New Method Captures the Influence of Mountain Topography on Snow Distribution
Climate modelers have assumed that when sunlight reaches Earth, it encounters a flat surface. This assumption works well when there is no large variation in topography across each grid. However, when there are steep mountains, the flat-surface assumption does not capture features, such as shading or bright reflection from snow on sloping surfaces, that affect how long snow persists and even feedback to local precipitation. Department of Energy climate researchers have investigated the effects of 3D mountains/snow on solar (sunlight) flux distributions and their impact on surface hydrology over mountains in the western United States. The Weather Research and Forecasting (WRF) model was used in conjunction with a 3D radiative transfer parameterization covering a time period from 2007-2008 during which abundant snowfall occurred. A comparison of the 3D WRF simulation with observations showed reasonable agreement. The investigators showed that 3D mountain features have a profound impact on the diurnal and monthly variation of surface radiative and heat fluxes, and on the consequent elevation-dependence of snowmelt and precipitation distributions. Deviations of snow due to 3D radiation effects range from an increase of 18% at the lowest elevations to a decrease of 8% at higher elevations. Since lower elevation areas occupy larger fractions of the land surface, the net effect of 3D radiative transfer is to extend snowmelt and snowmelt-driven runoff into the warm season. Because 60-90% of water resources originate from mountains worldwide, the differences in simulated hydrology due to 3D interactions between solar radiation and mountains/snow merit further investigation in order to understand the implications of modeling mountain water resources and these resources’ vulnerability to climate change.
Liou, K. N., Y. Gu, L. R. Leung, W. L. Lee, and R. G. Fovell. 2013. “A WRF Simulation of the Impact of 3-D Radiative Transfer on Surface Hydrology over the Rocky Mountains and Sierra Nevada,” Atmospheric Chemistry and Physics 13, 11709-721. DOI:10.5194/acp-13-11709-2013.