Regional Bowen ratio controls on afternoon moist convection: A large eddy simulation study

Abstract

This study examines the effect of the regional Bowen ratio β, the ratio of the domain‐averaged surface sensible heat flux (SHF) to latent heat flux (LHF), on afternoon moist convection. With a temporally evolving but spatially uniform surface available energy over a mesoscale domain under a weak capping inversion, we run large eddy simulation of the afternoon convective boundary layer (CBL). We first prescribe a small β of 0.56 (a wet surface) and then the reversed large β of 1.80 (a dry surface) by switching the SHF and LHF fields. The perturbation fields of the fluxes are prescribed with the Fourier spectra of κ− 3 (κ is horizontal wave number; strong mesoscale heterogeneity) and κ0 (homogeneity). The large β cases have strong vertical buoyancy fluxes and produce more vigorous updrafts. In the heterogeneous, large β surface case, with the removal of convective inhibition over a mesoscale subdomain of large SHF, deep convection develops. In the heterogeneous, small β surface case, convective clouds develop but do not progress into precipitating convection. In the homogeneous surface cases, randomly distributed shallow clouds develop with significantly more and thicker clouds in the large β case. (Co)spectral analyses confirm the more vigorous turbulent thermals in the large β cases and reveal that the moisture advection by the surface heterogeneity‐induced mesoscale flows makes the correlation between mesoscale temperature and moisture perturbations change from negative to positive, which facilitates the mesoscale pool of high relative humidity air just above the CBL top, a necessary condition for deep convection.

Semi‐Coupling of a Field‐Scale Resolving Land‐Surface Model and WRF‐LES to Investigate the Influence of Land‐Surface Heterogeneity on Cloud Development

“…The w-e wind case (Figure 14d) shows a strong preference for liquid water production in the southern 20 km of the domain, closely resembling the base heterogeneous case's aversion to cloud production over the moist patch but realigned to the w-e wind direction. The result that clouds show a preferential production over warm/dry areas in the presence of land-surface heterogeneity is widely reported (e.g., Avissar & Liu, 1996;Esau & Lyons, 2002;Hohenegger et al, 2009;Huang & Margulis, 2013;Kang, 2016;Lee et al, 2019;Taylor et al, 2012;van Heerwaarden & de Arellano, 2008). The persistence of the circulation with a background wind of the magnitude used here is relatively novel, but the general result that circulations orient themselves perpendicular to the background wind direction has been both demonstrated and explained in previous studies (e.g., Prabha et al, 2007;Raasch & Harbusch, 2001;Rochetin et al, 2017;Shen & Leclerc, 1995;Sühring et al, 2014;Weaver, 2004a), though with less of a consensus.…”

“…The necessary conditions of the land-surface heterogeneity to trigger secondary circulations are not fully established, though it is generally agreed that larger differences between the sensible heat fluxes in the warm and cool patches will produce stronger circulations. It is also generally agreed that the spatial scale of the coherent warm and cool patches must be of a sufficient size before circulations can be triggered, though with minimal consensus on more specific criteria (Albertson et al, 2001;Chen & Avissar, 1994;Hadfield et al, 1992;Han, Brdar, Raasch, & Kollet, 2019;Huang & Margulis, 2013;Kang, 2016Kang, , 2020Kang & Ryu, 2016;Shen & Leclerc, 1995;Patton et al, 2005;Sühring et al, 2014;Timmermans et al, 2008;Trier et al, 2004). Many studies conclude simply that larger spatial scales produce stronger circulations, while others find that there is an optimal scale of land-surface heterogeneity for cloud production after which further increases have a homogenizing effect.…”

The Importance of Soil‐Type Contrast in Modulating August Precipitation Distribution Near the Edwards Plateau and Balcones Escarpment in Texas

“…The resulting downward motion over the sandy soils suppresses the precipitation east of DFW while the upward motion over the clay soils triggers more precipitation west of the Balcones Escarpment (Figure b). The impact of different surface energy balance (or partition between sensible and latent heat fluxes) on precipitation shown in this study is also corroborated by large eddy simulations conducted by Kang (), in which a higher Bowen ratio (i.e., more sensible heat flux relative to latent flux) is shown to more likely trigger afternoon moist convection. This study is also consistent with previous observational studies (e.g., Taylor et al, ) that show more afternoon rainfall over areas with enhanced sensible heat flux.…”

Response of Convective Boundary Layer and Shallow Cumulus to Soil Moisture Heterogeneity: A Large‐Eddy Simulation Study

“…Moreover, clouds mainly form over the dry areas because of the induced mesoscale circulations. Soil moisture evaporates into the atmosphere over the wet surface and is advected to the dry surface by the induced mesoscale circulation (Kang, ; van Heerwaarden & de Arellano, ). The larger sensible heat flux over dry surface leads to more rising air parcels capable of forming clouds.…”