NUMERICAL SIMULATION STUDY OF CLOUD INTERACTION AND FORMATION MECHANISM OF HEAVY RAIN IN MIXED CONVECTIVE-STRATIFORM CLOUD

Using the numerical model of mixed convective-stratiform clouds(MCS)in the paper(Hong1997)and the averaged stratification of torrential rain processes,the evolution processes,interaction of the two kinds of clouds,structure and the precipitation features in the MCS toproduce heavy rain are simulated and studied,and the physical reasons of producing torrential rainare analysed.The results indicate that the stratiform cloud surrounding the convective cloudbecomes weakened and dissipates in the developing and enhancing of the convective cloud,and therainfall rate and water content in the stratiform cloud increase as the distance from the convectivecloud becomes larger.The numerical experiments find out that the stratiform cloud provides abenificial developing environment for the convective cloud,i.e.,the saturated environment and theconvergence field in the stratiform cloud help to lengthen the life cycle of the convective cloud,produce sustained rainfall with high intensity and intermittent precipitation with ultra-highintensity.These and the ice phase microphysical processes are the main factors for the torrentialrain formation and the MCS is a very effective precipitation system.     

Aircraft Measurements of the Microphysical Properties of Stratiform Clouds with Embedded Convection

The presence of embedded convection in stratiform clouds strongly affects ice microphysical properties and precipitation formation. In situ aircraft measurements, including upward and downward spirals and horizontal penetrations, were performed within both embedded convective cells and stratiform regions of a mixedphase stratiform cloud system on 22 May 2017. Supercooled liquid water measurements, particle size distributions, and particle habits in different cloud regions were discussed with the intent of characterizing the riming process and determining how particle size distributions vary from convective to stratiform regions. Significant amounts of supercooled liquid water, with maxima up to 0.6 g m~(-3), were observed between -3℃ and-6℃ in the embedded convective cells while the peak liquid water content was generally less than 0.1 g m~(-3) in the stratiform regions.There are two distinct differences in particle size distributions between convective and stratiform regions.One difference is the significant shift toward larger particles from upper -15℃ to lower -10℃ in the convective region, with the maximum particle dimensions increasing from less than 6000 μm to over 1 cm. The particles larger than 1 cm at -10℃ are composed of dendrites and their aggregates. The other difference is the large concentrations of small particles(25–205 μm) at temperatures between -3℃ and-5℃ in the convective region, where rimed ice particles and needles coexist. Needle regions are observed from three of the five spirals, but only the cloud conditions within the convective region fit into the Hallett-Mossop criteria.     

INHOMOGENEOUS FEATURES IN ME1-YU FRONTAL CLOUD SYSTEM

The structure of radar echo in stratiform cloud which was found in mei-yu frontal cloud system is generally inhomogeneous, especially in the structure of bright band echoes. The inhomogeneous structure of warm region in stratiform cloud and the shower feature of precipitation are closely related to the inhomogeneous structure of bright band and convective cells embedded in stratiform cloud.During Summer time the mei-yu cloud system is an important precipitating system in the southern part of China. To study its structure is of great significance for weather forecast and understanding the physical processes of cloud and precipitation. Therefore, we have observed mei-yu frontal cloud system by use of 711 type radar (3 cm) and airplane at Tunxi, Anhui Province since 1979. It was found that the structure of stratiform cloud, especially the structure of its warm region appears to be inhomogeneous1),2). This is a significant feature of cloud structure in mei-yu frontal cloud system. In this paper, we shall fu     

