Impact of 3-D topography on surface radiation budget over the Tibetan Plateau

Leung

et al. 2013

Self Cite

Abstract. We investigate 3-D mountains/snow effects on solar flux distributions and their impact on surface hydrology over the western United States, specifically the Rocky Mountains and Sierra Nevada. The Weather Research and Forecasting (WRF) model, applied at a 30 km grid resolution, is used in conjunction with a 3-D radiative transfer parameterization covering a time period from 1 November 2007 to 31 May 2008, during which abundant snowfall occurred. A 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 elevationdependence of snowmelt and precipitation distributions. In particular, during the winter months, large deviations (3-D-PP, in which PP denotes the plane-parallel approach) 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 the earlier morning. Over mountaintops higher than 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, which leads 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. Because 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 of modeling mountain water resources, and these resources' vulnerability to climate change and air pollution.

Section: Introductionmentioning

confidence: 90%

A WRF simulation of the impact of 3-D radiative transfer on surface hydrology over the Rocky Mountains and Sierra Nevada

Leung

et al. 2013

Self Cite

“…In conjunction with radiative transfer in mountains and snow regions, we have developed a Monte Carlo photon tracing program specifically applicable to intense and intricate inhomogeneous mountains and demonstrated that the effect of mountains on the surface radiative balance is substantial in terms of subgrid variability as well as domain average conditions (Liou et al, 2007;Lee et al, 2011Lee et al, , 2013. Because of the computational burden required by the 3-D Monte Carlo photon tracing program, an innovative parameterization approach has been developed in terms of deviations from PP radiative transfer results readily available in climate models for the five component of surface solar flux: direct and diffuse fluxes, direct and diffuse reflected fluxes, and coupled mountain-mountain flux (Lee et al, 2011).…”

Section: W-l Lee Et Al: a Global Model Simulation For 3-d Radiativmentioning

confidence: 99%

“…Because SRTM data cover the land surface between 56 • S and 60 • N, the parameterization is applied to all areas within this range. Moreover, Lee et al (2013) have shown that the parameterization can be applied to any grid box with a size larger than 10 × 10 km. Therefore, it is suitable for CCSM4 at a quarter-degree resolution.…”

Section: -D Radiation Parameterization In Ccsm4mentioning

confidence: 99%

A global model simulation for 3-D radiative transfer impact on surface hydrology over the Sierra Nevada and Rocky Mountains

Lee

et al. 2015

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Abstract. We investigate 3-D mountain effects on solar flux distributions and their impact on surface hydrology over the western United States, specifically the Rocky Mountains and the Sierra Nevada, using the global CCSM4 (Community Climate System Model version 4; Community Atmosphere Model/Community Land Model -CAM4/CLM4) with a 0.23 • × 0.31 • resolution for simulations over 6 years. In a 3-D radiative transfer parameterization, we have updated surface topography data from a resolution of 1 km to 90 m to improve parameterization accuracy. In addition, we have also modified the upward-flux deviation (3-D-PP (planeparallel)) adjustment to ensure that the energy balance at the surface is conserved in global climate simulations based on 3-D radiation parameterization. We show that deviations in the net surface fluxes are not only affected by 3-D mountains but also influenced by feedbacks of cloud and snow in association with the long-term simulations. Deviations in sensible heat and surface temperature generally follow the patterns of net surface solar flux. The monthly snow water equivalent (SWE) deviations show an increase in lower elevations due to reduced snowmelt, leading to a reduction in cumulative runoff. Over higher-elevation areas, negative SWE deviations are found because of increased solar radiation available at the surface. Simulated precipitation increases for lower elevations, while it decreases for higher elevations, with a minimum in April. Liquid runoff significantly decreases at higher elevations after April due to reduced SWE and precipitation.

“…In particular, we showed pertinent results for solar and IR radiative transfer for a 200 × 200 km 2 region centered at Lhasa over the Tibetan Plateau, employing 1 × 1 km 2 elevation data and 5 × 5 km 2 MODIS albedo data as boundary conditions in broadband flux calculations. A significant solar flux deviation of about 10-35 W m −2 from the flat surface of conventional GCMs and regional climate models would occur if realistic mountain features were not accounted for in surface energy balance modeling Lee et al, 2011Lee et al, , 2012). An error of such magnitude would have a substantial effect on the surface energy budgets over snow-covered mountains due to snow-albedo feedback, with significant ramifications for the seasonal variations of snowpack, runoff, and soil moisture, as well as the diabatic heating that drives regional and large-scale circulations.…”

Section: Introductionmentioning

confidence: 99%

Simulating 3-D radiative transfer effects over the Sierra Nevada Mountains using WRF

Lee

et al. 2012

Self Cite

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.