Analyzing the Interactions of Coupled Soil-Atmosphere Simulations on Turbulent Scales

Land-surface heterogeneity influences boundary-layer circulation. Several Large-Eddy Simulation (LES) studies of the last decades have sharpened our understanding of how different surface heterogeneities concerning mainly their length scale and amplitude affect the boundary-layer dynamics. In nature, besides fixed land-surface characteristics, the surface reacts to turbulent motions in the atmosphere. Therefore, the surface fluxes are highly dynamic according to the temporal and spacial scales of turbulence. Some recent publications attribute importance to this land-atmosphere feedback process for boundary-layer dynamics. Due to lack of proof in the aforementioned publication, we investigated how this so-called dynamic heterogeneity of surface fluxes which result from turbulent motions influence boundary-layer turbulence statistics.

We conducted an idealized case study with the LES model PALM with an implemented land-surface model (LSM), which solves the energy budget equation at the atmosphere-soil interface. A dry desert case is regarded with ten soil layers in total, the uppermost layer being in 1 mm depth to capture the short turbulent time scales. The atmosphere is simulated with ten meter grid size in a 10.8 km x 10.8 km domain for four hours until a quasi-stationary state is reached. A homogeneous control run is conducted with exactly the same energy input every time step as the LSM case. A comparison of both cases show no differences in average temperature of heat flux profiles. After four hours, the average boundary layer has grown five meters deeper in the case with dynamic heterogeneity, which is considered insignificant. The variance of horizontal wind speed, which is known to be very sensitive to turbulent motions is slightly decreased in the coupled LSM case. Overall we conclude that the small-scale and high-frequent soil-atmosphere feedback caused by turbulent eddies in the atmosphere is not significant for the state of the atmospheric boundary layer. This is in agreement with what we have expected, since it is known from literature that fixed heterogeneities at this small scale do not significantly alter the boundary layer characteristics