MESOSCALE CONVECTIVE SYSTEMS DURING 20-23 JUNE 2002 REVEALED BY SATELLITE OBSERVATION

The 20-23 June 2002 mesoscale convective systems,which produced heavy rainfalls over the middle and lower reaches of the Changjiang River Basin,are studied using satellite imagery, satellite products and conventional sounding data. Results demonstrate that the torrential rain was caused by three MαCSs (Meso-a scale Convective System) and some MβCS (Meso-β-scale Convective System) activities in succession. The TBB (black-body temperature) analysis depicts that several meso-β-scale cloud clusters and convective cells were embedded and alternately developed within an MαCS.As the strongest convection gradually decreased,the cold TBB area expanded quickly in hours before dissipation. However,the heavy rainfall occurred in MαCS developing and maturing stages.And the minimum TBB fluctuation matched well to the precipitation trend with the lower TBB for the heavier rainfall. A kind of favorable synoptic environment for MαCS genesis and activities could be described as follows.The West Pacific subtropical high is stable with its western ridge reaching to the west region of South China.To its south and west sides,there is the robust ITCZ (Intertropical Convergence Zone),the active Indian-Bengal monsoon cloud surges carry warm and moist air by low-level jet (LLJ) to mid-latitudes where a frontal zone existed for days,meanwhile a 500 hPa short-wave trough moved eastward.At the upper troposphere,the southeasterly divergent flow dominates the environment due to the South-Asian high enhancing and moving eastward,and a monsoon water vapor plume (WVP) is stretched from the Bay of Bengal to the Changjiang River Basin.With a certain favorable configuration including a monsoon WVP,a frontal cloud system,a shortwave trough cloud system,and monsoon cloud clusters,MαCSs could initiate and develop successively in the same region.     

Differences between Convective and Stratiform Precipitation Budgets in a Torrential Rainfall Event

Differences in rainfall budgets between convective and stratiform regions of a torrential rainfall event were investigated using high-resolution simulation data produced by the Weather Research and Forecasting(WRF) model. The convective and stratiform regions were reasonably separated by the radar-based convective–stratiform partitioning method, and the threedimensional WRF-based precipitation equation combining water vapor and hydrometeor budgets was further used to analyze the rainfall budgets. The results showed that the magnitude of precipitation budget processes in the convective region was one order larger than that in the stratiform region. In convective/stratiform updraft regions, precipitation was mainly from the contribution of moisture-related processes, with a small negative contribution from cloud-related processes. In convective/stratiform downdraft regions, cloud-related processes played positive roles in precipitation, while moisture-related processes made a negative contribution. Moisture flux convergence played a dominant role in the moisture-related processes in convective or stratiform updraft regions, which was closely related to large-scale dynamics. Differences in cloud-related processes between convective and stratiform regions were more complex compared with those in moisture-related processes.Both liquid-and ice-phase microphysical processes were strong in convective/stratiform updraft regions, and ice-phase processes were dominant in convective/stratiform downdraft regions. There was strong net latent heating within almost the whole troposphere in updraft regions, especially in the convective updraft region, while the net latent heating(cooling) mainly existed above(below) the zero-layer in convective/stratiform downdraft regions.     

Adjoint Sensitivity Experiments of a Meso-β-scale Vortex in the Middle Reaches of the Yangtze River

A relatively independent and small-scale heavy rainfall event occurred to the south of a slow eastward-moving meso-α-scale vortex. The analysis shows that a meso-β-scale system is heavily responsible for the intense precipitation. An attempt to simulate it met with some failures. In view of its small scale, short lifetime and relatively sparse observations at the initial time, an adjoint model was used to examine the sensitivity of the meso-β-scale vortex simulation with respect to initial conditions. The adjoint sensitivity indicates how small perturbations of initial model variables anywhere in the model domain can influence the central vorticity of the vortex. The largest sensitivity for both the wind and temperature perturbation is located below 700 hPa, especially at the low level. The largest sensitivity for the water vapor perturbation is located below 500 hPa, especially at the middle and low levels. The horizontal adjoint sensitivity for all variables is mainly located toward the upper reaches of the Yangtze River with respect to the simulated meso-β-scale system in Hunan and Jiangxi provinces with strong locality. The sensitivity shows that warm cyclonic perturbations in the upper reaches can have a great effect on the development of the meso-β-scale vortex. Based on adjoint sensitivity, forward sensitivity experiments were conducted to identify factors influencing the development of the meso-β-scale vortex and to explore ways of improving the prediction. A realistic prediction was achieved by using adjoint sensitivity to modify the initial conditions and implanting a warm cyclone at the initial time in the upper reaches of the river with respect to the meso-β-scale vortex,as is commonly done in tropical cyclone prediction.     

