Analysis On Two Typical Severe Preciptation Cases In Northeast Of China

Very short-range severe precipitation is a key and difficult point for weather forecasting because of the short duration,strong convection and burstiness as well as the serious secondary disasters.With the rapid development of observing technology and increase of the observation sites,the quality and the resolution of observational data have been greatly improved,which makes it possible to analyze the formation and development of the mesoscale convective system.The study mainly focuses on two typical very short-range severe precipitation cases,which are ‘2010.7' and ‘2013.8'.Firstly,based on the conventional and unconventional observation data,comparative analyzed of two cases are conducted including the characteristics of rainfall distribution,large-scale circulation associated with mesoscale features.Further,the mesoscale diagnostic analysis including the difference in the environmental conditions,triggering mechanisms,dynamic and thermodynamic characteristics of the mesoscale convective systems in these two cases are also performed by using the high temporal and spatial resolution simulation data from the mesoscal Weather Research Forecasting Modeling System(WRF).The main conclusions are summarized as follows:1)There are obvious differences between these two cases in the temporal and spatial distribution characteristics.The location of precipitation in case ‘2010.7' distributes in northeast-southwest direction,the rain belt concentrates and rainfall process mainly occurs at night.However,for case ‘2013.8',it indicates several strong rainfall centers and the rainfall mainly occurrs at daytime.The feature of hourly precipitation variation in two rainstorm cases implies there are some differences in the evolution of mesoscale systems in two cases.2)The large-scale circulation presents similarities between these two heavy rainfall processes.High-level jet appears at high level of troposphere,and low-level jet and low-level shear occurs at low level of troposphere.However,there are some differences in the development and evolution of high and low jets,especially for the circulation at 500 hPa,which is one of the reasons for the difference in precipitation area and intensity between two cases.3)The mesoscale characteristics of short-term heavy rainfall are also different under different large-scale environmental backgrounds.As the two processes are affected by different mesoscale convective cloud clusters,the radar echoes are different in intensity and structure at each stage.Firstly,for case ‘2013.8',the precipitational process is mainly affected by the M?CS system generated in local.The strong convergence of the wind velocity leads to the generation and sustention of M?CS.Instead,in case ‘2010.7',the process is affected by the belt mesoscale convective system with ‘train effect'.Secondly,both of the first stages for these two processes generate organized linear convection;the second stages generate flocculent convective echoes strengthened by shear or topographical influence.Although the intensity is not strong,the precipitation efficiency is higher.In addition,the weakening of MCS in case ‘2013.8' corresponds to an increase of positive flash frequency but decrease of negative flash frequency.4)The numerical simulation indicates: a)The simulated results are consistent with the observation.In addition,the model simulation with enhanced resolution can improve the hourly rainfall forecast.b)Although there are some differences in the energy front of the two heavy precipitational processes,the strong precipitation of both cases occurred in the warm region of energy front.c)Similarly,the trigger mechanism of meso-small scale convective system in these two cases is frontal gland lifting in front of low-level jet,but that is different at left pant of low-level jet.In case ‘2013.8',the trigger mechanism of meso-small scale convective system is the enhancement of cold advection which leads to an increase of the temperature advection difference.Instead,in case ‘2010.7',the trigger mechanism relates to the small-scale convergence,which is pulse of low-level jet.What's more,the secondary circulation promotes the development of the convective system.d)The moisture convergence is related to the angle between jet and shear rather than precipitation intensity.