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Journal of the Meteorological Society of Japan, Ser. II Vol. 72, No. 2, 1994, pp. 235-253

A Numerical Study on the Formation of Organized Convective Storms: Part I. Formation Patterns of Long-Lasting Cells

Ryohei Misumi1,2, Marjan Divjak1,3, Shuichi Tanahashi1,4 and Takao Takeda1

Abstract

A three-dimensional numerical simulation of a convective-cloud ensemble was performed in order to study the formation process of long-lasting precipitating convective cells. As the initial condition, random small thermals were given in the horizontally homogeneous atmosphere.

The convective cells were classified into three types: The first type (S-type) was short-lived and had a downshear-tilting updraft. The second type (F-type) was forced by the other cells and persisted for a long time, although the airflow structure was similar to S-type. The third type (L-type) was long-lasting and had an upshear-tilting updraft.

In the formation process of an L-type cell, the updraft root must move in a downshear direction faster than the rising air parcels in order to build an upshear-tilting updraft. The fast movement of the updraft root is initially forced by the neighboring cells. The results showed there are three patterns in this process. In Pattern I the updraft root moves with outflow diverging from one short-lived cell. In Pattern II it moves with outflow boundaries of several short-lived cells which form successively. In Pattern III it moves together with outflow diverging from a pre-existing L-type cell. Once an upshear-tilting updraft is built, the cell produces a strong cold air pool near the surface and is self-maintained at its edge.

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