Impact Study of Surface Albedo in US Navy Global Weather Prediction System

Albedo is the fraction of the incident radiation that is reflected from the surface. It plays a major role in the global energy balance as it defines the portion of absorbed solar radiation by the earth surface. The accuracy of radiative transfer modeling highly depends on the albedo of various surfaces from ocean, land to sea ice. Therefore, albedo is also an important component of surface boundary condition in a global weather/climate prediction system. The albedo package of the Global Forecast System (GFS) of the National Centers for Environmental Prediction has been implemented into the Navy Global Environmental Model (NAVGEM). It contains the global distributions of seasonal variations, based on which, instantaneous albedo is calculated with a dependence on the solar zenith angle including snow effect over land. In this study, GFS albedo is compared with NAVGEM albedo that takes different climate values and has no dependence on solar zenith angle. The modeled solar fluxes at the surface and the top of atmosphere are verified with satellite retrievals and ARM observations for two different approaches of albedo in NAVGEM by performing data-assimilation update cycles and free long forecasts. The comparisons reveal that the albedo with solar-zenith-angle dependence is a more accurate approach in predicting solar fluxes from time to time, which lead to more accurate net radiative fluxes. On the average, GFS albedo helps to reduce the surface temperature bias of NAVGEM, especially in long term forecasts.

Albedo is the fraction of the incident radiation that is reflected from the surface. It plays a major role in the global energy balance as it defines the portion of absorbed solar radiation by the earth surface. The accuracy of radiative transfer modeling highly depends on the albedo of various surfaces from ocean, land to sea ice. Therefore, albedo is also an important component of surface boundary condition in a global weather/climate prediction system. The albedo package of the Global Forecast System (GFS) of the National Centers for Environmental Prediction has been implemented into the Navy Global Environmental Model (NAVGEM). It contains the global distributions of seasonal variations, based on which, instantaneous albedo is calculated with a dependence on the solar zenith angle including snow effect over land. In this study, GFS albedo is compared with NAVGEM albedo that takes different climate values and has no dependence on solar zenith angle. The modeled solar fluxes at the surface and the top of atmosphere are verified with satellite retrievals and ARM observations for two different approaches of albedo in NAVGEM by performing data-assimilation update cycles and free long forecasts. The comparisons reveal that the albedo with solar-zenith-angle dependence is a more accurate approach in predicting solar fluxes from time to time, which lead to more accurate net radiative fluxes. On the average, GFS albedo helps to reduce the surface temperature bias of NAVGEM, especially in long term forecasts.