MC 6460
Speaker
Dan Kirshbaum, Department of Atmospheric and Oceanic Sciences, McGill University
Title
On the dynamics of deep-convection initiation
Abstract
Deep-convection initiation (DCI), or the formation of a first cumulonimbus cloud over a defined spatiotemporal region, stands as one of the most challenging problems in atmospheric science. It is an inherently multiscale, multiphase, nonhydrostatic, and nonlinear process, which demands a deep physical understanding to interpret and an extremely accurate and fine-scale numerical model to reliably predict. DCI is most commonly interpreted using adiabatic parcel theory, where a parcel drawn from the low-level environment is lifted adiabatically through the column. From such analysis, various convective metrics are evaluated, like the level of free convection (LFC, where the parcel becomes positively buoyant), and the level of neutral buoyancy (LNB, where the parcel buoyancy returns to zero). In general, environments with small convective inhibition (CIN, the vertically integrated negative buoyancy below the LFC) and large convective available potential energy (CAPE, the vertically integrated positive buoyancy between the LFC and LNB) are most supportive of DCI. However, this combination of parameters is only a necessary condition, and observations indicate that DCI does not always occur when these conditions are satisfied. Other critical factors in the DCI process include mixing between the cloud and its surroundings (entrainment and detrainment), which tends to dilute the cloud and modulate its mass flux, and vertical perturbation pressure gradients, which tend to offset buoyancy-induced vertical accelerations. The present study advances the understanding of DCI by evaluating its sensitivity to the properties of subcloud updrafts that lift low-level air parcels to their LFC. Specific properties of interest include updraft strength, width, and duration. Using idealized large-eddy simulations with an analytic subcloud circulation, these sensitivities are quantified and physically interpreted. It is found that increased updraft width, strength, and duration all favour DCI, but for different reasons. These reasons are explained through analyses of entrainment, detrainment, and vertical perturbation pressure distributions. A simplified model of the DCI process is proposed that captures most of the experimental trends.