On the Movements of Convective Storms, with Emphasis on Size Discrimination in Relation to Water-Budget Requirements

Abstract

In typical squall-line situations wherein the wind veers strongly with height, individual convective storms move as much as 60 deg right or 30 deg left of the direction of the mean wind in the cloud layer. It is shown that, on the average, the radar echoes having largest diameters move farthest to right of the wind. This behavior is consistent with physical considerations and with supply-and-demand requirements of the storm water budget. For a given rainfall intensity, the amount of water precipitated by a storm is proportional to its area or to the diameter squared. The amount of water vapor intercepted is proportional directly to the diameter, and to the velocity of the storm relative to the winds of the lower-tropospheric moist layer. A large storm must intercept more vapor in proportion to its diameter than a small one, requiring a larger migration velocity relative to the moist layer. This requirement is satisfied if (wind veering with height) large storms move toward the right of the mean wind. Based on these considerations, a simple expression is derived for the direction of storm motion as related to storm diameter. This describes the mean behavior fairly well, but there is considerable residual scatter. With this taken into account, an expression is given for the probability of storm passage over a given point as related to the initial storm location and size. Some characteristic patterns of development are illustrated. New convective elements tend to form on or amalgamate with the right-hand side or end of an existing storm cluster or squall line, somewhat on the up-wind side relative to the mean wind. This pattern of generation contributes to the movement of large storm clusters strongly toward the right of the winds, and also to make large storms move consistently more slowly than small ones.