Abstract:On June 25, 2020, Tianjin Xiqing was affected by a convective storm accompanied by a mesovortex (MV), which produced record-breaking gust (41.4 m?s-1) since the observation started in 1957. In order to improve the scientific understanding of extreme thunderstorm winds caused by such mesoscale vortex, the thermodynamic structure characteristics and maintenance mechanism of MV were analyzed by using the Variational Doppler Radar Analysis System (VDRAS) technique of radar data combined with multi-source observation data. The results show that the MV in this process was initially born at a height of 2.0 km, and the contracting and stretching vertical vortex rapidly descended to surface from 2.0 km height with the rotation speed increasing and vortex diameter contracting. During this process, the surface transition from warm and dry to cold and wet cyclonic vortex, extreme wind appeared in the overlap area of western of MV and rear inflow jet (RIJ). The evolution of MV was closely related to the different properties in convective storms. In the mature stage of MV, the vertical circulation was formed by tilt updraft (TUD), RIJ, front flank downdraft (FFD) and forward low-level inflow (FLI). During the strengthening and descent of MV, the rainwater evaporated and absorbed heat during its descent, leading to a significant enhancement in the intensity of RIJ and its continuous downward extension. The configuration of the cold pool and vertical wind shear plays crucial role in the evolution of MV: from the formation to development stage of MV, the cold pool and low-level vertical wind shear from 0-3 km height reach a stage of equilibrium; from the development to mature stage of MV, the cold pool and bulk vertical wind shear from 0-6 km height reach a balanced stage; from the mature to dissipation stage, the intensity of cold pool exceeds the bulk vertical wind shear, which is unfavorable for the storm development. Distinct from the ground-reaching RIJ associated with typical bow echo, the RIJ in this event did not reach the ground, but instead coupled with the vertical downdraft of MV in near an altitude of 1 km, generating a vertically downward perturbation pressure gradient force. Meanwhile, the drag effect of rainwater facilitated the strengthening of the downdraft. During its descent, evaporation and heat absorption weakened the cold pool, which in turn intensified the surface wind speed, collectively leading to the extreme gale.

Abstract:Aerosols mainly influence cloud formation and precipitation by regulating the radiative energy balance and altering cloud droplet properties through aerosol-cloud interactions. This paper provides a comprehensive review of the microphysical effects of aerosols. As cloud condensation nuclei or ice nuclei, aerosols can significantly alter the radiative and microphysical characteristics of clouds. An increase in aerosol concentration can lead to smaller cloud droplet sizes, increasing the cloud albedo to shortwave radiation, referred to cloud albedo effect. Simultaneously, aerosols can enhance the longwave radiation emitted by thiner clouds, blocking more longwave radiation in atmosphere, which is known as the cloud thermal emissivity effect. However, absorbing aerosols may promote cloud droplet evaporation, thus reducing the cloud albedo. Ferthermore, aerosols have significant impact on precipitation. When water vapor is insufficient or wind shear is strong, an increase in cloud droplet and a reduction in droplet effective radius can suppress precipitation and extend the cloud"s lifetime. However, when clouds develop more deeply, more and smaller droplets can be transported above the 0°C level, where freezing releases latent heat, promoting convective rainfall. Therefore, the microphysical effects of aerosols can suppress weak precipitation and enhance strong precipitation, leading to an increase in extreme weather events. However, many studies have observed phenomena that are inconsistent with these theories. To explain these discrepancies, this paper systematically presents four physical mechanisms behind aerosol-cloud interactions: condensation and evaporation effects, water vapor competition effects, collision and coalescence effects, and entrainment effects. The competition among these mechanisms leads to the varied results observed in studies. Finally, the paper discusses the challenges and future research directions, with an emphasis on enhancing observational data capabilities, developing a comprehensive framework for aerosol-cloud interactions under varying conditions, optimizing parameterization schemes, and advancing the application of artificial intelligence.

