Large-eddy Simulation of Surface Heterogeneity Induced Secondary Circulation with Ambient Winds

Land surface heterogeneity affects the distribution of energy from incoming solar radiation, and in conjunction with ambient winds, influences the convective atmospheric boundary layer development. In this study, experimental large-eddy simulations were carried out applying continuously-distributed soil moisture along a river corridor with idealized initial atmosphere conditions at a spatial scale on the order of kilometers. Simulations were performed with ambient wind ranging from 0 to 16 m/s and for different directions, which are cross-valley and parallel-valley. After decomposing the simulated winds into the ensemble-averaged wind, phase wind, and turbulence, the results show that soil moisture heterogeneity induces a well-organized secondary circulation structure with the horizontal mesoscale phase wind approaching some 2 m/s. The secondary circulation structure persists under the parallel-valley wind conditions independently of the wind speed, but is destroyed when the cross-valley wind is stronger than the horizontal mesoscale phase wind. We explored the relationship between the secondary circulation strength, expressed as the normalized maximum of the vertical phase wind variance, and dimensionless variables such as Bowen ratio and stability parameter (ratio of boundary layer depth and Obukhov length). With the mean of these dimensionless variables, we found a distinct relationship between the strength of the secondary circulations with respect to the ambient wind.