Large Eddy Simulation of Surface Heterogeneity Induced Secondary Circulation with Background Winds

Land surface heterogeneity affects the surface heat flux distribution and induces secondary circulations at a certain scale. Background wind may significantly influence the effect of surface heterogeneity on secondary circulation. In this study, we investigate how background wind affects the evolution of the atmospheric boundary layer, focusing on the influence of the formation of secondary circulation. We used a coupled ICON-LES (Icosahedral Nonhydrostatic Large Eddy Simulation mode) with a land surface model (TERRA-ML) to simulate the development of the atmospheric boundary layer over a river corridor mimicked by continuously distributed soil moisture under different background wind conditions. The atmospheric domain size is 4.8 km x 4.8 km x 4.2 km in X, Y, and Z directions with a horizontal and vertical spatial grid spacing of 50 m using double-periodic boundary conditions. All simulations have the same initial well-mixed atmospheric conditions and constant incoming radiation of 700 Wm^-2   with varying background winds with different wind speeds (0 to 16 ms^-1) and directions (cross-valley, parallel-valley, or mixed).

The atmospheric states are decomposed into three parts: ensemble-averaged, mesoscale, and turbulence. We show that wind speed and surface heterogeneity jointly affect the surface energy distribution, independent of the wind direction. The secondary circulation structure persists under the parallel-valley wind regardless of wind speed but is destroyed when the cross-valley wind is stronger than the mesoscale horizontal wind speed. The maximum mesoscale vertical wind variance reflects the secondary circulation strength. We show that the secondary circulation strength positively correlates with the Bowen ratio and stability parameter (-Zi/L)  under cross-valley wind and mixed conditions.