Abstract:Extreme winds disaster occurred in Nanchang, Jiangxi Province in early morning on March 31, 2024, which is responsible for the fall of three residents from a high building. Based on SA dual-polarization Doppler weather radar observations from Nanchang and surface observations, this study analyzed the fine structure of the severe storm near the accident building. The results revealed that a bookend vortex was embedded in the northern end of the bow echo within a quasi-linear convective system around the time of the accident, with its horizontal scale and vertical vorticity comparable to that of a strong mesocyclone. Tornado vortex signatures (TVS) with tornado debris signatures (TDS) were identified in the center of the bookend vortex which passed over the accident building. The rotational velocity of the TVS exceeded 40 m·s-1. Damage surveys conducted along the tornado path identified by TVS and TDS revealed multiple tornadic damages, including strong cyclonic rotational winds, localized convergent rotational winds, debarked trees and complete collapse of stone bridges. This indicated that an EF2 tornado occurred very close to the accident building, most likely responsible for the fatal fall. It is noteworthy that the tornado-related vortices include a bookend vortex and several leading-edge misocyclones embedded within it, which emerged and intensified almost simultaneously in this event. The observational study on the formation mechanisms indicated that those tornado-related vortices resulted from the tilting of the horizontal vorticity. The horizontal vorticity is associated with the strong low-level vertical shear of the elevated rear-inflow jet and the baroclinic vorticity related to the gust front.

Abstract:Tengchong is located in the Yunnan-Guizhou Plateau with complex terrain and is a typical mountain city. The planetary boundary layer (PBL) and its cross-border transport of pollutants caused by its special geographical location urgently need attention, so it is of great significance to study the long-term boundary layer structure and the changing characteristics of meteorological conditions in Tengchong. In this paper, the L-band radiosonde data of Tengchong and the atmospheric boundary layer height (PBLH) calculated by Richardson number method are used to verify the ERA5 reanalysis data. Based on the ERA5 reanalysis data, the temporal characteristics and influencing factors of PBLH in Tengchong during 1980-2023 are analyzed by statistical method. The results indicate that the PBLH of ERA5 has a good correlation with the observed PBLH in Tengchong, and the average PBLH of ERA5 is 3.7m and 279.4m lower than the observed PBLH at 08:00 and 20:00, respectively.The PBLH in Tengchong showed significant fluctuations, with an upward trend in summer and autumn (0.7 m/yr, 0.2 m/yr), a downward trend in spring and winter (-0.1 m/yr, -0.2 m/yr), and an upward trend in dry and wet seasons (0.05 m/yr, 0.3 m/yr). PBLH was the order in spring (1504m) > winter (871m) > Autumn (572m) > summer (468m) at daytime, while at night, PBLH was the order in summer (52m) > Autumn (23m) > Spring (27m) > Winter (19m). PBLH in wet season (44m) was higher than that in dry season (21m). PBLH fluctuates greatly during the daytime and is relatively stable at night, reaching the daily peak at 14:00 or 15:00 in the afternoon. The PBLH can exceed 3500m in dry season and 2000m in wet season. The daily PBLH fluctuates the most in spring and dry season. On the annual scale, PBLH was positively correlated with 10m wind speed, surface sensible heat flux, ground-air temperature difference, surface temperature and 2m air temperature, and negatively correlated with relative humidity, total cloud cover, surface latent heat flux and 2m dew point temperature. On the seasonal scale, the PBLH in spring, autumn, winter and dry season was mainly driven by 10m wind speed, while in summer and wet season it was mainly driven by high latent heat flux, total cloud cover, relative humidity and low sensible heat flux. In the diurnal variation, 10m wind speed, surface sensible heat flux and relative humidity in daytime were the main factors affecting the development of PBLH, while the surface latent heat flux at night was the only significant factor affecting the development of PBLH.

Abstract:This study examines the impact of direct assimilation of FY-3D satellite MWHS II microwave humidity sounder data on the prediction of extreme rainfall, using the July 31, 2023, heavy rainstorm in Beijing as a case study. Comparative experiments were conducted before and after data assimilation, and the WRF numerical prediction model was applied to analyze the effects across multiple scales and variables. The results show that the assimilation of MWHS II data significantly improved the simulation of extreme rainfall. It successfully captured the maximum rainfall center, exceeding 550 mm, and provided a more accurate simulation of rainfall distribution. The study also highlights the effect of assimilation on large-scale systems. It improved the large-scale environmental field, creating conditions that favored the extreme rainfall event. Key improvements included a strengthened temperature gradient in critical areas, optimized water vapor distribution, especially over the eastern sea, and an increased north-south pressure gradient. Together, these factors maintained a stable large-scale background that supported precipitation. On a smaller scale, the impact on convective systems was even more noticeable. Over the Beijing area, the vertical vorticity structure was optimized, with enhanced negative vorticity in the mid-to-upper atmosphere and increased positive vorticity in the lower levels, which promoted upward motion. The atmosphere became more unstable, with increased relative humidity in the lower levels, decreased humidity in the mid-levels, and a steeper vertical temperature gradient. These factors contributed to the triggering and maintenance of strong convection. Additionally, the microphysical processes were improved. More snow and graupel particles formed in the mid-to-upper layers, and the conversion of cloud water to rainwater accelerated in the lower levels, enhancing the overall precipitation efficiency. These effects were most prominent during the first 36 hours of the simulation, emphasizing the critical role of data assimilation during the early and developing stages of precipitation.

