Statistical analysis of drylines in Northeast China

Based on conventional surface observations, soundings, satellite and radar data, temporal and spatial distributions and meteorological elements of the drylines in the warm season (May—August) in Northeast China (40°—53°N, 115°—135°E) during 2003—2017 are analyzed. The research shows that the drylines in Northeast China are mainly located in the Northeast China Plain and western Liaoning province, and the occurrence frequency of drylines is higher in the south than in the north. Most of the drylines are oriented along the southwest—northeast direction, with a width of 90—120 km and a length of 100—800 km. The annual average occurrence frequency of drylines in the warm season in Northeast China is 15.5%. The frequency of dryline occurrence shows no obvious interannual change, but the annual change is significant and presents a sinusoidal curve, with the peaks of more than 20% occurring from mid-late May to late June and mid-late August and the bottom of less than 10% occurring in July. The air pressure on wet side of the dryline is slightly higher than that on the dry side by 1 hPa, and the temperatures on both sides are mostly above 24℃. The temperature difference between the two sides is generally between 1 and 4℃. The dew point gradient and specific humidity gradient ranges are 9.6—15℃/(100 km) and 4.5—8.3 g/(kg·100 km), respectively. The equivalent potential temperature gradient is about 9.6—19 K/(100 km). The time series of the above the elements and their gradient values on both sides of the dryline change significantly. The temperature and humidity on both sides and the gradient of humidity increase from May to August, while the absolute value of temperature gradient decreases. Among the total 286 drylines, 40% of the drylines triggered convection. In the middle and early June, the percentage of the drylines that can trigger convection is the highest, reaching up to more than 60%. Whether the dryline can trigger convection has little to do with its location. The humidity gradient of convective drylines is slightly larger than that of non-convective drylines. The most significant difference of sounding elements between non-convective drylines and convective drylines lies in the value of CAPE on the wet side. The former is below 1200 J/kg on the wet side, while the latter is above 1200 J/kg on the wet side, with the highest value reaching about 3000 J/kg.