The influence of solar zenith angle and cloud type on cloud radiative forcing at the surface in the Arctic

Measurements of the long- and shortwave incident radiation taken from the U SCGC Polar Sen during a research cruise to the Northeast Water Polynya duri ng the summer of 1993 are analyzed together with observations of cloud type and amount to determine the effects of summertime Arctic clouds on the sur face radiation budget. it is found that the solar zenith angle is critical in determining whether clouds heat or cool the surface. For large solar zen ith angles (>similar to 80 degrees) the infrared heating effect of clouds i s greater than the reduction in insolation caused by clouds, and the surfac e is heated by the presence of cloud. For smaller zenith angles, cloud cove r cools the surface, and for intermediate zenith angles, the surface radiat ion budget is insensitive to the presence of, or changes in, cloud cover.