Extended Abstract (480K)P4.7
Impact of MODIS BRDF and Albedo on Land Surface Models
Zhuo Wang, University of Arizona, Tucson, AZ; and X. Zeng, M. Barlage, R. Dickinson, C. Schaaf, and F. Gao
The land surface albedo is strongly dependent on the solar zenith angle (SZA) and the 3-D structure of vegetation canopy. In the NCAR Community Climate System Model (CCSM2), it is calculated based on a two-stream approximation, which does not include the effect of 3-D vegetation structure on radiative transfer. The model monthly averaged albedo and SZA dependence of albedo are evaluated using the Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) and albedo data acquired during July 2001 - July 2002. The model monthly averaged albedos in February and July are close to the MODIS white-sky albedos (within 0.02 or statistically significant) over about 40% of the global land between 60S and 70N. However, CCSM2 significantly underestimates albedo by 0.05 or more over deserts (e.g., the Sahara Desert) and some semiarid regions (e.g., part of Australia). For eleven model grid cells with different dominant plant functional types, the model diffuse NIR albedo is higher by 0.05 or more than the MODIS white-sky albedo in five of these cells. Direct albedos from the model and MODIS (as computed using the BRDF parameters) increase with SZA, but model albedo increases faster than the MODIS data. The bare soil albedo is assumed to be a function of soil color and soil moisture but independent of SZA in most land surface models. However, analyses of MODIS BRDF and albedo data over thirty desert locations indicate that bare soil albedo does vary with SZA. In particular, bare soil albedo normalized by its value at 60( SZA can be adequately represented as (1+C)/(1+2C*cos(SZA)). Using the MODIS and in-situ data, the empirical parameter C is determined to be 0.16 which is much smaller than 0.4 as estimated in previous studies. The SZA dependence of soil albedo is also found to significantly affect the modeling of land surface energy balance over a desert site. These analyses and the MODIS BRDF/albedo data provide a starting point towards improving model treatment of land surface albedo.
Poster Session 4, LAND SURFACE ALBEDO AND ITS IMPACTS ON SURFACE CLIMATE POSTERS
Thursday, 13 January 2005, 9:45 AM-11:00 AM
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