Atmos. Chem. Phys. Discuss., 13, 19389-19419, 2013
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
A WRF simulation of the impact of 3-D radiative transfer on surface hydrology over the Rocky–Sierra Mountains
K. N. Liou1, Y. Gu1, L. R. Leung2, W. L. Lee3, and R. G. Fovell1
1Joint Institute for Regional Earth System Science and Engineering, Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA
2Pacific Northwest National Laboratory, Richland, WA, USA
3Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan

Abstract. Essentially all modern climate models utilize a plane-parallel (PP) radiative transfer approach in physics parameterizations; however, the potential errors that arise from neglecting three-dimensional (3-D) interactions between radiation and mountains/snow on climate simulations have not been studied and quantified. This paper is a continuation of our efforts to investigate 3-D mountains/snow effects on solar flux distributions and their impact on surface hydrology over the Western United States, specifically the Rocky and Sierra-Nevada Mountains. We use the Weather Research and Forecasting (WRF) model applied at a 30 km grid resolution with incorporation of a 3-D radiative transfer parameterization covering a time period from 1 November 2007 to 31 May 2008 during which abundant snowfall occurred.

Comparison of the 3-D WRF simulation with the observed snow water equivalent (SWE) and precipitation from Snowpack Telemetry (SNOTEL) sites shows reasonable agreement in terms of spatial patterns and daily and seasonal variability, although the simulation generally has a positive precipitation bias. We show that 3-D 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. In particular, during the winter months, large deviations (3-D–PP) of the monthly mean surface solar flux are found in the morning and afternoon hours due to shading effects for elevations below 2.5 km. During spring, positive deviations shift to earlier morning. Over the mountain tops above 3 km, positive deviations are found throughout the day, with the largest values of 40–60 W m−2 occurring at noon during the snowmelt season of April to May. The monthly SWE deviations averaged over the entire domain show an increase in lower elevations due to reduced snowmelt, leading to a reduction in cumulative runoff. Over higher elevation areas, positive SWE deviations are found because of increased solar radiation available at the surface. Overall, this study shows that deviations of SWE due to 3-D radiation effects range from an increase of 18% at the lowest elevation range (1.5–2 km) to a decrease of 8% at the highest elevation range (above 3 km). Since lower elevation areas occupy larger fractions of the land surface, the net effect of 3-D radiative transfer is to extend snowmelt and snowmelt-driven runoff into the warm season. Additionally, because about 60–90% of water resources originate from mountains worldwide, the aforementioned differences in simulated hydrology due solely to 3-D interactions between solar radiation and mountains/snow merit further investigation in order to understand the implications to modeling mountain water resources and their vulnerability to climate change and air pollution.

Citation: Liou, K. N., Gu, Y., Leung, L. R., Lee, W. L., and Fovell, R. G.: A WRF simulation of the impact of 3-D radiative transfer on surface hydrology over the Rocky–Sierra Mountains, Atmos. Chem. Phys. Discuss., 13, 19389-19419, doi:10.5194/acpd-13-19389-2013, 2013.
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