Microphysical Characteristics of Extreme-Rainfall Convection over the Pearl River Delta Region, South China from Polarimetric Radar Data during the Pre-summer Rainy Season

During the pre-summer rainy season,heavy rainfall occurs frequently in South China.Based on polarimetric radar observations,the microphysical characteristics and processes of convective features associated with extreme rainfall rates(ERCFs)are examined.In the regions with high ERCF occurrence frequency,sub-regional differences are found in the lightning flash rate(LFR)distributions.In the region with higher LFRs,the ERCFs have larger volumes of high reflectivity factor above the freezing level,corresponding to more active riming processes.In addition,these ERCFs are more organized and display larger spatial coverage,which may be related to the stronger low-level wind shear and higher terrain in the region.In the region with lower LFRs,the ERCFs have lower echo tops and lower-echo centroids.However,no clear differences of the most unstable convective available potential energy(MUCAPE)exist in the ERCFs in the regions with different LFR characteristics.Regardless of the LFRs,raindrop collisional coalescence is the main process for the growth of raindrops in the ERCFs.In the ERCFs within the region with lower LFRs,the main mechanism for the rapid increase of liquid water content with decreasing altitude below 4 km is through the warm-rain processes converting cloud drops to raindrops.However,in those with higher LFRs,the liquid water content generally decreases with decreasing altitude.

Assimilating Surface Observations in a Four-Dimensional Variational Doppler Radar Data Assimilation System to Improve the Analysis and Forecast of a Squall Line Case

This paper examines how assimilating surface observations can improve the analysis and forecast ability of a four- dimensional Variational Doppler Radar Analysis System （VDRAS）. Observed surface temperature and winds are assimilated together with radar radial velocity and reflectivity into a convection-permitting model using the VDRAS four-dimensional variational （4DVAR） data assimilation system. A squall-line case observed during a field campaign is selected to investigate the performance of the technique. A single observation experiment shows that assimilating surface observations can influence the analyzed fields in both the horizontal and vertical directions. The surface-based cold pool, divergence and gust front of the squall line are all strengthened through the assimilation of the single surface observation. Three experiments--assimilating radar data only, assimilating radar data with surface data blended in a mesoscale background, and assimilating both radar and surface observations with a 4DVAR cost function--are conducted to examine the impact of the surface data assimilation. Independent surface and wind profiler observations are used for verification. The result shows that the analysis and forecast are improved when surface observations are assimilated in addition to radar observations. It is also shown that the additional surface data can help improve the analysis and forecast at low levels. Surface and low-level features of the squall line-- including the surface warm inflow, cold pool, gust front, and low-level wind--are much closer to the observations after assimilating the surface data in VDRAS.

Variability of microphysical characteristics in the “21·7” Henan extremely heavy rainfall event

In this study, significant rainfall microphysical variability is revealed for the extremely heavy rainfall event over Henan Province in July 2021(the “21·7” Henan EHR event) using a dense network of disdrometers and two polarimetric radars.The broad distributions of specific drop size distribution(DSD) parameters are identified in heavy rainfall from the disdrometer observations, indicating obvious microphysical variability on the surface. A K-means clustering algorithm is adopted to objectively classify the disdrometer datasets into separate groups, and distinct DSD characteristics are found among these heavy rainfall groups. Combined with the supporting microphysical structures obtained through radar observations, comprehensive microphysical features of the DSD groups are derived. An extreme rainfall group is dominantly formed in the deep convection over the plain regions, where the high number of concentrations and large mean sizes of surface raindrops are underpinned by both active ice-phase processes and efficient warm-rain collision-coalescence processes in the vertical direction. Convection located near orographic regions is characterized by restricted ice-phase processes and high coalescence efficiency of liquid hydrometeors, causing the dominant DSD group to comprise negligible large raindrops. Multiple DSD groups can coexist within certain precipitation episodes at the disdrometer stations, indicating the potential microphysical variability during the passage of convective system on the plain regions.

The T-TREC Technique for Retrieving the Winds of Landfalling Typhoons in China

In this study, an extension of the TREC （Tracking Radar Echo by Correlations） technique, named Tropical Cyclone （TC） circulation TREC （T-TREC）, is developed to retrieve the winds of landfalling typhoons in China. The T-TREC analysis is performed on a polar grid centered at the TC center, using arc-shaped correlation cells and an arc-shaped search area. The search for the best correlation match is confined along the cyclonic direction with a limited search distance in the radial direction based on the cyclonic circulation characteristics of TCs in the Northern Hemisphere. The TC center is determined objectively using reflectivity data while the Doppler radar radial velocities are incorporated to estimate the search range and create a velocity correlation matrix as auxiliary constraints. The T-TREC was applied to the landfalling Typhoon Saomai （0608） observed by Chinese next generation Weather Surveillance Radar 1998 Doppler （CINRAD WSR-98D） on the southeast coast of China. The results show that the T-TREC has the ability to estimate the typhoon circulation with an average bias of 4 m s -1 . The incorporation of radial velocity data could distinctively improve wind retrievals for intense typhoons, especially by reducing the underestimation caused by fairly uniform reflectivity patterns in the vicinity of the eyewall and the outer rainband. A quantitative evaluation of the influence of typhoon center and cell size on the wind estimation demonstrates that the quality of the T-TREC retrieved wind circulation depends on the estimation of the typhoon center. A 4-km deviation of the TC center may result in a 10% increase in the retrieved wind error. The effect of cell size depends on the typhoon scale： better wind retrieval results can be obtained for a smaller typhoon.

