To investigate the impact of sea surface to a squall line over Bohai sea in the evening on 25 July 2016, which developed over sea、moved slowly and had a long life, Weather Research and Forecasting (WRFV3.8) model is used. This squall line develops over sea, moves slowly and has a long life time. The control runs and a sensitivity experiment is designed. Results show that:(1) WRF model can simulate the formation process of the squall line reasonably. East wind blowing from the sea is the key factor of convection before the system goes into the sea. In enhancement phase of squall line in Bohai Sea, the north wind and the southwest wind converge at sea and form the strong convergence zones, leading to the reorganizing and developing of squall line. North cold air goes down south and triggers the release of unstable stratification energy, providing energy for convection. The pulsation of the southwest low level jet has a positive effect in the development of the system. Strong vertical wind shear in the deep and low level creates a favorable environment for squall line to forming and developing into linear convection; (2) Smooth Bohai Sea surface strengthens the wind near the ground. Southwest wind enhances after going into the sea, strengthening the convergence with north ground wind, and making the convection more active. This effect also hinders the movement of convergence and slows down the speed of squall line moving southward; (3) Bohai Sea surface has less sensible heat and latent heat exchange than land surface at daytime, turbulence activity in sea surface is weak. Latent heat exchange of Bohai Sea is higher, but boundary layer height is lower, that is not conducive to the enhancement of low vertical wind shear, hindering the development of squall line.
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