| Atmospheric convection has a strong impact on the earth’s atmospheric system,which will lead to the exchange of heat,momentum and water vapor between the lower troposphere and the upper troposphere,or even between the troposphere and the stratosphere.Extremely strong,fast and disaster prone are the main characteristics of atmospheric convection activities,especially strong convection activities often bring strong disaster weather such as instantaneous gale,hail,tornado,thunderstorm and short-term heavy precipitation.Therefore,the full expression of atmospheric convection activity process is one of the key problems of the prediction performance of the weather and climate numerical model.The spatial change of the land surface process in the transitional zone affected by summer monsoon was remarkable,and the local thermal and dynamic effects were extremely uneven.In addition,due to the combined action of other factors such as topography,the particular environmental conditions could easily stimulate convection activities.The strong heterogeneity of convection activities and their precipitation made this region one of the most seriously affected areas by weather and climate disasters such as drought and strong convection in China.Moreover,the climatic characteristics of convection activities in this region were quite different from those in other climatic regions.At present,the research on convection activities in this region was mostly aimed at typical weather cases and a certain type of typical strong convective weather.However,most studies of convection activities focused on the effects of atmospheric circulation factors,the effects of land-air interactions were often overlooked.In the existing numerical models,most land surface parameters were fixed values,which could not reflect the real changes in land surface conditions,and this simple assumption of the initial state of land surface characteristics often leaded to large errors.In view of this,improving the understanding of the fine climate characteristics of convective activities in the transition zone affected by summer monsoon were urgent needed,exploring the influence of land-air interaction on the initiation and development of convection,and obtaining the driving mechanism of local mesoscale circulation on convective activities,so as to provide important reference for improving the accuracy of convective activities prediction in this area.This paper focused on the characteristics of convective activity in the transition zone affected by summer monsoon and the influence mechanism of soil moisture on it.Firstly,the applicability of non-soil moisture products in northern China was evaluated,and the temporal and spatial distribution characteristics of warm-season convective activity（deep convective activity）in different climate zones in northern China was analyzed by using long-time series of static satellite observation data.Secondly,based on the ground-based radar observation data,the temporal and spatial distribution and propagation characteristics of the typical regional convection system and convection initiation in the transition zone affected by summer monsoon were obtained.Then,two kinds of products with good applicability,GLDAS and CLDAS,were selected to discuss the influence law of soil moisture on convection initiation.Finally,based on CMA-MESO model,a batch of simulated return experiments were carried out to reveal the influence mechanism of soil moisture changed on local convection activities.The main conclusions were as follows:（1）The applicability characteristics of soil moisture products in different climate zones in northern China is obtained.The correlation between CRA40 and observed values was the best,ERA5 was better for dry center simulation,ERA5-Land was better for wet center simulation,GLDAS-Noah was better for wet center simulation of soil moisture,CLDAS was mainly for wet soil areas,and NCEP-DOE R2 and GLDAS-CLSM were worse.CRA40,ERA5,ERA5-Land,NCEP-DOE R2 and GLDAS-Noah were all simulated positive deviations in all seasons,and CRA40,ERA5-Land and ERA5-Land in summer and autumn had good correlation with the observed values,and the correlation between the simulated winter soil moisture and the observed values of different products was the smallest in the whole year.Different products were mainly used to simulate the soil moisture in arid areas,and GLDAS-Noah had the best ability to describe the change trend of soil moisture.GLDAS-Noah,CRA40 and ERA5 had good applicability in monsoon areas,and most products could simulate the emergence time of dry soil and wet soil in the transition zone affected by summer monsoon,while GLDAS-Noah and CRA40 had better simulation effects.（2）The climate characteristics and main influencing factors of warm season convective activity in northern China were revealed.The slope area of Qinghai-Tibet Plateau,Northeast China and North China Plain were active areas of convection and deep convection in northern China,while the Gobi and desert areas in arid areas in northwest China were inactive areas,and the interannual changes fluctuate significantly.The frequency of convective activity and deep convective activity was the highest in May,and the lowest in September.Convective activity gradually increased from late May to mid-June and develops northward,and then develops southward in mid-July.Convective activity in Qinghai-Tibet Plateau and the slope area was more active in mid-May and late July,with obvious intermittent characteristics,and the seasonal variation characteristics of convective activity in Northeast China were more obvious.The diurnal fluctuation of convective activity in the arid area of western China was small,and the peak value generally appears from evening to the first half of the night.The frequency of convective activity in the eastern monsoon area was obviously higher than that in other climate areas,and the diurnal variation of convective activity in the transition area affected by summer monsoon was quite different.The main influencing factors of convective activities were different in different climate zones,but the subtropical high in the western Pacific,the polar vortex in the northern hemisphere and the Qinghai-Tibet Plateau had strong influences on convective activities in all regions.