Abstract:The microphysical characteristics of the melting layer in clouds have a critical impact on cloud structure and precipitation formation. Utilizing the Airborne Ka-Band Precipitation Cloud Radar (KPR) and in-situ observation instrument onboard the King Air 350 aircraft of the Weather Modification Center, CMA. This study analyzed the cloud microphysical characteristics near the melting layer during stratiform precipitation with embedded convection in east China on September 15, 2023. The results show that above 0℃, the particles in the cloud mainly gather and grow through rime, and the aggregated graupel particles are the main ones. When the ice phase particles falling and melting after 0℃, graupel particles and large ice crystals melting and gradually transform into small droplets. With the decreasing of the altitude, the radial velocity and spectrum width of KPR gradually increased, indicating that the size of precipitation particles in the cloud increased, the relative velocity of particles soared, and the particles are more likely to collide and grow, and then small droplets grow into large droplets through the collision process. By fitting cloud and precipitation particle spectrums, it is showed that Gamma distribution can fit cloud and precipitation particle spectrums well. Above the 0℃ layer, the slope of cloud droplets spectrum increases with decreasing-altitude, and the precipitation particle spectrum decreases with decreasing-altitude. However, below the 0°C layer, the slopes of both spectra initially increase and then decrease with decreasing-altitude.

Abstract:Enhancing the understanding of the characteristics and causes of extreme precipitation anomalies in the Jianghuai region is crucial for comprehending regional climate change patterns and improving disaster prevention and mitigation capabilities. Based on gauged precipitation data by China Meteorological Administration and NCEP/NCAR reanalysis data, this study analyzes the circulation configuration and evolutionary characteristics during different types of extreme precipitation events through percentile and K-means clustering methods,atmospheric dynamic-thermodynamic diagnostics technique.. The research indicates that extreme precipitation in the Jianghuai region can be categorized into four types according to the location of precipitation center: C-Type, N-Type, S-Type, and E-Type. The first three types concentrate during periods associated with the seasonal northward shift of the rain belt, while the fourth type is more scattered. During the occurrence of the four types of extreme precipitation, the main precipitation zones are located below the divergence area in the upper troposphere where is between the strong South Asian High and the westerly jet stream, with increased vertical velocity, enhanced temperature gradients in the upper and middle troposphere, greater meridional circulation in mid-high latitudes, intensified subtropical high, and enhanced water vapor transport from oceanic regions. The first three types of precipitation are different from the fourth type in East Asian teleconnection pattern (EAP), low-level jet, and water vapor transport. During the first three types, EAP is more typical, and low-level jet is obvious. The water vapor transport mainly comes from the Western Pacific and South China Sea. The temperature gradient in the upper troposphere is a dipole anomaly structure with colder temperatures to the north and warmer temperatures to the south. When E-type occurs, there is atypical EAP, no obvious low-level jet, obvious easterly water vapor transport along the East Asian coast, and only a unipolar warm center in the upper troposphere. The low-frequency evolution of thermal-dynamic conditions for the four extreme precipitation types differs as follows: C-Type precipitation occurs during the northeastward movement of subtropical high and the downward propagation of the EAP wave train; in the N-type case, the warm center of the near-surface temperature moves southward from the Yangtze-Yellow River to the south of the Yangtze River, and the temperature over the Jianghuai region is cold up and warm down; S-Type occurs during the southwestward movement of subtropical high, with the temperature gradient at 300hPa transitioning from cold north and warm south to warm north and cold south, and positive height values dominating throughout the entire troposphere; E-Type occurs during the northeastward movement followed by the southwestward movement of subtropical high, with convective activity strengthening from low to middle latitudes, and almost entirely warm areas at 200hPa and below. Further analysis demonstrates that changes of subtropical high and thermal effects have better indicative significance for low-frequency prediction of extreme precipitation events.