Abstract:El Ni?o-Southern Oscillation (ENSO) is the strongest interannual climate mode in the atmosphere-ocean coupling system over the tropical Pacific. Westerly wind bursts (WWBs) are an important precursor of the ENSO events. In the past few decades, several statistical and dynamics-based models have been used to simulate and predict ENSO events. However, these models have some difficulties in simulating WWBs, leading their simulation and prediction performance for ENSO events that are also limited. This study constructs U-Net models that are driven by several atmospheric and oceanic data for zonal wind anomalies (ua) in the tropical Pacific. Furthermore, the spatiotemporal characteristics of WWBs are identified and their relationship with ENSO phenomenon are analyzed. The results indicate that the U-Net model can effectively simulate the spatiotemporal distribution characteristics of WWBs during the testing period (2003-2022). The deviation between the occurrence frequency and accumulated days of WWBs reconstructed by the model and the observed values is less than 4.0 %, and the correlation coefficient between the time series of WWBs occurrence probability (P) simulated by the model and observed values 0.87. Meanwhile, the U-Net model can effectively capture the non-linear relationship between WWBs and ENSO, and there is a significant lead-lag correlation between WWBs and ENSO in both U-Net models and reanalysis data. Besides, the duration, zonal width and the average maximum amplitude of WWBs reach their peak during El Ni?o events, and the peak of probability (P) of WWBs during El Ni?o events reconstructed by the U-Net model is much closer to that of observation. In conclusion, compared with the traditional WWBs parameterization scheme that relies on establishing an approximate linear relationship between WWBs and sea surface temperature fields, the U-Net model has significant advantages in representing WWBs.

Abstract:Upper-level aviation turbulence is triggered by turbulence in the free atmosphere. It is a major concern for aircraft operations. Under a warming climate, the turbulence over China shows a trend of increasing. To better understand the sources and characteristics of the upper-level aviation turbulence over China, two sets of Aircraft Meteorological Data Relay (AMDAR) are used to establish a one-year global turbulence observation dataset in the form of eddy dissipation rate (EDR) through quality controls and consistency processing. Based on the aviation, FY-4A satellite observations and the ERA5 reanalysis, the spatial distribution, as well as the seasonal and diurnal variations of three types of upper-level aviation turbulence, clear-air turbulence (CAT), near-cloud turbulence (NCT), and convective-induced turbulence (CIT) over China, are analyzed. It is found that the active zone of the clear-air turbulence is prone to occur in the upper troposphere near the tropopause in the vertical direction. Near-cloud turbulence over China has 4 active zones, which are located in eastern, northern China, and the South Sea, as well as over Japan. The northern to eastern slopes of the Qinghai Tibet Plateau are regions with a high probability of near-cloud turbulence in western China. The convective-induced turbulence concentrated more over southern China. All three types of turbulence exhibit typical seasonal and diurnal variations. Clear-air turbulence is more common in the afternoon during winter and spring seasons, while near-cloud and convective-induced turbulence are more likely to occur in the morning during summertime.

Abstract:The reversal of temperature anomalies from warm to cold and vice versa in the East Asian region is one of the most significant climatic phenomena during the winter season under the context of global warming. Compared to the variability of seasonal-mean winter temperature anomalies, the climate prediction uncertainty for the reversal phenomenon is greater, and its adverse impacts are more significant. Based on atmospheric reanalysis data from the winters of 1980/81 to 2022/23, this study employs seasonal empirical orthogonal decomposition and composite analysis methods to investigate the interannual variability of the East Asian winter temperature anomaly reversal mode (TARM). The results indicate that although the different processes of the East Asian winter TARM are related to intraseasonal reversals of the Siberian High, the Eurasian teleconnection and the Arctic vortex anomalies, the interannual variability of the "warm to cold" mode is significantly stronger than that of the "cold to warm" mode, suggesting a pronounced asymmetry in the interannual variability of TARM. In the "warm to cold" mode, the lower-level Siberian High strengthens over time, with the positive phase of East Atlantic-West Russia-like teleconnection in early winter shifting to a negative phase in late winter, leading to more active cold air. Conversely, in the "cold to warm" mode, the intensity of the winter Arctic vortex shifts from weak to strong, the Siberian High weakens after that, favoring the appearance of temperature anomalies that are cold in early winter and warm in late winter. Further analysis shows that North Atlantic and tropical Indian Ocean sea surface temperature anomalies with La Ni?a are important factors influencing the "warm to cold" mode, while a reduction in November Barents-Kara sea ice acts as a precursor signal for the "cold to warm" mode. Therefore, the diversity of atmospheric underlying conditions is an important reason for the asymmetry of the East Asian winter TARM. Therefore, climate prediction for the East Asian winter TARM should consider the diversity of atmospheric underlying conditions.

