling wind vibration. Additionally, the cable forces of the TSS significantly decrease under thunderstorm downbursts, with a loss of about 68%. The residual cable forces are uniformly distributed for the original structure and show a minor correlation with cable locations, whereas the residual cable forces of the structure with self-centering braces have a higher correlation with the cable locations. The residual cable force is significantly reduced after installing the self-centering braces, with a maximum reduction of 57.22%. Furthermore, the self-centering braces have a full hysteresis curve under thunderstorm downbursts, which indicates that it has high energy dissipation capacity and a good self-centering effect, which is conducive to controlling the wind vibration response of the structure under thunderstorm downbursts. • Vertical peak displacement of upper chord nodes exhibits a "V" shaped distribution. • Vertical residual displacement follows an "S" shaped distribution. • Wind-induced vibration at the cantilevered end is much stronger than the mid-span. • Wind vibration response can be greatly decreased by adding self-centering braces. [ABSTRACT FROM AUTHOR]