Abstract:Affected by the residual vortex of Typhoon Haikui (2311), an extreme rainstorm occurred in the southeast coast of Guangxi from September 10 to 11, 2023, with large rainfall intensity, overlapping falling areas and obvious night rain characteristics, and the rainfall in many places broke the observation records since the establishment of the station. Based on the multi-source observations and ERA5 reanalysis data, the cause of the extreme rainstorm and the possible mechanism of residual vortex maintenance were analyzed. The results are as follows: Under the background of the weakening of the continental high and the stable maintenance of the western Pacific subtropical high, the "Haikui" residual vortex between the two is weakly guided by the steering flow, remaining stationary over Guangxi, which cooperated with the strong southwest monsoon flow and contributed to the extreme rainstorm. There is obvious strong precipitation potential near the rainstorm center, and the precipitable water is abnormally large before the heaviest precipitation. The boundary layer jet on the southeast side of the residual vortex is the key influencing system for extreme rainstorm, which plays a cross-scale pivotal role in connecting the residual vortex with local extreme rainstorm. Its development at night is related to the local kinetic energy increase driven by work done by the meridional pressure gradient force in the southeast side of the residual vortex.The negative meridional potential height horizontal gradient on the southeast side of the residual vortex combined with southerly wind increases the local kinetic energy, resulting in enhanced development of boundary layer jet at night. The development of boundary layer jet is conducive to the formation of strong vertical helicity through low-level convergence uplift combined with positive vorticity circulation of residual vortex, on the one hand, and the frontogenesis forcing of convergence and deformation at low level provides dynamic conditions for the development of heavy precipitation. On the other hand, is conducive to the rainstorm center maintains high temperature and high humidity at low level, and thicker wet layer depth. The deep warm advection heating layer and the middle troposphere latent heating provide favorable thermal conditions for the maintenance of residual vortex circulation. At night, the perturbation downslope wind on the eastern slope of the Yunnan-Guizhou Plateau and the perturbation onshore wind in the Beibu Gulf cooperate with each other to point to the center of the residual vortex, enhance the convergence effect of the wind field, and provide dynamic conditions for the development and maintenance of the cyclonic vorticity of the residual vortex.

Abstract:Based on the tropical regional atmospheric model system of China Meteorological Administration (CMA-TRAMS), the forecast of tropical cyclone (TC) formation for the No. 3 Typhoon Chaba in 2022 are analyzed, as well as the comparison of prediction between CMA-TRAMS and European Centre for Medium-Range Weather Forecasts (ECMWF). The reasons of the successful genesis prediction of CMA-TRAMS for Typhoon Chaba are investigated from different aspects such as the development environment of tropical cyclone (TC) embryos, physical processes within TC embryos, embryo structure and the corresponding development. The results indicate that numerical models are required to have good descriptive ability in the above aspects to perform well in TC generation forecast. This study is beneficial for us to understand the main physical processes and factors closely related to TC generation prediction in numerical models, and it would provide clues for the follow-up model development and improvement. The results indicate that the significant difference in the cyclonic circulation pattern of the monsoon trough in the western Philippines between the 72-120 h forecasts is the direct cause of the difference in TC generation forecasts between the two models. CMA-TRAMS predicts the generation, development, and merging of multiple mesoscale convective systems (MCSs) or mesoscale convective vortices (MCVs) in a positive vorticity environment between 96-120 h, and the organization of circulation to form warm core structures, which is important in its successful prediction of the formation of "Chaba The accuracy of wind field forecasting in the western monsoon trough of the Philippines may have a significant impact on the prediction of TC formation in the South China Sea. The continuous convergence and merging of MCSs and MCVs, and the organization of cyclone circulation are important physical processes for TC formation. The results of this study have deepened our understanding of the main physical processes involved in TC formation, enhanced our knowledge of the influencing factors of numerical model prediction for TC formation, and provided clues for improving model forecasts.

Abstract:Multiple mesoscale vortices often coexist or alternate in the process of torrential rainfall in southwest China. The southwest vortices and its closely related mesoscale vortices (if any) which occur during the same period is defined as the southwest vortex group in this article. The southwest vortex group include both traditional southwest vortices and general mesoscale vortices. Based on ERA5 reanalysis data and precipitation data obtained by an automatic observation station, a torrential rainfall event occurs from 12-14 August 2020 (Coordinated Universal Time, the same below) was studied, wherein southwest vortex group is found. The results show that: a total of 16 mesoscale vortices were found (numbered Vortex I-Vortex XVI), which were the dominant weather system for this torrential rainfall event. Among southwest vortex group, Vortex I, Vortex II, Vortex IV and Vortex XVI are southwest vortices, and others are general mesoscale vortices. The large-scale background is conducive to this event, mainly manifested as strong upper-level divergence which related to the South Asia High, a middle-tropospheric warm advection which related to a Westerlies shortwave trough, and a vigorous southwesterly low-level jet which brought abundant water vapor and caused a strong low-level convergence. Vortex I and Vortex II are both southwest vortices and the main members of the southwest vortices, which are characterized by long life span, deep vertical extension, and strong related precipitation. The thermal structures of the two are significantly different, with cold and warm core structures, respectively. Most of the air particles that formed Vortex I and Vortex II were sourced from the lower troposphere. Within 5 to 7 hours before their formation, the air particles significantly ascend, accompanied by strong precipitation processes. The vertical stretching caused by convergence makes the cyclonic vorticity increasing significantly, while the tilting effect decreasing the cyclonic vorticity. Air particles from the Yunnan-Guizhou Plateau made the larger contribution of cyclonic vorticity than which from the Tibetan Plateau. Vorticity budget shows that there are significant similarities and differences in the factors that dominate the evolution of Vortex I and Vortex II. Vertical stretching due to convergence and vertical advection of cyclonic vorticity due to convection, are the most favorable factors for the generation, development, and maintenance of the Vortex I, whereas tilting effects and horizontal advection dominate its extinction. For Vortex II, convergence and vertical advection are also the favorable factors for its generation, whereas the horizontal and vertical advection of cyclonic vorticity are the dominant factors of its maintenance, during its demise period, except vertical advection other factors all accelerated the dissipated.