Quantitative Analysis of Meso-β-scale Convective Cells and Anvil Clouds over North China

This paper proposes several quantitative characteristics to study convective systems using observations from Doppler weather radars and geostationary satellites. Specifically, in order to measure the convective intensity of each system, a new index, named the ``Convective Intensity Ratio' (CIR), is defined as the ratio between the area of strong radar echoes at the upper level and the size of the convective cell itself. Based on these quantitative characteristics, the evolution of convective cells, surface rainfall intensity, rainfall area and convectively generated anvil clouds can be studied, and the relationships between them can also be analyzed. After testing nine meso-β-scale convective systems over North China during 2006--2007, the results were as follows: (1) the CIR was highly correlated with surface rainfall intensity, and the correlation reached a maximum when the CIR led rainfall intensity by 6--30 mins. The maximum CIR could be at most ～30 mins before the maximum rainfall intensity. (2) Convective systems with larger maximum CIRs usually had colder cloud-tops. (3) The maximum area of anvil cloud appeared 0.5--1.5 h after rainfall intensity began to weaken. The maximum area of anvil cloud and the time lag between maximum rainfall intensity and the maximum area of anvil cloud both increased with the CIR.     

Numerical Simulations of Impacts of Urbanization on Heavy Rainfall in Beijing Using Di erent Land-Use Data

A summer strong convective precipitation event on 10 July 2004 over Beijing is numerically simulated in this paper, and the impact of urban heat island (UHI) on summer convective rain is investigated. The analysis reveals that a mesoscale convective cloud cluster system leads to this heavy rainfall event, suggesting the supply of moisture by the large scale circulation before the initiation of precipitation, a generally weaker UHI of 2-3℃ existed in the urban area. Much like a sea breeze, the anomalously warm urban air created relatively low pressure, inducing the inflow of cooler rural air towards the urban center, which is favorable to the ascending motion and the formation of convective precipitation over the urban area. In addition,the numerical simulation of the strong convective precipitation event suggests that the simulated result of precipitation using the 2002 LANDSAT-7 land-use data with 30-m resolution is much better than that using the 1992-1993 USGS land-use data with 1-km resolution, whether in the magnitude of rainfall or in the location of precipitation. The simulation confirms to some extent that the UHI has a significant role in causing extreme rainfall event.     

Adjoint Sensitivity Experiments of a Meso-β-scale Vortex in the Middle Reaches of the Yangtze River

A relatively independent and small-scale heavy rainfall event occurred to the south of a slow eastwardmoving meso-α-scale vortex. The analysis shows that a meso-β-scale system is heavily responsible for the intense precipitation. An attempt to simulate it met with some failures. In view of its small scale, short lifetime and relatively sparse observations at the initial time, an adjoint model was used to examine the sensitivity of the meso-β-scale vortex simulation with respect to initial conditions. The adjoint sensitivity indicates how small perturbations of initial model variables anywhere in the model domain can influence the central vorticity of the vortex. The largest sensitivity for both the wind and temperature perturbation is located below 700 hPa, especially at the low level. The largest sensitivity for the water vapor perturbation is located below 500 hPa, especially at the middle and low levels. The horizontal adjoint sensitivity for all variables is mainly located toward the upper reaches of the Yangtze River with respect to the simulated meso-β-scale system in Hunan and Jiangxi provinces with strong locality. The sensitivity shows that warm cyclonic perturbations in the upper reaches can have a great effect on the development of the meso-β-scale vortex. Based on adjoint sensitivity, forward sensitivity experiments were conducted to identify factors influencing the development of the meso-β-scale vortex and to explore ways of improving the prediction. A realistic prediction was achieved by using adjoint sensitivity to modify the initial conditions and implanting a warm cyclone at the initial time in the upper reaches of the river with respect to the meso-β-scale vortex, as is commonly done in tropical cyclone prediction.     

