A Large-Eddy Simulation Study of Moist Convection Initiation over Heterogeneous Surface Fluxes

Abstract

This study uses large-eddy simulations to investigate processes of moist convection initiation (CI) over heterogeneous surface fluxes. Surface energy balance is imposed via a 1808 phase lag of the surface moisture flux (relative to the sensible heat flux), such that the relatively warm surface is relatively dry (and the relatively cool surface is relatively wet). As shown in previous simulations, a mesoscale circulation forms in the presence of surface-flux heterogeneity, which coexists with turbulent fluctuations. The mesoscale convergence zone of this circulation develops over the relatively warm surface, and this is where clouds first form. Convection initiation occurs sooner as the amplitude of the heterogeneity increases, and as the surface moisture increases (i.e., Bowen ratio decreases). Shallow clouds initiate when boundary layer heights (z i ) become greater than the lifting condensation level (LCL). Deep precipitating clouds initiate when the LCL and level of free convection (LFC) are roughly the same when averaged over the relatively warm surface, which is equivalent to the mean convective inhibition (CIN) becoming nearly zero. From the perspective of the entire (mesoscale) domain, cases with strongly heterogeneous surfaces have a wider distribution of both z i and LCL. Thus, a comparison of z i with LCL over a mesoscale area (i.e., within one mesoscale model grid box) may lead to misleading conclusions about CI and cloud-base height. It is also shown that as the amplitude of the surfaceflux heterogeneity increases the mesoscale convergence zone becomes narrower and stronger. Furthermore, CI occurs earlier over relatively wet surfaces partly because turbulent eddies are more vigorous owing to slightly greater buoyancy.

Assessing the uncertainty of soil-moisture impact on convective precipitation by an ensemble approach

“…The redistribution of soil moisture increases the large-area heterogeneity, but decreases the small-area heterogeneity. This is in accordance with Adler et al (2011);Kang and Bryan (2011), who found an influence of redistribution of soil moisture on the location of convective inition. Therefore, area "blue", mainly containing small convective cells, is more influenced than area "red" with the large advected precipitation band.…”

“…Despite to the systematic effect on partitioning of the heat fluxes, precipitation reacts less systematically on soil moisture variations Hohenegger et al, 2009). The distribution and inhomogeneity of soil moisture patterns may initiate secondary circulation (Clark et al, 2004;Adler et al, 2011;Kang and Bryan, 2011;Dixon et al, 2013;Maronga and Raasch, 2013;Froidevaux et al, 10 2014).…”

A Simple Method for Simulating Wind Profiles in the Boundary Layer of Tropical Cyclones

“…Here we use the numerical model "Cloud Model version 1" (CM1) that has been used for several LES studies in recent years (e.g., Kang and Bryan 2011;Kang et al 2012;Wang 2014;Nowotarski et al 2014;Markowski and Bryan 2016). Details of the model used here, including a new "two part" subgrid model near the surface, are provided in the Appendix.…”

The Influence of Land Surface Heterogeneities on Cloud Size Development

“…Garcia-Carreras et al (2011) explained the reasons for a preferred location of shallow convective clouds on the front of the mesoscale circulations. Recently, Kang and Bryan (2011) studied the effect of the amplitude of the surface heterogeneity on the transition to deep convection by prescribing sinusoidal surface fluxes of various amplitudes. As the amplitude becomes larger the area of mesoscale convergence becomes narrower and stronger and will lead to an earlier transition to deep convection.…”