The development of organized convection in a simplified squall-line model

Squall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration, precipitation output and area of influence. Time-dependent models have identified surface outflow boundaries and low-level environmental wind shear as the crucial elements in squallline formation and/or maintenance. Steady-state models have shown how properties of the downdraught reservoir, or ‘cold pool’, must be matched to the far-field wind and thermodynamic profiles to sustain the convection. However, the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain. For help with these issues, a time-dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions. The simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far-field environments. The convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by ‘storm fronts’. Forced (or neutral) ascent occurs at a surface ‘gust front’ on the upwind side of the cold pool. We distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts. It is found that subsidence over the cold pool disorganizes the updraughts, whereas a deep overturning circulation downshear from the gust front has an organizing effect. Hence, factors which allow the initial subsidence to occur ahead of the gust front, namely contrasting lowerand upper-level winds and large potential buoyancy, favour squall-line development. It is argued that, in cases with large convective potential energy, the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught.