Abstract
Abstract. A surface solar radiation parameterization based on deviations between 3-D and conventional plane-parallel radiative transfer models has been incorporated 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 testbed, we show that mountain effect could produce up to −50 to + 50 W m−2 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 snow albedo associated with more solar insolation due to mountain effect. Soil moisture increases on the sunny side of the mountains due to enhanced snowmelt, while decreases on the shaded side. Substantial differences are found in the morning hours from 8–10 a.m. and in the afternoon around 3–5 p.m., while differences around noon and in the early morning and late afternoon are comparatively smaller. Variation in the surface energy balance can also affect atmospheric processes, such as cloud fields, through the modulation of vertical thermal structure. Negative changes of up to −40 g m−2 are found in the cloud water path, associated with reductions in the surface insolation over the cloud region. The day-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 m−2. Changes in sensible and latent heat fluxes and surface skin temperature follow the solar insolation pattern. Differences in the domain-averaged diurnal variation over the Sierras show that the mountain area receives more solar insolation during early morning and late afternoon, resulting in enhanced upward sensible heat and latent heat fluxes from the surface and a corresponding increase in surface skin temperature. 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. The hourly differences of the surface solar insolation in higher elevated regions, however, show smaller magnitude in negative changes during the middle of the day and possibly more solar fluxes received during the whole day.
Highlights
The energy emitted by the sun and received by the Earth’s surface is determined by three sets of factors
Using the Sierra-Nevada in the Western United States as a testbed, the model simulation results show that the mountain effect would produce up to −50∼50 W m−2 deviations in the surface solar fluxes, resulting in a temperature change of up to ±1 ◦C
Upward surface sensible and latent heat fluxes are modulated to compensate for the changes in surface solar fluxes
Summary
The energy emitted by the sun and received by the Earth’s surface is determined by three sets of factors. The purpose of the present parameterization effort is to produce relative deviations of the fluxes from those of a flat surface with the same elevation, given subgrid scale topographic information, which includes the mean values for the sky view factor, the terrain configuration factor, and the cosine of the solar zenith angle, and conventional topographic parameters for each 10 × 10 km domain involving mean elevation and slope in multiple linear regression analysis, along. The current regression equations developed for Fd∗if, Fr∗dif,andFc∗oup, which are small values as compared to Fd∗ir and Fr∗dir, could serve as the first-order approximation for corrections to surface solar fluxes determined from plane-parallel radiative transfer models over mountains covered by clouds. Since the value Fc∗oup is not a linear function with respect to surface albedo, 4 sets of coefficients that cover a range of albedo are employed in determining the regression coefficients
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