The Effect of Land Surface Heterogeneity and Background Wind on Shallow Cumulus Clouds and the Transition to Deeper Convection

Abstract

Idealized large-eddy simulations (LESs) with prescribed heterogeneous land surface heat fluxes are performed to study the impact of the heterogeneity length scale and background wind speed on the development of shallow cumulus and the subsequent transition to congestus/deep convection. We study the impact of land surface heterogeneity in an atmosphere that favors shallow convection but is also conditionally unstable with respect to deeper convection. We find that before the convection transition, larger and thicker shallow cumulus clouds are attached to moisture pools near the PBL top over patches with low evaporative fraction (referred to as “DRY”). This feature is attributable to a surface-induced secondary circulation whose development depends on the heterogeneity size and the background wind speed. With large patches (≥5 km) under zero ambient wind, the secondary mesoscale circulation promotes the vertical transport of moisture forming a moisture pool over DRY patches, while with smaller patches, no such circulation develops. The influence of the background wind on the secondary circulation is strong such that any wind stronger than 2 m s−1 effectively eliminates the impact of surface heterogeneity on the PBL and brings no secondary circulation. This is because the triggered secondary circulation is not strong enough to withstand the imposed background wind. Based on these, we propose two criteria for the convection transition, namely, that the heterogeneity length scale is greater than 5 km and that the background wind speed is less than Uc0, where Uc0 is the near-surface cross-patch wind speed of the secondary circulation under zero background wind for a given patch size and is about 1.5 m s−1 in our cases.

The Impact of Variable Land‐Atmosphere Coupling on Convective Cloud Populations Observed During the 2016 HI‐SCALE Field Campaign

“…These results are consistent with Lee et al (), in which they used an idealized LES configuration with square patches that have either higher or lower prescribed latent heat fluxes to mimic wet and dry soil conditions, respectively. In that study, clouds preferentially formed over the dry regions when the ambient winds were less than 2 m/s.…”

“…Previous idealized and real‐world modeling studies have demonstrated the importance of soil moisture variations in creating secondary circulations that influence cloud development (e.g., Chen & Avissar, ; Lee et al, ; Segal & Arritt, ; Xiao et al, ). For idealized soil moisture variations and no synoptic forcing during the day, warmer (cooler) air over dry (wet) soil regions will set up convergence (divergence) zones near the surface and rising (sinking) motions in the boundary layer.…”

“…Aloft, the wind patterns are reversed. Clouds will therefore preferentially form over the dry soil moisture regions demonstrated by Lee et al (2019) and other studies. However, the secondary circulations are more complex and difficult to identify in real-world conditions.…”

Quantifying feedbacks in the plant-atmosphere-cloud continuum

“…They imply from their results that moisture convergence might be more important than local surface fluxes to trigger deep convection (Hohenegger & Stevens, 2012). Furthermore, as discussed by Avissar & Schmidt (1998) and Lee et al (2018), a weak background wind of 2 to 2.5 m s -1 was sufficient to significantly decrease the effects of static surface heterogeneities by enhanced horizontal mixing, thereby hampering moist convection.…”

“…Investigations concerning static heterogeneity are often performed by prescribing the land-surface in dry or wet patches following a checkerboard pattern (e.g., Lee et al, 2018). Following from a difference in surface temperature caused by the patch's moisture content, surface energy fluxes are altered.…”