3D Radiative Transfer Parameterization and the Effect on Surface Processes Based on WRF Simulations

A surface solar radiation parameterization has been developed on the basis of deviations between the 3D Monte Carlo photon-tracing program and conventional plane-parallel radiative transfer models. The downward surface solar flux over mountains are calculated by means of multiple linear regression equations with topographic information, including the solar incident angle, sky view factor, and terrain configuration factor, as input variables. We have incorporated the parameterization into the Weather Research and Forecasting (WRF) model to understand the solar insolation over mountain/snow areas and to investigate the impact of the spatial and temporal distribution and variation of surface solar fluxes on land-surface processes. Using the Sierra Nevada in the western United States as a test bed, we show that mountain effect could produce up to -50 to +50 W/m2 deviations in the surface solar fluxes over the mountain areas, resulting in a temperature increase of up to 1°C on the sunny side. Upward surface sensible and latent heat fluxes are modulated accordingly to compensate for the change in surface solar fluxes. Snow water equivalent and surface albedo both show decreases on the sunny side of the mountains, indicating more snowmelt and hence reduced albedo associated with more solar insolation absorbed by the surface due to mountain effect. The daily-averaged deviations in the surface solar flux are positive over the mountain areas and negative in the valleys, with a range between -12 ~ 12 W/m2. Differences in the domain-averaged diurnal variation over the Sierra Nevada show that the mountain area receives more solar insolation during early morning and late afternoon. During the middle of the day, however, the surface insolation and heat fluxes show negative changes, indicating a cooling effect. Hence overall, the diurnal variations of surface temperature and surface fluxes in the Sierra Nevada are reduced through the interactions of radiative transfer and mountains.