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.