Structure and developing mechanism of micro-scale phenomena associated with cumulonimbus clouds

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A supercell storm and an associated major tornado were successfully reproduced in a semi-compressible non-hydrostatic meso-scale numerical model having a calculation domain of 66.4km × 66.4km × 15.1km, horizontal mesh size of 70m and 45 vertical levels. The storm was initiated by imposing a thermal bubble in a horizontally uniform environment that was adopted from the upper-air sounding on 20 May 1977 near Del City, Oklahoma in the United States. After 50 min from its initiation, a meso-low is formed at 2km above the ground level (AGL) as a result of the interaction between the storm updraft and the vertical shear of the horizontal flow. The upward pressure gradient accelerates the updraft below 2km AGL and produces a mesocyclone at 1.2km AGL by 60 min through tilting of horizontal vorticity and subsequent stretching. The rotation field lowers the pressure near the mesocyclone center, which further accelerates the updraft to reach 40m/s near 1km AGL. On the other hand, a gust front where rain-cooled air flowing out from a precipitating region and a warm moist environmental inflow meet is formed near the ground. The gust front is accompanied by vertical vorticity due to horizontal shear. When the strong updraft in the low-level mesocyclone comes right above the gust front, the vertical vorticity is stretched to form a tornado. A supercell tornado therefore is found to develop qualitatively in the similar way to a landspout which often develops along a meso-scale front where both convergence and horizontal shear are strong.