STRUCTURE OF MESO-β AND -γ-SCALE ON SOUTH CHINA HEAVY RAINFALL ON 12～13 JUNE 2005 USING DUAL-DOPPLER RADAR

The three-dimensional wind fields of the heavy rain on 12-13 June 2005 in Guangdong province are retrieved and studied with the volume scan data of the dual-Doppler radar located in the cities of Meizhou and Shantou. It is shown that the meso-β-scale and meso-γ-scale convergence lines located in the convective system at the low and middle layer play an important role in the heavy rainfall. The convergence line is the initiating and maintaining mechanism of the rain. A three dimensional kinematic structure model is also given.     

In-Situ Measurements of Cloud-Precipitation Microphysics in the East Asian Monsoon Region Since 1960

A large number of in-situ measurements of cloud-precipitation microphysical properties have been made since 1960, including measurements of particle size distribution, particle concentration, and liquid water content of clouds and rain. These measurements have contributed to considerable progress in understanding microphysical processes in clouds and precipitation and significant improvements in parameterizations of cloud microphysics in numerical models. This work reviews key findings regarding cloud-precipitation microphysics over China. The total number concentrations of various particles vary significantly, with certain characteristic spatial scales. The size distributions of cloud droplets in stratiform clouds can generally be fit with gamma distributions, but the fit parameters cover a wide range. Raindrop size distributions(RSDs)associated with stratiform clouds can be fit with either exponential or gamma distributions, while RSDs associated with convective or mixed stratiform-cumuliform clouds are best fit with gamma distributions.Concentrations of ice nuclei(IN) over China are higher than those observed over other regions, and increase exponentially as temperature decreases. The particle size distributions of ice crystals, snow crystals, and hailstones sampled at a variety of locations can be reliably approximated by using exponential distributions,while aerosol particle size distributions are best described as the sum of a modified gamma distribution and a Junge power-law distribution. These results are helpful for evaluating and improving the fidelity of physical processes and hydrometeor fields simulated by microphysical parameterizations. The comprehensive summary and analysis of previous work presented here also provide useful guidelines for the design of future observational programs.     

Variations of Precipitation Structure and Microwave Tbs During the Evolution of a Hailstorm from TRMM Observations

In this paper, a hailstorm occurring on 9 May 1999 in Huanghuai region was studied by using the combined data from the precipitation radar (PR), microwave image (TMI), and visible infrared scanner (VIRS) on the Tropical Rainfall Measuring Mission (TRMM) satellite. According to the 3-orbit observations of 5-h duration from the TRMM satellite, the variation characteristics of the precipitation structures as well as cloud top temperature and microwave signals of the precipitating cloud were comprehensively analyzed during the evolution of hailstorm. The results show that the precipitation is obviously converted from early hail cloud with strong convection into the later storm cloud with weak convection. For hail cloud, there exists some strong convective cells, and the heavy solid precipitation is shown at the middle-top levels so that the contribution of rainfall amount above the freezing-layer to the column precipitation amount is rather larger than that within the melting-layer. However, for storm cloud, the convective cells are surrounded by the large area of stratiform precipitation, and the precipitation thickness gradually decreases, and the rainfall above the freezing-layer obviously reduces and the contribution of rainfall amount within the melting-layer rapidly increases. Therefore, the larger ratio of rainfall amount above the freezing layer to column precipitation amount is, the more convective the cloud is; reversely, the larger proportion of rainfall below the melting layer is, the more stable the stratiform cloud is. The different changing trends of microwave signals at different precipitation stages show that it is better to consider the structures and stages of precipitating cloud to choose the optimal microwave channels to retrieve surface rainfall.     

