The dynamics and simulation of tropical cumulonimbus and squall lines

Abstract Techniques of theoretical analysis and numerical simulation are combined to produce a dynamical model of tropical cumulonimbus convection which features a close cooperation between the updraught and downdraught circulations. The cloud‐scale dynamics determine the structure and transfer properties. Sub‐cloud‐scale transfer is unimportant. A steady‐state dynamical model shows that the upshear or down‐shear propagation speed, c , of a cumulonimbus cell relative to the mid‐level flow is determined as a function of the convective available potential energy, CAPE , and weakly influenced by the windshear through a non‐dimensional number, R , of the large‐scale flow. This propagation speed is almost constant for a wide range of R , with c ≃ 0.3 CAPE , but only possible if R ≳ 2.8 (small shear). This contrasts with a previous result for another regime of convection, obtained by Moncrieff and Green (1972), if R ⩽ 1 (high shear). The transfer of momentum is distinctive and of large magnitude. The initiation and growth of the convective circulation represent an essentially nonlinear, finite‐amplitude process, whose properties are closely related to the wind profile in the tropical atmosphere. The numerical simulations attain a quasi‐steady state of a complex, three dimensional nature, basic features of which can be represented in terms of the steady‐state analysis. Moreover, the outflow of the downdraught air is closely related to the attainment of a steady circulation and suggests a mechanism both for the maintenance and the eventual breakdown of the convective regime.