IMPACT OF SOIL HETEROGENEITY IN A MIXED-LAYER MODEL OF THE PLANETARY BOUNDARY-LAYER

The dynamics of soil moisture and temperature states are forced by lan d surface fluxes that form the coupling between the soil and the atmos phere. This two-way interaction between the soil and the atmosphere re sults in feedback mechanisms that affect the temporal patterns of vari ability of the system. To understand these characteristics of soil moi sture and temperature variability, a one-dimensional coupled mixed-lay er model of the land surface and the planetary boundary layer energy a nd humidity budgets is constructed. To allow integrations on the order of days, the model includes both the growth phase and the collapse ph ase of mixed layer evolution. The model includes the radiative effects due to the presence of clouds. Given the incoming shortwave radiation , lapse rates of temperature and humidity above the mixed layer and la teral windspeed, the model predicts all longwave radiative and turbule nt surface heat fluxes. The model is shown to capture accurately obser vations taken during the FIFE field experiment. With this model, the i mpact of soil heterogeneity on the evolution of the surface and mixed- layer energy and humidity budgets is examined, using hydraulic propert ies from sand, loam and clay soils. It is assumed that the small corre lation length scale of soil hydraulic properties and surface inhomogen eities are averaged out in the mixed layer. Relative to a uniform soil , heterogeneity increases the spatial mean latent heat flux for conduc tive soils and decreases it for unconductive soils, due to decreased a nd increased percolation respectively. Locally decreasing latent heat fluxes cause a warmer and drier mixed layer and through that an increa se of the latent heat flux over other areas. This mixed-layer feedback reduces the impact of heterogeneous surface fluxes.