A NUMERICAL STUDY ON THE FORMATION OF ORGANIZED CONVECTIVE STORMS .1.FORMATION PATTERNS OF LONG-LASTING CELLS

A three-dimensional numerical simulation of a convective-cloud ensembl e was performed in order to study the formation process of long-lastin g precipitating convective cells. As the initial condition, random sma ll 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 t ime, although the airflow structure was similar to S-type. The third t ype (L-type) was long-lasting and had an upshear-tilting updraft. In t he 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 b uild an upshear-tilting updraft. The fast movement of the updraft root is initially forced by the neighboring cells. The results showed ther e are three patterns in this process. In Pattern I the updraft root mo ves 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 b uilt, the cell produces a strong cold air pool near the surface and is self-maintained at its edge.