Effects of Soil Moisture Variations on Boundary Layer Characteristics: Numerical Simulations using WRF

Soil moisture is one of the key factors in land surface processes. In this paper a series of numerical experiments are conducted using the Weather Research and Forecasting model (WRF) to understand how the changes in soil moisture affect the surface flux partitioning, boundary layer structure, and cloud processes. The results are then compared with observations. The domain size is of 1800 km by 1440 km with the resolution of 12 km by 12 km, centering at the ARM SGP site (36.605oN, 97.485oW). Forty two full sigma levels are specified in the vertical direction with 19 levels below 850 hPa. The pressure at the top of the model is set to 100 hPa. The physical parameterization schemes in WRF includes Noah land surface scheme, Mellor-Yamada-Janjic TKE scheme, Betts-Miller-Janjic convection scheme, RRTM longwave radiation scheme, the simple short wave radiation scheme, and the Lin et al. microphysics scheme. Our results indicated that soil moisture has a great impact on the surface heat flux partitioning. A decrease in soil moisture leads to an increase in sensible heat flux and a reduction in latent heat flux (i.e., an increase in Bowen ratio). The consequences of these changes have large implications on boundary layer development; increased sensible heat flux causes increased turbulence kinetic energy that drives boundary layer development. As a result, a higher and drier boundary layer is obtained through a stronger active BL turbulence and entrainment. As a consequence, boundary layer clouds show distinctive characteristics. With dominant sensible heat flux resulting from decreased soil moisture, higher cloud base with decreased total cloud water content as well as enhanced updrafts in the cloud layer is obtained from our simulations.