Abstract:The precise simulation of interannual variability of East Asian summer monsoon (EASM) is still a challenge for the state-of-the-art models. In this study, based on the recent proposed potential vorticity circulation (PVC) theory, we find that the second mode of the multivariate empirical orthogonal function (MV-EOF) for the observed East Asian Summer Monsoon (EASM) has a close relationship with the first mode of the cross-equatorial PVC (CEPVC) and the cross-tropopause PVC (CUPVC). Based on observational analysis, we further evaluate the simulation capability of the second mode of the East Asian Summer Monsoon (EASM) in July from the FGOALS-f3-L historical experiment and analyze the potential causes of the bias. Results indicate that the model fails to reproduce the meridional dipole pattern of geopotential height and precipitation of EASM which is closely related to the simulation of the CUPVC bias. Further analysis suggests that the correct simulation of the convergence and divergence of the water vapor flux which related to the changes in CUPVC is the key physical process for the model to accurately simulate the second mode of EASM. The results also indicate the improvement in the simulation of the CUPVC related to the South Asian High (SAH) phenomenon could improve the simulation skill of the interannual variability of EASM.

Abstract:The geostationary orbit microwave sounding satellite carrying a microwave sounder utilizes the relatively stationary characteristics of the platform, as well as the microwave"s ability to penetrate clouds and rain, to achieve high-frequency, all-day, and all-weather observations of the atmosphere, providing high-value data for numerical weather forecasting and disaster weather research. This paper analyzes the sounding capability of the Chinese FengYun-4 geostationary orbit microwave satellite currently under development. The observation method and product system are introduced. Its millimeter wave sub-millimeter wave sounder has added the 55 GHz microwave hyperspectral frequency band and 380-425 GHz terahertz channel that are not available on existing low orbit satellites, greatly expanding the sounder"s ability to observe atmospheric status and cloud parameters. The unique observation geometry characteristics of the satellite are shown through simulation technology. Based on the ARMS fast mode and atmospheric backgrounds, the brightness temperature of typical channels for detection of temperature and humidity, as well as the terahertz channels, are simulated under typhoon background.

Abstract:Based on the conventional ground observation data and ERA5 reanalysis data, the vertical structure and evolution mechanism of the low vortex during the heavy precipitation process in North China during July 19-21, 2016 (case 1) and July 11-13, 2021 (case 2) were compared and analyzed in this paper. The results show that the two vortex systems are generated in a similar circulation background of low west and high east, and the upper-level jet stream in case 1 is significantly stronger than that in case 2, which is the obvious difference between the two vortex structures in large-scale circulation background. The vertical structure of the two low vortices is different. In case 1, the low vortices develop deeply in the vertical direction, and there are two positive vorticity centers in the upper layer (300 hPa) and the lower layer (850 hPa), respectively. With the movement of the low vortex, the positive vorticity center in the lower layer gradually weakens, while the positive vorticity center in the upper layer gradually increases, and the temperature distribution is warm at the top and cold at the bottom. There is a weak warm core structure at 400 hPa, under which there is a cold air mass, and the dry and cold air on the west side of the central axis of the vortex. In case 2, the development of low vorticity is relatively shallow, with only one positive vorticity center in the lower layer, and it is always located at 850 hPa. The middle troposphere presents an obvious warm core structure between 700 and 300 hPa, and the cold core near the ground. The development of the two low vortex systems is related to the downward transmission of the upper potential vortex and the transport of warm and humid air at the lower level. However, in case 1, there is the intrusion of dry cold air, and there is obvious frontogenesis and convective instability in the upper and lower levels, which is conducive to the generation of upward motion and the enhancement of positive vorticity. In case 2, the whole troposphere is dominated by warm advection, and the center of the potential vortex and the center of the warm advection are both below 850 hPa, and they almost overlap, resulting in upward motion and the development of the low vortex system. The vorticity budget analysis reveals that during various evolution stages of the two vortices, the budget on the northern and eastern sides of the vortex centers is positive, indicating a tendency for the vortices to move eastward and northward. The horizontal divergence term and the tilting term play a primary positive role in the vorticity growth at the lower levels of the two North China vortices during their development stages. The advection term and the divergence term contribute significantly and positively during the strength and attenuation stages. The difference between the two cases lies in the fact that, for Case 1, the vertical term contributes positively in the strength and attenuation stages, particularly in the vortex center and its eastern side, whereas for Case 2, the vertical term remains negative throughout the entire evolution process.

