On the Sensitivity of Convective Cold Pools to Mesh Resolution

Abstract

It is well recognized that triggering of convective cells through cold pools (CPs) is key to the organization of convection. Yet, numerous studies have found that both the characterization and parameterization of CP effects in numerical models is cumbersome—in part due to the lack of numerical convergence with respect to the horizontal mesh resolution, Δx, obtained in typical cloud‐resolving simulators. Within a comprehensive numerical convergence study we systematically increase the horizontal resolution in a set of idealized large‐eddy simulations. Our analysis captures key CP processes, namely free propagation, frontal collision and merging of gust fronts. We characterize the numerical convergence of tropospheric moistening rates, gust front vortical strength and propagation speed, and the amplitude of the lobe‐and‐cleft instability. The understanding gained from this analysis may help develop robust subgrid models for CP dynamics.

U‐Net Segmentation for the Detection of Convective Cold Pools From Cloud and Rainfall Fields

“…Refined measures could be designed that still assign a score to a minimally displaced CP pixel. However, physically relevant CPs, for example, in terms of collision effects (Fiévet et al., 2023; Meyer & Haerter, 2020) and intense precipitation (Jensen et al., 2022) tend to cover larger patch fractions and the IOU score is systematically high—again with best performances for the pseudo‐3D models.…”

“…At Earth’s surface, CPs spread horizontally as density currents (Drager & van den Heever, 2017; Droegemeier & Wilhelmson, 1985; Zuidema et al., 2017). While expanding radially along the surface, CPs can be characterized as consisting of (a) a head, which can measure hundreds to several thousand meters vertically and (b) a shallower interior, which is separated from the head by a wake region (Benjamin, 1968; Cafaro & Rooney, 2018; Droegemeier & Wilhelmson, 1987; Fiévet et al., 2023; Kneller et al., 1999; Kruse et al., 2022; Markowski & Richardson, 2011; Meyer & Haerter, 2020; Simpson, 1980).…”

How Variable are Cold Pools?

“…Understanding this variability is crucial to assessing cold pool representation in large-eddy simulation (LES), numerical weather prediction, and climate models, and to understanding their many atmospheric and societal impacts. Simulated cold pool properties, and how they interact with other components of the earth system, are known to be sensitive to the model grid spacing, as shown in previous modeling studies (Straka et al, 1993;Bryan et al, 2003;Grant & van den Heever, 2016;Huang et al, 2018;Hirt et al, 2020;Fiévet et al, 2023). Many of these studies have demonstrated more intense, longer-lived, and faster-propagating cold pools at finer model resolutions.…”

Detecting Cold Pool Family Trees in Convection Resolving Simulations

“…Cyclic boundary conditions are chosen in both horizontal dimensions. We found this horizontal mesh resolution a suitable compromise, where relatively large domain sizes can be simulated for several days, yet, key CP effects, such as the dynamic gust front, can be simulated at satisfactory detail (Bryan et al., 2003; Fiévet et al., 2023; Hirt et al., 2020; Meyer & Haerter, 2020; Straka et al., 1993). The mesh is discretized along the vertical direction z using 64 levels of increasing depth Δ z , ranging from Δ z = 50 m at the first level ( z = 25 m) to 1,000 m at the domain's top boundary ( z = 26 km).…”

“…This may be due to an incomplete understanding of the fundamental processes affecting CP interaction, dynamics, and thermodynamic modifications of the boundary layer, as well as the pre‐moistening of the lower free troposphere. Process‐focused studies that leave out parts of the components involved (Fiévet et al., 2023; Haerter & Schlemmer, 2018; Meyer & Haerter, 2020; Romps & Jeevanjee, 2016), may help improve parameterizations further.…”