Global Model Simulation for 3-D Radiative Transfer Impact on Surface Hydrology
Orographic forcing is an efficient and dominant mechanism for harnessing water vapor into consumable fresh water in the form of precipitation, snowpack, and runoff. Mountain water resources not only support human activities, but are also vital to diverse terrestrial and aquatic ecosystems. To study the long-term effect of solar radiation effect over three-dimensional (3-D) mountains and snow on surface energy and hydrology, the 3-D radiative transfer parameterization developed for the computation of surface solar fluxes has been incorporated into the Community Climate System Model version 4 [(CCSM4); Community Atmosphere Model version 4 (CAM4)/Community Land Model version 4 (CLM4)] global model and applied at a resolution of 0.23°x0.31° over the Rocky Mountains and Sierra Nevada areas in the western United States. In the 3-D radiative transfer parameterization, the surface topography data have been updated from a resolution of 1 km to 90 meters to improve parameterization accuracy. In addition, the upward-flux deviation [3D–plane-parallel (PP)] adjustment has also been modified to ensure that energy balance at the surface is conserved in global climate simulations based on 3-D radiation parameterization. Findings show that deviations of the net surface fluxes are not only affected by 3-D mountains, but also influenced by feedbacks of clouds and snow in conjunction with long-term simulations. Deviations in the sensible heat and surface temperature generally follow the patterns of net surface solar flux. Including 3D-mountain effects significantly increases (decreases) solar radiation at higher (lower) elevations, leading to increased (reduced) snowmelt. Combined with precipitation changes influenced by changes in the surface fluxes, runoff is significantly reduced in mountainous regions after the snow accumulation peaks in April. The 3-D mountain effects could have an important impact on vegetation by changing the energy and water available to plants. With the larger differences in solar radiation, soil moisture, and soil temperature developing in late spring and early summer, changes in photosynthetic rate and plant phenology may affect leaf area index and gross primary production. These findings will be further investigated in the future using longer simulations to quantify the 3-D mountain effects on radiation and the impacts on water and carbon cycles and vegetation globally.
Lee, W.-L., Y. Gu, K. N. Liou, L. R. Leung, and H.-H. Hsu. 2015. “A Global Model Simulation for 3-D Radiative Transfer Impact on Surface Hydrology over the Sierra Nevada and Rocky Mountains,” Atmospheric Chemistry and Physics 15, 5405-13. DOI: 10.5194/acp-15-5405-2015.