Impact of soil heterogeneity in a mixed-layer model of the planetary boundary layer

Abstract

The dynamics of soil moisture and temperature states are forced by land surface fluxes that form the coupling between the soil and the atmosphere. This two-way interaction between the soil and the atmosphere results in feedback mechanisms that affect the temporal patterns of variability of the system. To understand these characteristics of soil moisture and temperature variability, a one-dimensional coupled mixed-layer model of the land surface and the planetary boundary layer energy and humidity budgets is constructed. To allow integrations on the order of days, the model includes both the growth phase and the collapse phase 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 lateral windspeed, the model predicts all longwave radiative and turbulent surface heat fluxes. The model is shown to capture accurately observations taken during the FIFE field experiment. With this model, the impact of soil heterogeneity on the evolution of the surface and mixed-layer energy and humidity budgets is examined, using hydraulic properties from sand, loam and clay soils. It is assumed that the small correlation length scale of soil hydraulic properties and surface inhomogeneities are averaged out in the mixed layer. Relative to a uniform soil, heterogeneity increases the spatial mean latent heat flux for conductive soils and decreases it for unconductive soils, due to decreased and increased percolation respectively. Locally decreasing latent heat fluxes cause a warmer and drier mixed layer and through that an increase of the latent heat flux over other areas. This mixed-layer feedback reduces the impact of heterogeneous surface fluxes.