（3）The refined distribution and propagation characteristics of warm season convective activities in the transition zone affected by the northern summer monsoon in northern China were clarified.The distribution of warm-season convection activities in the hilly region of the Loess Plateau in central Gansu was mostly related to topography.Deep convection was mainly distributed in the northern slope of Taizi Montain and the eastern part of Dingxi City,while shallow convection was distributed in the middle of the study area and Wushaoling Montain.Convection activities were highly occurred in July,and there was little difference between June and August.An obvious diurnal variation of convective activity existed,which was mostly highly occurred in the afternoon,and less in the in the early hours of the morning.The diurnal variation was mainly influenced by radiation heating and topographic uplift,and the convective activity from night to early morning was mainly dominated by synoptic-scale dynamic forcing conditions.In convective storms,"convective bubbles"with a life history of less than 36 minutes accounted for the largest proportion,while large-scale and systematic convective activities accounted for a relatively small proportion.The area and moving distance of convective storms had a significant linear relationship with the duration.In June,it moved eastward and southward,and in July,it moved northward to southeast.In August,the convective storm mainly moved westward and northward.Up to 70.6%of convective storms move slower than the rate of 30 km·h-1.（4）The response of convective initiation to soil moisture in the transition zone affected by the summer monsoon in northern China is discovered.The frequency of convective initiation in warm season in hilly region of Loess Plateau in central Gansu was the least in June and the most in August,and topography was an important factor leading to convective initiation.The convective initiation diurnal variation was a single peak structure,with the high incidence period from 13:00 to 19:00,and the peak value appeared around 15:00.The daily change in July was more intense,and less fluctuating in August.Under the forced condition of weak weather system,more convective bubbles generated in the hilly area of Loess Plateau in central Gansu,and convective initiation occurred most frequently under strong weather system forcing conditions.The frequency of early afternoon was the highest in all weather system forcing types,and the frequency of convective initiation was lowest in strong weather system forcing conditions.More than 90%of convective initiation occurred in the region with soil moisture of 0.15～0.35 m3·m-3,and the region with soil moisture of 0.20～0.30 m3·m-3was the area with high incidence of convective initiation.The soil moisture corresponding to the maximum frequency of convective initiation would gradually decreased with the increase of weather system forcing.Under different weather forcing conditions,the maximum frequency of convective initiation was concentrated in the region where the soil moisture gradient was less than 0.001 m3·m-3·km-1,and the occurrence frequency decreased with the increase of the corresponding soil moisture gradient.Convection initiation mostly occurred in the transition zone from wet soil in the north to dry soil in the south,which was obviously wetter than the surrounding soil.However,under different weather forcing conditions,the average soil moisture at the location of convection initiation decreased with the strengthening of the weather system.The region with a smaller gradient along the latitude was the transition zone from positive gradient in the east to negative gradient in the west,and the convection initiation occurred in the region with a significant negative gradient from the south to positive gradient in the middle along the longitude.The formation of convective activities under weak weather forcing conditions depended on the positive and abnormal time variation of soil moisture,while the influence of weather system on convective initiation was dominant under strong or strong weather forcing conditions.（5）The influence mechanism of soil moisture change on local atmospheric convection activity and precipitation is clarified.A positive feedback on the accumulated precipitation by soil moisture was existed,which had little effect on the accumulated precipitation at the initial stage of simulation.The positive feedback of soil moisture on precipitation gradually strengthened with the passage of time,and the feedback intensity weaken with the forcing of weather system.The influence of soil moisture on the combined reflectivity was nonlinear.Under weak and strong weather system forcing conditions,the increase of soil moisture often leaded to the increase of the reflectance factor,but the decrease of soil moisture also leaded to the increase of the reflectance factor in most areas,resulting the increase of the spatial gradient of the reflectance factor.Under strong weather system forcing conditions,soil moisture changes had little effect on combined reflectance.A positive feedback on the frequency of convective activities caused by soil moisture,but the feedback intensity getting weak with the weather system forcing.Soil moisture had negative feedback on surface temperature,2 m altitude temperature and surface heat flux,and the feedback intensity increased with the increase of solar radiation.Most Bowen ratios also showed negative feedback,but the feedback intensity and influence period were quite different under different weather forcing conditions.The 700 h Pa specific humidity was positively correlated with soil humidity in most periods,and the feedback intensity was enhanced with the increase of solar radiation,but the influence of soil humidity on 700 h Pa specific humidity was negatively correlated from morning to noon under the forced conditions of strong weather system.The increase of soil humidity leaded to the strong humidification in the lower layer,the weak humidification in the middle layer and the enhanced ascending motion in the lower layer,and it also leaded to the increase（decrease）of the humidity enthalpy in the boundary layer.There was a consistent and significant negative correlation between soil moisture and Lifting Condensation Level（LCL）.With the strengthening of solar radiation,the negative feedback intensity increased,and the decrease of soil moisture leaded to an abnormally high LCL,which was not conducive to the triggering of convection activities.The effect of negative feedback on the level of free convection（LFC）was the most significant under weak weather system forcing.Convective available potential energy（CAPE）was significantly positively correlated with soil moisture,and the maximum CAPE increased with the weather system forcing strengthening,and convection inhibition（CIN）was positively correlated with soil moisture. |
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