Abstract:In this paper, a comprehensive effect assessment of a convective?stratiform mixed cloud precipitation enhancement operation in Anhui Province on June 17, 2023 was carried out using hourly rainfall data from national and regional automatic rainfall stations in Anhui Province, S-band radar data and sounding data in Anqing area. The results showed that shortly after cloud seeding, a narrow, intense echo area was observed within 1 km above the seeding height. This area expanded 24 minutes after the cloud seeding, formed an intense echo center at the seeding height. Additionally, the echo shape changed from strip to block, accompanied by a concentrated area of severe convection. The proper echo units were identified and tracked by the Centroid Optimization Matching method and the Lagrangian method, and the best comparison unit was selected based on the similarity measurement. Time-series variations of the five radar physical parameters of the seeded unit and the comparison unit were further analyzed. The results showed that following the operation, the radar physical parameter values of the seeded unit increased significantly, reaching their peaks within 42 minutes, and then remained stable. Conversely, the corresponding parameter values in the contrast unit exhibited a decreasing trend. The double-ratio values of the five radar physical parameters were greater than 1 within 1 h after the operation, indicating that the echo intensity of the contrast unit gradually decreased, while the seeded unit developed more vigorously with prolonged lifespan, demonstrating the obvious seeding effect. In addition, we optimized the estimation of natural rainfall in the operational impact area by using a cluster-based historical regression method for the floating area. Firstly, the K-Medoids clustering algorithm combined with PCA dimensionality reduction technology was used to accurately classify the precipitation characteristics in southern Anhui Province. Then the performances of six regression models were evaluated through cross-validation, and the results showed that the ElasticNet regression model had the best performance in predicting the rainfall in the area of influence. Finally, the regression model was applied in the individual cloud seeding case, providing the results of 2.92mm rainfall increase in 3h after operation and 22.3% rainfall enhancement effect, the results of the one-sample t-test showed that the precipitation enhancement effect in the affected area was significant at the 95% confidence level.

Abstract:“Warm Arctic-cold Eurasia” (WACE) pattern is one of the key modes of winter Arctic-Eurasia climate variability, existing on multiple time scales. After 2012, the weakening of the “Arctic warming-Eurasia cooling” trend has sparked intense discussions on whether the Arctic-Eurasia climatic linkage has disappeared or weakened. Diagnosis analyses based on multi-source data indicate that the time scale of the close connection between the Arctic and Eurasia climates shifts from the winter-mean to subseasonal variations, presenting the increasing and strengthening phase reversal of WACE between early and late winter. WACE pattern can lead to a weakened large-scale meridional temperature gradient, forming persistent and large-amplitude atmospheric circulation anomalies, thereby triggering the extreme cold waves. More importantly, the WACE reversal can drive the extreme cold-warm transitions in the Mongolian region and eastern China. As Mongolia is a major dust source for China, rapid wintertime phase reversals lead to loose, bare ground surfaces, providing ample material conditions for severe sandstorms in North China.SThe “Warm Arctic-Cold Eurasia” on the subseasonal scale offers a clearer physical features of the variations in early and late winter. The WACE in late winter also significantly impacts the haze pollution in North China and plays a key role in predicting Arctic wildfires. Future research on WACE pattern urgently needs to focus on its trend changes and uncertainties, clarify the trigger mechanism for its phase reversal, and improve the simulation and prediction performance of climate models, in order to enhance the prediction ability for winter-spring extreme climates in mid-low latitudes.