Study On The Effect Of Hangzhou Bay On The Development Of Squall Line In North Zhejiang Province

To enhance the understanding of the squall line maintaining mechanism under influence of Hangzhou Bay, with doppler radar, general and encrypted surface observed data,NCEP GFS data, the diagnosing analyses are made of Squall line in Northern Zhejiang on 27 July ,2014. Focusing on the evolution of convection moved into Hangzhou Bay from the north and south coast and the effect on the overall development of squall lines. On this basis,To explore the impact of sea and land surface on the squall line,the WRF model is used to simulate the squall line process,and carried out the comparative test of modify the sea and land types.The result indicate that north-south convergence lines and moderate strength of deep vertical wind shear are the key environmental factors of Squall line generation and development. Convections are strengthened after moving into Hangzhou bay from north and south coast, lead to squall line maintaining.The better moisture conditions and instability energy conditions on Hangzhou Bay are important reason that the further strengthening of convection moved from the north shore of Hangzhou Bay , the connection of two linear convections in southern Jiangsu, northern Zhejiang and the sustainable development of squall line. The temperature and humidity on the surface of Hangzhou Bay is equal to the surface of south coast,and the strong windshear in Hangzhou Bay lead to the maintaining strength of convection moved into Hangzhou Bay from south coast. Under the interaction of cold pool and rear inflow,pushing forward system center convection, developed into bow echo on the sea,playing an important precondition to squall line maintaining.The results show that the WRF model can simulate the development process of the squall line.After changing the Hangzhou Bay to cultivated land, the squall line convection intensity increased obviously.The main reason is summarized as follows,compared with the sea,the land surface has an influence on the surface flux,temperature and humidity and other thermal conditions,making the instability energy of pre-period convection development enhanced.The change of wind field near the ground lead to low level vertical wind shear increase and the rotation motion enhance,which is more favorable to the formation of internal strong and stable vertical circulation,Making the convection intensity stronger during the squall line generation and development stages.Thus, the influence of the sea and land on the squall line is firstly by changing the low level flux, temperature and humidity field,flow field and other thermodynamic conditions,and then affect the instability stratification, lifting conditions and environmental vertical wind shear conditions,changing the small and medium scale structural features and making the different strength variation characteristics of squall line.