Cold Pools as Conveyor Belts of Moisture

Herman F Fuglestvedt,Jan O Haerter

doi:10.1029/2020gl087319

Herman F Fuglestvedt, Jan O Haerter

Open Access

https://doi.org/10.1029/2020gl087319

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Abstract

AbstractObservations and simulations have found convective cold pools to trigger and organize subsequent updrafts by modifying boundary layer temperature and moisture as well as by lifting air parcels at the outflow boundaries. We study the causality between cold pools and subsequent deep convection in idealized large‐eddy simulations by tracking colliding outflow boundaries preceding hundreds of deep convection events. When outflow boundaries collide, their common front position remains immobile, whereas the internal cold pool dynamics continues for hours. We analyze how this dynamics “funnels” moisture from a relatively large volume into a narrow convergence zone. We quantify moisture convergence and separate the contribution from surface fluxes, which we find to play a secondary role. Our results highlight that dynamical effects are crucial in triggering convection, even in radiative‐convective equilibrium. However, it is the low‐level convergence resulting from this dynamics that removes inhibition, moistens the atmosphere aloft, and ultimately permits deep convection.

Highlights

Cold pools (CPs) form when a fraction of convective precipitation reevaporates as it falls to the surface
The key to triggering new deep convection is attributed to a combination of low convective inhibition (CIN) and high convective available potential energy (CAPE) at the outflow boundary after the CP cold anomaly was removed by surface fluxes, a mechanism that has found support in later studies (Langhans & Romps, 2015; Torri et al, 2015)
CPs travel along the surface and their outflow boundaries are visible as sharp spikes of positive vertical velocity

Summary

Introduction

Cold pools (CPs) form when a fraction of convective precipitation reevaporates as it falls to the surface. The classic and intuitive view is that forced lifting along the advancing outflow boundaries helps low-level air parcels overcome convective inhibition (CIN) and reach the level of free convection (LFC) (Droegemeier & Wilhelmson, 1985; and more recently, Jeevanjee & Romps, 2015; Torri et al, 2015). This view was challenged by Tompkins (2001). The key to triggering new deep convection is attributed to a combination of low CIN and high convective available potential energy (CAPE) at the outflow boundary after the CP cold anomaly was removed by surface fluxes, a mechanism that has found support in later studies (Langhans & Romps, 2015; Torri et al, 2015)

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