河套地区典型干线的形成及其在对流触发中的作用

利用高空、加密地面、EC-ERA5（0.25o×0.25o）再分析和FY-2系列静止气象卫星云图（可见光星下点分辨率1.25 km，红外5 km）等资料，对黄河河套地区3次典型干线的形成及其在对流触发中的作用进行了详细分析。结果表明:（1）3次干线触发对流出现在中高纬度高空槽东移诱发蒙古气旋发展的背景下，高低空系统配置为前倾槽、高空西北气流及其携带的冷平流叠加在低层河套暖区之上，为大范围强对流天气提供了有利的环境条件；（2）3次干线均为蒙古气旋形势下黄土高原西高东低缓坡地形特有的现象，具有明显的地域特征，长600—800 km、宽80—100 km的显著干线呈北东北—南西南走向，与河套区域内海拔1300 m等高线走向基本吻合。受近于干绝热的下沉升温及高原西部下垫面非绝热加热快速升温降湿影响，在河套西部形成干热空气，其与东部暖湿气团在河套地区交汇是导致干线形成的主要原因之一；（3）干线具有明显的日变化特征，白天干线以西加热升温快，干线向东移动；夜间到凌晨干线西侧辐射降温快于东侧，干线向西后退。干线最强时段出现在14时（北京时，下同）前后，两侧露点梯度达10℃/（100 km）或以上，且伴有明显的偏西风和偏南风汇合（辐合）流场；（4）在干线及伴随的汇合（辐合）流场的作用下， 13—14时初始对流在干线附近生成，随后在干线湿侧附近加强，形成明显的线状对流云带，在高空偏西气流引导下，对流云带东移发展并逐渐远离干线，在河套东部有利的环境条件下，线状对流云带继续东移并扩展至地面干线以东约500 km的范围内，导致陕西中北部和华北部分地区出现大范围雷暴大风、局地冰雹甚至龙卷等强对流天气。根据河套地区3次典型干线的共同特征，给出了河套地区干线形成和强对流易发区的天气学模型，为今后同类天气形势下干线触发对流天气分析和预报提供参考和借鉴。 

The formation of typical dry lines in Hetao and its role in triggering convection

Not many researches have been conducted on typical cases of convection triggered by dry lines in the Hetao region of China.The formation of three typical dry lines and their roles in triggering convection in the Hetao region of Yellow River are analyzed in detail based on upper air soundings,intensive surface observations,EC-ERA5(0.25o×0.25o)reanalysis data and FY-2 meteorological satellite images(the resolution of subsatellite point in the visible images is 1.25 km while that in the infrared images is 5 km).The results are as follows.(1)The three cases occurred under the background of the development of the Mongolia cyclone,which was induced by the eastward movement of the upper trough in the middle and upper troposphere.The structure of the forwardtilting trough indicates that the upper-level northwesterly flows and cold advection were superimposed on the lower-level warm zone,providing a favorable environmental condition for the occurrence of severe convection over large areas.(2)All the three cases occurred along the gentle slope of the Loess Plateau,which is high in the west and low in the east,and under the condition of the Mongolia cyclone development,presenting significant regional characteristics.Besides,the dry lines with a length of about 600-800 km and a width of about 80-100 km were oriented along north-northeast to south-southwest direction and coincided with the orientation of the 1300 m contour line in the Hetao region.Furthermore,the main cause for dry lines generation is the effect of the dry-warm air produced by the near adiabatic descending warming and the rapid diabatic warming and dehumidification in the western Hetao area.(3)The dry lines show obvious diurnal characteristics.The western part of the dry line warmed up rapidly in the daytime and the dry line moved eastward,while it cooled off faster than the eastern part from the nighttime to early morning and the dry line retreated westward.The dry lines present their most marked features around 14:00 BT with the dew point temperature difference between wet and dry sides reaching 10℃/(100 km)or more with an obvious convergent flow field of the westerly and the southerly wind.(4)Due to impact of the dry line and its associated convergent flow field,the initial convection was generated near the dry line from 13:00 to14:00 BT in the afternoon,which were then reinforced on both sides of the dry line and the linear convective band formed.The convective band moved eastward and gradually stayed away from the dry lines under the steering of upper-level westerly airflows and continued to move eastward,expanding to a range of about 500 km to the east of the ground drylines under favorable environmental conditions in the eastern Hetao region.Severe convections such as large-scale thunderstorms,strong winds,local hails and even tornadoes subsequently developed over central and northern Shannxi province and some areas in North China.Finally,the synoptic conceptual model of the generation of typical dry lines in Hetao and the areas prone to severe convection are summarized according to the common characteristics of the three typical dry lines in Hetao area.This study provides a reference for analysis and forecast of dry lines that could trigger convections in the similar situation.