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 USCGC Polar Sen during a research cruise to the Northeast Water Polynya during the summer of 1993 are analyzed together with observations of cloud type and amount to determine the effects of summertime Arctic clouds on the surface radiation budget. it is found that the solar zenith angle is critical in determining whether clouds heat or cool the surface. For large solar zenith angles (>similar to 80 degrees) the infrared heating effect of clouds is greater than the reduction in insolation caused by clouds, and the surface is heated by the presence of cloud. For smaller zenith angles, cloud cover cools the surface, and for intermediate zenith angles, the surface radiation budget is insensitive to the presence of, or changes in, cloud cover.