Generation mechanism and prediction of near surface gusts generated by deep convection.

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The purpose of this research is to investigate the mechanism of the near surface gusts generated by deep convection by the use of cloud resolving model (CRM)s. There are two processes to produce gusts near the surface. One is the buoyancy driven downdrafts and the other is the initial wind in the upper level of the downdraft air. In order to investigate the mechanism, the CRM must well simulate the gust produced by the downdrafts in the deep convection. The models used in this research are (1) Met-Office CRM and (2) ARPS developed by the Center for Analysis and Prediction of Storms (CAPS), University of Oklahoma. In the GCSS (GEWEX Cloud System Study) WG-4 model intercomparison projects, it is shown all the models well simulate the characteristic feature of the squall line. However, the details, such as the maximum updraft speed, rainfall intensity, released latent heat, differ from model to model. The convection in ARPS is not so active as those in other models, because of the large numerical diffusion coefficients used in ARPS. The comparison are also made in the simulation of the rainband in the Baiu Front. The models are used to simulate the gusts for two cases. For the analysis of the levels of the origin of the downdrafts. artificial tracer was put in the models. The results show that the levels differs from case to case, because of the stratification, humidity, and wind profile. Furthermore, the relative importance of the buoyancy driven downdrafts and that of the initial wind at the original upper level differs from case to case. It is shown that it is important to combine the two processes to predict the gusts generated by deep convection.