Large‐eddy simulation of catchment‐scale circulation

Abstract

The impact of soil moisture heterogeneity on the convective boundary layer (CBL) development was studied. Based on results from large‐eddy simulation (LES) applying soil moisture patterns along a river corridor and idealized atmospheric vertical profiles as initial conditions, this study provides insight in the influence of spatial scale of soil moisture heterogeneity on catchment‐scale circulations (CCs) and the ensuing growth of the CBL. The simulation results show that the intensity of organized circulations resulting from soil moisture heterogeneity is nonlinearly dependent upon soil moisture heterogeneity scale λ (SMHS) and horizontal gradient. Because of the large SMHS and strong soil moisture contrast, none of the simulations has reached a true steady state even after 24 h of simulation time. The intensity of organized circulations shows a sigmoidal dependence on SMHS. The optimal SMHS for horizontal transport is on the order of 19.2 km, while optimal SMHS for vertical motions occurs at 2.4 km. In these cases, the CCs also exert a strong influence on the boundary‐layer structure and the entrainment layer. The potential temperature is not constant with height due to a weak mixing in the boundary layer for large SMHS cases. Differences in sensible heat flux profiles between the heterogeneous cases increase with increasing height and reach a maximum at the top of the CBL. Interestingly, boundary‐layer height changes strongly with changing horizontal soil moisture gradient and SMHS while domain means, variances, and amplitudes of land surface energy fluxes are all almost identical. The entrainment flux and subsidence at the top of the CBL are jointly responsible for the CBL height variation.

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

“…The buoyancy flux difference between unstructured cases and structured cases is small below the entrainment layer and becomes significant near the entrainment zone, and the difference is as much as 20 W/m 2 . The difference in entrainment flux can be attributed to the influence of the induced mesoscale circulation and has been reported previously by several studies (Han et al, ; Sühring et al, ). Moreover, the buoyancy flux difference is pronounced in the cloud layer due to the different rates of entrainment in clouds between different cases.…”

“…Large‐eddy simulation (LES) has been used to study land‐atmosphere interactions and the impact of the land surface heterogeneity on atmospheric boundary characteristic (Avissar & Schmidt, ; Hadfield et al, , ; Han et al, ; Patton et al, ; Raasch & Harbusch, ; Shen & Leclerc, ; Sühring et al, ; van Heerwaarden et al, ), on the shallow convection development (H. Y. Huang & Margulis, ; Kang & Ryu, ; Raasch & Harbusch, ; van Heerwaarden & de Arellano, ) and on the transition from shallow to deep convection (Kang & Bryan, ; Lee et al, ; Rieck et al, ; Rochetin et al, ) for the past two decades. One main concern of these studies has been the optimal heterogeneity scale to induce a mesoscale circulation, which is commonly suggested to be at a mesoscale but varies considerably.…”

“…Using an LES and land surface model (LSM) coupled model, Patton et al () found that the optimal scale to induce a mesoscale circulation was 4 to 9 times the boundary layer height with 1‐D, stripe‐like heterogeneity in soil moisture. With an LES and LSM coupled model and continuous soil moisture variations, Han et al () reported that the optimal scale for horizontal transport was about 18 times the boundary layer height, while the optimal scale for vertical motions was about 2 times the boundary layer height. Recently, Lee et al () studied the effect of heterogeneity scale on the transition to deep convection by prescribing 2‐D chessboard surface flux patches.…”

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 general consensus from LES studies of heterogeneous land-atmosphere interactions is that surface patterns comprised of marked areas of either high sensible heat flux or high latent heat flux (typically resulting from, or an idealization of, underlying patterns of soil moisture and/or vegetation) will lead to secondary mesoscale circulations. These circulations tend to transport moist air from areas with high latent heat fluxes to areas with high sensible heat fluxes where it can be lifted through the ABL, leading to cloud production over the drier land surfaces (Avissar & Liu, 1996;Cheng & Cotton, 2004;Esau & Lyons, 2002;Garcia-Carreras et al, 2011;Hadfield et al, 1991;Han, Brdar, Raasch, & Kollet, 2019;Hohenegger et al, 2009;Huang & Margulis, 2013;Lee et al, 2019;Shen & Leclerc, 1995;van Heerwaarden & de Arellano, 2008). The clouds produced by the aforementioned circulation process tend to be deeper and more localized than those produced by homogeneous surfaces, leading to larger overall liquid water path (LWP) values but lower overall cloud cover percentages.…”

“…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.…”