Effects of snow physical parameters on shortwave broadband albedos

摘要

Snow pit work of several-day intervals was performed simultaneously with radiationbudget observations during two winters in eastern Hokkaido, Japan. From these data weinvestigated the effects of elapsed time after snowfall (snow aging), air temperature, snowsurface temperature, snow grain size, and snow impurities on the visible and the nearinfrared albedos for improving the snow albedo scheme in the land surface process froman empirical model to a physically based model. The dependence of albedos on elapsedtime after snowfall could be clearly classified by dividing the snow-covered period into adry snow season and a wet snow season rather than by snow surface temperature. Thealbedo reduction by snow aging statistically depends on the snow surface temperature,which is often used to predict the snow albedo in the empirical model of land surfaceprocess. However, the albedo reduction rate was very scattered for snow surfacetemperatures above 10 C. This is because the snow albedo reduction essentially dependson the snow grain size and the concentration of snow impurities. Using the radiativetransfer model for the atmosphere-snow system, the effects of these snow physicalparameters on broadband albedos are calculated and compared with the observed ones.The measured broadband albedos fell close to the range of theoretically calculated ones asfunctions of these snow physical parameters. In particular, the measured near infraredalbedo agreed well with the theoretically calculated ones both for the dependence of snowgrain size and snow impurities but not as well for the visible albedo in detail. In the nearinfrared region the light absorption by ice is strong, and thus the snow albedo contains theinformation of snow physical parameters near the surface where these parameters aremeasured. In contrast, the visible albedo contains the snow information in the deeper layerbecause the ice is relatively transparent in the visible region. This suggests the necessity ofthe multiple-snow-layer model for the visible region in the physically based snow albedomodel.