3-D Radiative Transfer Impact on Surface Hydrology Over Sierra Nevada and Rocky Mountains in WRF and CCSM4

Wednesday, May 14, 2014 - 07:00
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We have developed a Monte Carlo photon tracing program specifically applicable to intense and intricate inhomogeneous mountains, followed by an efficient regression parameterization in reference to deviations from plane-parallel radiation results readily available in climate models for direct and diffuse solar fluxes in terms of a number of topographic parameters. The regression-based parameterization has been successfully incorporated into WRF in connection with the Fu-Liou-Gu radiation scheme included in the WRF physics package. We investigated 3-D mountain/snow effects on solar flux distribution and their impact on surface hydrology over the western United States, specifically Rocky Mountains and Sierra Nevada using a 30 km resolution WRF covering a time period from 11/1/2007 to 5/31/2008. Comparison of the simulation with the observed SWE and precipitation from SNOTEL sites shows reasonable agreement in terms of spatial patterns and daily and seasonal variability. 3-D mountain features have a profound impact on the diurnal and monthly variation of surface radiative and heat fluxes and on the elevation-dependence of snowmelt and precipitation distributions. SWE deviations due to 3-D effects range from an increase of 18% at the lowest elevation to a decrease of 8% at the highest elevation. Since lower elevation areas occupy larger fractions of the land surface, the net 3-D radiation effect is to extend snowmelt and snowmelt-driven runoff into the warm season. An updated 3-D radiation parameterization has also been incorporated into CCSM4 with a 0.25o resolution for a 3-year run to study the mountains/snow effect on climate simulations. Deviations of net surface solar flux depict patterns opposite to those of cloud fraction. Changes in sensible heat and surface temperature follow the patterns of net surface solar flux. Also, monthly SWE deviations averaged over the entire domain show an increase at lower elevations due to reduced snowmelt, leading to a reduction in cumulative runoff. Over higher elevation areas, negative SWE deviations are seen because of increased availability of surface solar fluxes. Simulated precipitation increases at lower elevations while decreasing at higher elevations. Runoff significantly decreases at higher elevations after April due to reduced SWE and precipitation. Additionally, we have examined the 3-D mountain effect on vegetation in CCSM4 results. Changes in the vegetation absorbed solar flux, sensible heat, and temperature basically follow the deviation patterns in net surface solar flux. The leaf area index increases at all elevations - the greatest increases occur at the mountain top induced by the largest available solar flux.