On the mechanisms of deep convection initiation

Dan Kirshbaum

McGill University

Wednesday, Feb 28, 2024, 11:00 am MT
DSRC Room 2A305

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Abstract

Deep convection initiation (DCI) challenges both conceptual understanding and numerical weather prediction (NWP) owing to its multi-scale, multi-phase, and turbulent nature. At its core, DCI is a nonlinear threshold problem, where multiple factors come together in localized regions to produce clouds that successfully transition from shallow to deep. Once DCI occurs, it shapes the development of clouds and precipitation over a broader mesoscale region.

The necessary conditions for DCI are familiar from adiabatic parcel theory: deep moist instability (positive convective available potential energy and parcel buoyancy over a deep layer), combined with sufficient vertical displacement to release it. While these conditions are simple and reasonably predictable in modern NWP models, they only indicate regions where and when DCI is possible. Within such regions, more intricate and complex processes determine precisely if, when, conditions and locally sufficient to produce DCI.

These processes include, among others, vertical perturbation pressure gradients, entrainment and detrainment, precipitation and cold pools, and external circulations that interact with active clouds (e.g., turbulent eddies, internal gravity waves, elevated downdrafts). While some of these processes are inherently unpredictable, others can be better understood to push the relevant science forward.

This presentation reviews the DCI problem and highlights efforts to build understanding in arguably the most challenging aspect of it: the cloud-scale controls on DCI. Specific topics of interest include the understanding and quantification of vertical-shear effects, cumulus entrainment and detrainment, boundary-layer controls on incipient cumuli, and the explicit representation of cumulus convection in NWP models.

About Dan Kirshbaum:
Dan did his Ph.D. with Dale Durran at the University of Washington, after which he served as an Advanced Studies Program fellow at the National Center for Atmospheric Research (NCAR) and then a postdoc at Yale University. After doing a brief lectureship at University of Reading, he moved to McGill in 2011 as an Assistant Professor. Dan's research group focuses on processes driving cumulus cloud formation and development, including boundary-layer circulations both over and away from complex topography.