Cloud Radiative Forcing of the Arctic Surface: The Influence of Cloud Properties, Surface Albedo, and Solar Zenith Angle

Abstract

An annual cycle of cloud and radiation measurements made as part of the Surface Heat Budget of the Arctic (SHEBA) program are utilized to determine which properties of Arctic clouds control the surface radiation balance. Surface cloud radiative forcing (CF), defined as the difference between the all-sky and clear-sky net surface radiative fluxes, was calculated from ground-based measurements of broadband fluxes and results from a clear-sky model. Longwave cloud forcing (CFLW) is shown to be a function of cloud temperature, height, and emissivity (i.e., microphysics). Shortwave cloud forcing (CFSW) is a function of cloud transmittance, surface albedo, and the solar zenith angle. The annual cycle of Arctic CF reveals cloud-induced surface warming through most of the year and a short period of surface cooling in the middle of summer, when cloud shading effects overwhelm cloud greenhouse effects. The sensitivity of CFLW to cloud fraction is about 0.65 W m?2 per percent cloudiness. The sensitivity of CFSW to cloud fraction is a function of insolation and ranges over 0?1.0 W m?2 per percent cloudiness for the sun angles observed at SHEBA. In all seasons, liquid-containing cloud scenes dominate both LW and SW radiative impacts on the surface. The annual mean CFLW (CFSW) for liquid-containing and ice-only cloud scenes is 52 (?21) and 16 (?3) W m?2, respectively. In the LW, 95% of the radiatively important cloud scenes have bases below 4.3 km and have base temperatures warmer than ?31°C. The CFLW is particularly sensitive to LWP for LWP < 30 g m?2, which has profound implications in the winter surface radiation balance. The CFSW becomes more negative as surface albedo decreases and at higher sun elevations. Overall, low-level stratiform liquid and mixed-phase clouds are found to be the most important contributors to the Arctic surface radiation balance, while cirrus clouds and diamond dust layers are found to have only a small radiative impact on the Arctic surface.