Seasonal and Inter-annual Variations of Top-of-Atmosphere Irradiance and Cloud Cover over Polar Regions Derived from CERES Data Set

CERES data taken over Arctic and Antarctic from March 2000 to February 2004 are analyzed to determine seasonal and inter-annual variations of top-of-atmosphere shortwave and longwave irradiances and cloud cover. The shortwave and longwave irradiances are derived from CERES radiances measurements by CERES angular distribution models. The daily mean shortwave irradiance is estimated using CERES directional models. Clouds are identified by the CERES cloud algorithm using MODIS radiances. The annual mean top-of-atmosphere albedo over Arctic (60N to 90N) and over Antarctic (60S to 90S) is 0.472 and 0.570, respectively. The annual mean top-of-atmosphere longwave irradiance is 201 W m^-2 for Arctic and 180 W m^-2 for Antarctic. The annual mean net irradiance of both regions are nearly equal, -93.5 W m^-2 for Arctic and -90.9 W m^-2 for Antarctic. The daytime cloud cover over Arctic between March 2000 and February 2004 increased at a rate of 1.5% per decade while the daytime snow and sea ice cover derived from microwave instruments decreased at a rate of 3.6% per decade during the same period. The shortwave and nighttime longwave irradiance over Arctic derived from CERES radiance measurements show no significant trend during this period. Because the cloud and snow and sea ice covers are two predominant properties affecting the top-of-atmosphere albedo, the result indicates that the albedo decrease due to the sea ice cover change is likely offset by the albedo increase due to the cloud cover change.