Environmental Conditions of Three Squall Lines in the North Part of Zhejiang Province

Based on radar data and intensive surface observations, combined with JMA reanalysis data, a typical case with three consecutive squall lines is analyzed, which occurred in the north part of Zhejiang Province on 2 July 2008. Corresponding atmospheric conditions are investigated in detail. Special emphasis is given on the relationship between convection current and underlying surface state such as temperature, humidity and convergence of wind, especially the impact of land-sea boundary on rebirth and strengthening of convection current. Besides, the forming processes of the third typical bow echo, including development, attenuation, effluent, inflow, rebirth and dissemination of convection cell are studied in horizontal and vertical direction. After that, favorable conditions for all evolutionary stages and interrelation of three squall lines are summarized.It shows that there are specific places where newly-born convections and convection reinforcements are likely to be found, such as high temperature region, high humidity region, frontal surface, convergence line and coastline. In general, it's favorable for convection when the ground temperature is more than 32℃, the dew point temperature is greater than 23℃, the ground temperature gradient is greater than 0.1℃/km, or the ground level wind shear is greater than 5 m/s. Severe convective systems also react on underlying surface, and then convective systems are influenced as well, severe thunderstorm causes strong divergent outflow and cool pool is formed on ground layer. Cold air in the front of thunderstorm diverges outwards and causes gust front, which lifts the pre-frontal warm and moist. New convective cells develop close to the gust front, so that convective systems can diffuse forward. The question about convective systems' change crossing coastlines is complex. If they move to the sea by day, the temperature of underlying surface will descend and system's intensity would be weaken easily, and the situation will become opposite by night. In addition, the convergence of wind and intensity of convective systems enhance over the sea on account of small frictional forces and strong wind speed. Severe thunderstorm generates and strengthens at coastlines frequently, and particularly at the junction between the gust front and the coastline due to convergence caused by wind discontinuity around coastlines. Finally, the convection weather concept model before the trough is summarized:The area is within the scope of subtropical high before the trough at 500 hPa. There is strong southwest jet and warm wet tongue at low level, which forms unstable stratification. There is a big wind belt at 500 hPa, which forms a larger vertical wind shear from 0 to 6 km level, adding to the potential instability, and a strong convection system is triggered if a cold front comes.