Cloud forcing of surface radiation in the North Water Polynya during NOW '98

Measurements taken from the CCGS Pierre Radisson in the North Water Polynya in spring and summer 1998, are used to study the effects of clouds on the longwave and shortwave components of the Arctic surface radiation budget. T he clear-sky baseline is provided by parametrizations taken from the litera ture, tuned to the clear-sky measurements taken during the cruise. The effe cts of clouds are examined in terms of cloud cover, cloud type, solar zenit h angle and time of day. A diurnal signal in the cloud cover is found, with minimum values at local noon. The cloud forcing of the surface radiation w as found to be mostly negative (i.e., clouds predominantly cool the surface when the sun is in the sky), with strong dependences on the solar zenith a ngle, cloud amount and whether the sun is obscured by clouds or is in clear sky; the influence of cloud type was secondary to these parameters.