A Numerical Simulation of Quasi‐Stationary Tropical Convective Bands

A method of imposing large‐scale ascent on a three‐dimensional numerical model is introduced and this is shown to be crucial in maintaining the quasi‐stationary convective bands observed in GATE. the effects of ascent on the overall convective development are to enhance updraughts and particularly to maintain the development of new convection cells. the magnitutude of the forcing is varied to show the sensitivity of growth on convergence. A quasi‐stationary convective band, aligned approximately parallel to the low‐level wind shear, is simulated in an initial environment defined from soundings obtained from the GATE ships Researcher and Oceanographer on 3 September 1974; the organization is therefore quite distinct from that of fast‐moving squall lines aligned at right angles to the shear. the simulated convective system has marked dynamical similarity to the observed bands; these consisted of a number of cumulonimbus cells with new cells developing on the eastern end of the hand with deeper cells to the west. the cells generally travel along the line. The large shear due to the strong low‐level jet is crucial in influencing the band development and orientation. the dynamical structure of the convective band is investigated by a trajectory analysis, which indicates an organized three‐dimensional structure. The transports of heat, moisture and momentum are calculated, and a quasi‐balance between large‐scale ascent and cloud‐induced descent on a time scale of a few hours is shown to exist. Apparent heat sources and moisture sinks are calculated and these have a bi‐modal vertical structure, in general agreement with those in observed tropical cloud clusters.