| dc.description.abstract | This study utilized IBM_VDRAS (Immersed Boundary Method_Variational Doppler Radar Analysis System) to analyze the afternoon thunderstorm on 7 July 2017. Before we analyzed this case, a few new features were implemented to IBM_VDRAS. They included Coriolis force, radar beam blockage, assimilation of clear air echo, and a new minimizer called LBFGS-B (Limited-memory BFGS for bound-constrained). Furthermore, in order to resolve the problems associated with the overestimation of wind speed at the lowest level of IBM_VDRAS, we changed the lower boundary condition from free-slip to no-slip type, and compared the results against the observations. The results showed that no-slip type boundary condition is able to reduce the problem of wind speed overestimation, and generate more accurate wind directions than those from the free-slip type boundary. In the case study of this research we compared the differences between a fast moving convection over the plain and an ordinary convection developed in the mountainous area, and attempted to find out the reason causing such differences.
From the analyses of IBM_VDRAS, it can be seen that the structure of the moving convection was asymmetric, and was more like a squall line, with strong wind descending behind the convection to the ground. This is also confirmed by the surface station observations as the wind speed increased when the convection passed the station. In addition, the cold pool’s moving speed was approximately equal to the strong wind. Therefore, it is speculated that the movement of this moving convective system was driven by the advection of the strong wind. On the other hand, the ordinary convection developed vertically in the mountainous area without strong wind field, and was almost stationary. The difference might be attributed to the strength of the environmental wind shear. From the ERA5 reanalysis data, it was shown that the maximum wind speed in the plain area occurred at 2.0 ~ 3.0 km, and the low level wind shear below 3.0 km was about 8ms^(-1). By contrast, the wind speed above mountain was weaker, with a low level wind shear below 3.0 km of only 3.5ms^(-1).
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