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 forward-tilting 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 to 14:00 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.