Kinematics and Microphysical Characteristics of the First Intense Rainfall Convective Storm Observed by Jiangsu Polarimetric Radar Network

The polarimetric radar network in Jiangsu Province has just been operationalized since 2020. The first intense precipitation event observed by this polarimetric radar network and disdrometer occurred during August 28-29, 2020 and caused severe flooding and serious damage in eastern Jiangsu Province. The microphysics and kinetics for this heavy precipitation convective storm is diagnosed in this study, in order to promote the application of this polarimetric radar network. Drop size distribution (DSD) of this event is estimated from measurements of a ground disdrometer, and the corresponding three-dimensional atmospheric microphysical features are obtained from the multiple polarimetric radars. According to features of updraft and lighting, the evolution of the convective storm is divided into four stages: developing, mature with lightning, mature without lightning and dissipating. The DSD of this event is featured by a large number of raindrops and a considerable number of large raindrops. The microphysical characteristics are similar to those of warm-rain process, and ice-phase microphysical processes are active in the mature stages. The composite vertical structure of the convective storm indicates that deep ZDR and KDP columns coincide with strong updrafts during both mature stages. The hierarchical microphysical structure retrieved by the Hydrometeor Identification Algorithm (HID) shows that depositional growth has occurred above the melting level, and aggregation is the most widespread ice-phase process at the -10℃ level or higher. During negative lightning activity, the presence of strongest updrafts and a large amount of ice-phase graupel by riming between the 0℃ and -35℃ layers generate strong negative electric fields within the cloud. These convective storms are typical warm clouds with very high precipitation efficiency, which cause high concentration of raindrops, especially the presence of large raindrops within a short period of time. The ice-phase microphysical processes above the melting layer also play an important role in the triggering and enhancing of precipitation.     

OBSERVATIONAL STUDY OF MICROPHYSICS IN THE STRATIFORM REGION AND TRANSITION REGION OF A MIDLATITUDE MESOSCALE CONVECTIVE COMPLEX

Cloud microphysical data observed with PMS probes have been combined with radar and other in-situdata collected by a NOAA P-3 aircraft that flew through the stratiform and transition regions of a mesoscaleconvective complex(MCC).The combined data have been analyzed with respect to the mescscale structureof the storm systems.The characteristics of ice particles in the transition and stratiform regions were quitediffereat.The ice particle concentrations in the transition region were about 4 to 6 times that found in thestratiform region,and the size of ice particles in the stratiform region was about twice that in the transitionregion.The relatively lower radar reflectivity in the transition region is a result of smaller particle sizes.Themain precipitation particle growth mechanisms are riming and aggregation in the transition region ard theaggregation process predominates in the stratiform region referred from the microphysical structures.The ag-gregation starts in the upper,colder lev(?)ls but becomes more efficient as the particles approach the melting layer.     

层状云微观结构模型的观测和模拟初证

The microphysical ＂three-layer＂ model for stratiform clouds over a midlatitude location in Northwest China is investigated by combining in situ airborne Particle Measuring Systems, Inc. （PMS）, radar measurements, and the NCAR/Penn State Mesoscale Model Version 5 （MM5） simulation with a two-moment microphysics scheme. The coexistence of measured supercooled liquid water and small ice particles produces snow particles below the cloud top in the second layer. Peak number concentration and mean diameter of cloud water and raindrop appear in the third warm layer. A thin dry layer just below the melting layer is also observed. The predicted precipitation is tested by equitable threat score. The melting layer is clearly defined in the radar image and model radar reflectivity output is agreement with the observations. The model results provide features of the microphysical structure for every layer of ＂three-layer＂ model at Yan＇an station. For both observation and model simulation, the ＂three-layer＂ model explains the stratiform precipitation formation completely and comprehensively.     

Convective and Stratiform Cloud Rainfall Estimation from Geostationary Satellite Data

The Bayes Decision (BD) method was used to distinguish the corrective and stratiform components of cloud sys-tems from GMS-4 satellite data. A technique originally developed by Adler and Negri (1988, hereafter abbreviated AN) was improved for estimating the convective and stratiform cloud precipitation areas and rates of cloud systems from GMS satellite imagery. It has been applied to a tropical cyclonic cloud cluster observed over east coast area of China on September 23, 1992, which brought about flood disaster in that region. Overlaid 6-hour surface rainfall ob-servations show that the rainfall areas and amounts match with results from improved AN technique. The successful application of the Adler and Negri’s technique to convective and stratiform clouds provides encouragement for the use of this method over large region of mid-latitude China where radar data are not fully covered.     

