The Shallow-to-Deep Convective Transition Over an Idealized Mesoscale Convergence Zone

Abstract:

The initiation of deep convection within a conditionally unstable atmosphere requires air parcels to reach their level of free convection, above which latent-heat release generates positive buoyancy. However, air parcels that satisfy this condition often fail to undergo deep ascent due to mitigating factors such as entrainment and adverse perturbation vertical pressure gradients. Recent studies suggest that the transition from shallow-to-deep convection may additionally require the formation of evaporative cold pools, a sufficient cloud-layer lapse rate, gradual moistening of the mid-troposphere through shallow-cumulus detrainment, and/or the fortuitous development of new cumuli through the remnants of their predecessors. In this study, we use cloud-resolving simulations of cumulus convection over an idealized surface-based convergence zone to study the mechanisms and sensitivities of deep-convection initiation forced by mesoscale ascent. The surface convergence forms in response to a localized diurnal heating anomaly over an otherwise homogenous and unheated surface, producing an organized boundary-layer updraft with a maximal amplitude of around 3 m/s over the center of the heat source. It gives rise to a line of cumuli that gradually deepens and, in some cases, transitions into deep convection. Detailed analysis of the simulations reveals several notable aspects of cumulus evolution, including that its vertical development is not uniquely determined by the combination of surface forcing and surface-based convective inhibition (CIN), as some have suggested. In addition, the specific mechanism by which deep convection first develops is sensitive to the environmental conditions, which implies that no single mechanism always explains the shallow-to-deep transition. For example, with a thermal tracking algorithm developed for the purpose, it can be shown that, in a marginally unstable atmosphere, the total water perturbation induced locally by decaying cumuli has an effect in helping the following thermal to transition to deep convection.