Single-Doppler Radar Analysis of a Mesocyclone in the Taiwan Strait

In this study, the kinematic and precipitation structures of a mesocyclone associated with a hook echo were analyzed using single Doppler radar data. The mesocyclone was embedded in a mesoscale convective rainband near northern Taiwan coastline on 10 September 2004. The synoptic environment was characterized by a moderate convective available potential energy （CAPE） and a moderate ambient vertical shear from surface to 5 km. In addition, a pronounced low-level mesoscale shear/convergence zone, which resulted from the interaction of two tropical depressions, was also identified in the northwest coast of Taiwan, providing a favorable dynamic condition for the development of the mesocyclone. Analyzing single Doppler dipole signature shows that this mesocyclone formed initially at low levels, then deepened and strengthened rapidly into mature stage with the vertical depth exceeding 8 km. The diameter of the mesocyclone decreased with the height at the time of vortexgenesis, and then evolved into columnar structure accompanied with the broader diameter in middle layer. The mesocyclone lasted for about 2 h. The Ground-Based Velocity Track Display （GBVTD） method was applied to retrieve the ax- isymmetric circulation of the mesocyclone. The GBVTD-derived primary circulation showed the radius of maximum wind （RMW） of the mesocyclone was about 5-6 km and varied from inward tilting to outward tilting with time. The axisymmetric radial wind field was initially characterized by a low-level inflow inside the RMW and outflow outside the RMW, respectively. The strongest reflectivity was associated with a stronger updraft near the RMW, and a weak downdraft was located at the center of the mesocyclone. Subsequently the downdraft and reflectivity near the mesocyclone center strengthened obviously, accompanied with the low-level outflow, strong updraft as well as high reflectivity extending outside the RMW. The relative tangential wind initially exhibited a wavenumber 1 asymmetric structure with the maximum wind region at the left portion of the meso cyclone and shifted counterclockwise with height. The axisymmetric tangential wind strengthened and reached its maximum intensity with a value about 20 m s^-1 at z=1 km. After that the axisymmetric tangential wind decreased rapidly, meanwhile the wave-1 asymmetric structure redeveloped with the maximum wind at the left-front of motion. In summary, the evolution and structure of the mesocyclone is similar to that observed within a non-supercell mesocyclone. It is worth to mention that the axisymmetric circulation characteristics of the mesocyclone at its mature stage are very similar to those observed in a mature typhoon. However, there are significant differences, i.e., the size is much smaller, the lifetime is much shorter, and the downdraft in the center is produced by precipitation instead of compensating subsidence.

RADAR OBSERVATION OF PRECIPITATION ASYMMETRIES IN TROPICAL CYCLONES MAKING LANDFALL ON EAST CHINA COAST

This study explores,for the first time,the asymmetric distribution of precipitation in tropical cyclones(TCs) making landfall along east China coast using reflectivity data collected from coastal Doppler radars at China's Mainland and Taiwan.Six TCs(Saomai,Khanun,Wipha,Matsa,Rananim and Krosa) from 2004 to 2007 are examined.The temporal and spatial evolution of these TCs’ inner and outer core asymmetric precipitation patterns before and after landfall is investigated.The radius of inner-core region is a function of the size of a TC apart from a fixed radius(100 km) adopted in previous studies.All six TCs possessed distinct asymmetric precipitation patterns between the inner-and outer-core regions.The amplitude of asymmetry decreases with the increasing TC intensity and it displays an ascending(descending) trend in the inner(outer) core.In the inner-core region,the heavy rainfall with reflectivity factor above 40 d BZ tends to locate at the downshear side before landfall.Four cases have precipitation maxima on the downshear left side,in agreement with previous studies.As TCs approaching land(~ 2 hr before landfall),their precipitation maxima generally shift to the front quadrant of the motion partly due to the interaction of TC with the land surface.In the outer-core region,the precipitation maxima occur in the front quadrant of the motion in five of the six cases before landfall.After landfall,the precipitation maxima shift from the right-front quadrant clockwisely to the right-rear quadrant of the motion collocated well with the mountainous areas along the coast,which indicates the impact of topography forcing on the precipitation distribution.This study illustrated how the precipitation asymmetry in the inner-and outer-core at different stages of TC landfall is affected by storm motion,vertical wind shear and topography.