A NUMERICAL MODEL OF MIXED CONVECTIVESTRATIFORM CLOUD

A 2-D slab-symmetric model of mixed convective-stratiform cloud is developed bysuperimposing convective cloud-size field on the convergence field,in order to simulate and studythe mixed clouds consisting of stratiform cloud and convective cloud.A deep convective,anelasticand conservative system of equations with basic variables(V,θ,π')is solved by a new method tocalculate dynamic field.The water substance in the cloud is divided into 6 categories and themicrophysical processes are described in spectrum with two variable parameters and morereasonable particle number/size distributions.To compare with measured radar echo intensity andstructure,the model may calculate echo intensity of the model cloud observed by radar.     

An Analysis of Thermally-Related Surface Rainfall Budgets Associated with Convective and Stratiform Rainfall

Both water vapor and heat processes play key roles in producing surface rainfall.While the water vapor effects of sea surface temperature and cloud radiative and microphysical processes on surface rainfall have been investigated in previous studies,the thermal effects on rainfall are analyzed in this study using a series of two-dimensional equilibrium cloud-resolving model experiments forced by zonally-uniform,constant,large-scale zonal wind and zero large-scale vertical velocity.The analysis of thermally-related surface rainfall budget reveals that the model domain mean surface rain rate is primarily associated with the mean infrared cooling rate.Convective rainfall and transport of hydrometeor concentration from convective regions to raining stratiform regions corresponds to the heat divergence over convective regions,whereas stratiform rainfall corresponds to the transport of hydrometeor concentration from convective regions and heat divergence over raining stratiform regions.The heat divergence over convective regions is mainly balanced by the heat convergence over rainfall-free regions,which is,in turn,offset by the radiative cooling over rainfall-free regions.The sensitivity experiments of rainfall to the effects of sea surface temperature and cloud radiative and microphysical processes show that the sea surface temperature and cloud processes affect convective rainfall through the changes in infrared cooling rate over rainfall-free regions and transport rate of heat from convective regions to rainfall-free regions.     

NUMERICAL SIMULATION OF CLOUD MICROPHYSICAL CHARACTERISTICS OF LANDFALL TYPHOON KROSA

In this study,the super typhoon KROSA(2007)was simulated using a mesoscale numerical model Global and Regional Assimilation and Prediction System(GRAPES)with a two-moment mixed-phase microphysics scheme.Local rainfall observations,radar and satellite data were also used to analyze the precipitation structure and microphysical features.It was shown that low-level jets and unstable temperature stratification provided this precipitation process with favorable weather condition.Heavy rainfall centers were located in the north and east part of KROSA with the maxima of 6-hourly total rainfall during the simulation more than 100 mm.The quantities of column solid water and column liquid water were generally equivalent,indicating the important role of ice phase in precipitation formation.Results of CloudSat showed that strong convection occurred in the eyewall around the cyclonic center.According to the simulation results,heavy precipitation in the northeast part of the typhoon was mainly triggered by convective clouds,accompanied by the strongest updraft under the melting level.In the southwest part of KROSA,precipitation intensity was rather homogeneous.The ascending center occurred in high-level cold clouds,favoring the formation and growth of ice particles.     

Numerical Simulation of Torrential Rainfall and Vortical Hot Towers in a Midlatitude Mesoscale Convective System

A cloud-resolving model simulation of a mesoscale convective system （MCS） producing torrential rainfall is performed with the finest horizontal resolution of 444 m. It is shown that the model reproduces the observed MCS, including its rainfall distribution and amounts, as well as the timing and location of leading rainbands and trailing stratiform clouds. Results show that discrete convective hot towers, shown in Vis5D at a scale of 2-5 kin, are triggered by evaporatively driven cold outflows converging with the high-θe air ahead. Then, they move rearward, with respect to the leading rainbands, to form stratiform clouds. These convective towers generate vortical tubes of opposite signs, with more intense cyclonic vorticity occurring in the leading convergence zone. The results appear to have important implications for the improvement of summertime quantitative precipitation forecasts and the understanding of vortical hot towers, as well midlevel mesoscale convective vortices.