Abstract:Based on our prior research, this paper carries out the simulation of three typical short-wavelength radar detections (X-band-9.5GHz-3cm, Ka-band-35GHz-8mm, and W-band-94GHz-3mm) for non-precipitation cloud particles, particularly ice crystals. It further compares and analyzes the radar reflectivities stemming from variations in the particles" physical attributes. Additionally, the study explores the feasibility of employing these three radar wavelengths to discern the morphology of ice crystals. The results show that, despite the W-band"s short wavelength and heightened sensitivity, the backscattering characteristics of ice crystals exhibit a considerably more intricate pattern than those observed in the X-band and Ka-band. Variations in incident angle, particle morphology, and particle size elicit dramatic and complex alterations in backscattering, posing a formidable challenge to traditional inversion methods that rely on statistical or empirical fitting approaches to extract cloud particle microphysical properties. It has also been shown that utilizing the characteristics of the differences between multi-wavelength radar observations can help to provide microphysical information about particles in clouds. For example, in the ice cloud, a pronounced disparity in reflectivity factor between the W-band and either the Ka-band or X-band, particularly when it dips below -3 dBZ, suggests the likelihood of prolate particle presence. This offers invaluable insights and practical experience for refining and enhancing the accuracy of algorithms designed to invert cloud microphysical features based on triple-frequency radar data.

Abstract:Based on multi-source observations and ERA5 reanalysis data, we present in detail the environment conditions, storm evolution and possible formation mechanisms of the Fuxin EF1 tornado and the Kaiyuan EF2 tornado on June 1 2023 in this paper. The results show that the northern part of Liaoning Province was affected by the upper trough and lower-level shear line at the rear part of a Northeast China Cold Vortex. The strong vertical temperature lapse rate between the surface (or 850 hPa) and 500 hPa was the extreme environmental condition factor that caused this process. The Fuxin tornado storm had hook echo, descending reflectivity core (DRC) and tornado vortex signature (TVS), formed by the merger of two isolated storms generated on the dry-line. The strong echo on the east side of tornado storm generated the strong cold pool with the surface temperature dropped by more than 6 ℃ within one hour. The outflow of cold pool merged with the dry-line, forming a triple point at the edge or the strong cold pool. When the hook echo moved to the triple point, a moderately strong downdraft produced a cold pool with suitable intensity, coupled with the strong convergence and uplift at the triple point, the tornado generated under the DRC and TVS. The Kaiyuan tornado storm was a linear convective system, formed by the merger of multiple storms along the surface convergence line. Due to Kaiyuan"s location within the low-level jet and its stronger low-level wind shear, it was conducive to the initial formation of a misocyclone on the ground convergence line. The cold pool with a temperature dropped by more than 10°C within an hour was produced by the storm at the west side of the misocyclone. The cold pool outflow merged with the surface convergence line to form a triple point, resulting in stronger convergence in Kaiyuan than that of Fuxin. When the misocyclone passed through the triple point, under the strong convergence and stretching effect, the low-level rotation of misocyclone reached its strongest in the life cycle which first met the TVS standard and the Kaiyuan tornado generated. The Northeast cold vortex process had extreme conditions of instability, the storms generated on the dry-line and surface convergence line, produced a strong cold pool within an area of about 30 km near the tornado genesis zone. At the edge of the strong cold pool where the ground temperature dropped less than 4°C within an hour, the humidity conditions of the lower atmosphere were improved. The triple point was also formed to strengthen the ground convergence. When the TVS or the misocyclone passed through these triple confluence points, tornadoes were formed respectively.