The Characteristics And The Forecast Evalution Of Mesoscale System Precipitation Over North China

The precipitation characteristics of the Mesoscale Convective System(MCS)over North China in the warm season(May-October)are complex.Studying the spatiotemporal distribution and different evolution processes of MCS precipitation and revealing the forecast ability of Convection-Permitting Model(CPM)in MCS precipitation can provide important references for understanding the weather and climate characteristics in North China and the improvement of the prediction in heavy precipitation.In this paper,based on the objective method and the satellite precipitation products,the MCS rain cell in the warm season of North China is identified from 2015 to 2020.The spatio-temporal distribution of MCS precipitation,the characteristics,and life evolution of short-and long-lived MCS in different seasons are analyzed.Aiming at MCSs in mid-summer(July and August),combined with the Black Body Temperature(TBB),MCSs are divided into two types: initialize and end over local mountains(LM),and initialized over mountains but propagate to plain(MP).Then the evolution of convection precipitation and environmental conditions of the two-type MCS are revealed.Based on the observational analysis,the forecast ability of CMA＿Meso(3km)operational models for the MCS precipitation characteristics and different MCS processes in mid-summer from 2020 to 2021 are evaluated.The main conclusions of the paper are as follows:(1)The spatio-temporal distributions and characteristics of MCS precipitation and their seasonal variations are revealed.MCSs most frequently initiate on the eastern slope of Taihang(TH)mountain.Notable MCS precipitation centers are located around mountain slopes and downstream areas.MCSs In July and August occur southward compared to other months,with the precipitation center expanding towards the plain and MCS precipitation contributes the most to the total precipitation(～70%).The diurnal peak time of MCS rainfall manifested as a southeastern delay from mountainous to downstream plains has seasonal variations.In July and August,the peaks of MCS precipitation to the east of 114°E are generally later than those in May and June.In terms of precipitation characteristics,MCSs in July and August have comparable rainfall area and moving speed to those in May and June but feature the strongest precipitation intensity.MCSs in September and October have the largest size and the fastest speed but with the weakest intensity.Long-lived MCSs have stronger precipitation intensity,larger rainfall area,and faster speed than short-lived MCSs.MCS has asymmetrical life evolution,which is most obvious in July and August.Long-lived MCSs occur with more pronounced upper troposphere warming and lowlevel anomalous southwesterly.Meanwhile,higher specific humidity in the low-and mid-level troposphere provides a more favorable environment for the maintenance of MCS precipitation.(2)The different cloud and precipitation quantities and the circulation condition of LM type and MP type MCSs when propagating over terrain are analyzed.Afternoon-evening and midnight-morning are two high-frequency periods of MCS generation in North China.Compared with LM MCSs,MP MCSs feature a longer lifetime,lower TBB,and intenser precipitation.For LM and MP type MCSs initiated from afternoon to evening,the TBB evolution trends are obviously different,while for those initiated from midnight to early morning,their trends are relatively consistent.For MP type MCSs initiated in different periods,there are obvious differences in the precipitation center,the shape of rainfall area,and the moving speed.The increase of precipitation intensity when moving eastward to the hillside and the disappearing speed of the system after crossing the mountain also show differences.Comparing the synthesis circulation anomaly fields of different types of MCS precipitation,it is found that when MCS occurs in the afternoon and evening,the lower troposphere is warm with high CAPE over plains,and the middle and upper troposphere are cold low-pressure anomalies.For LM and MP type MCSs,there are differences in the location and intensity of cold low-pressure anomalies,and the anomalous wind directions throughout the troposphere are different;When MCS occurs from midnight to early morning,there are warm high-pressure anomalies in the middle-upper troposphere with anomalous divergence.The anomalous southwest wind in the lower troposphere converges in North China,which causes deeper wet anomalies.There are significant differences in the intensity of warm high-pressure anomalies,the magnitude of wind speed and shear in the lower troposphere,and the location of the center of specific humidity anomalies,which affect the maintenance and development of MCS(3)The forecast ability of operational convection-permitting CMA＿Meso model for the MCS precipitation characteristics and different MCS processes in summer are evaluated.CMA＿Meso can reproduce the southwest-northeast rain belt east of 112°E.Compared with GPM,the model overestimates the MCS precipitation on the eastern slope of the TH mountain but underestimates the MCS precipitation and frequency over the downstream plains.The model can reproduce the afternoon peak of MCS precipitation in the mountainous areas and the early morning peak in the southeast of the NCP,but the evening precipitation on the slope and the midnight precipitation at the mountain foot are significantly underestimated.CMA＿Meso can reproduce the movement speed of MCSs but underestimated the rainfall area.The evaluation based on the evolution characteristics of MCSs indicates that the model can reproduce that the precipitation intensity peak occurs ahead of the largest area,and the life evolution of mass water carrying can be forecasted accurately.Comparing the forecast of different types MCSs show that the prediction of MP MCSs has the bias that the peak time of precipitation intensity and area in the lifetime is later,and the decay rate is faster after peaking.In addition,the model has a better ability to depict the precipitation intensity differences at different terrain heights than that of rainfall sizes.