An annual cycle of Arctic surface cloud forcing at SHEBA

Abstract

We present an analysis of surface fluxes and cloud forcing from data obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, conducted in the Beaufort and Chuchki Seas and the Arctic Ocean from November 1997 to October 1998. The measurements used as part of this study include fluxes from optical radiometer sets, turbulent fluxes from an instrumented tower, cloud fraction from a depolarization lidar and ceilometer, and atmospheric temperature and humidity profiles from radiosondes. Clear‐sky radiative fluxes were modeled in order to estimate the cloud radiative forcing since direct observation of fluxes in cloud‐free conditions created large statistical sampling errors. This was particularly true during summer when cloud fractions were typically very high. A yearlong data set of measurements, obtained on a multiyear ice floe at the SHEBA camp, was processed in 20‐day blocks to produce the annual evolution of the surface cloud forcing components: upward, downward, and net longwave and shortwave radiative fluxes and turbulent (sensible and latent heat) fluxes. We found that clouds act to warm the Arctic surface for most of the annual cycle with a brief period of cooling in the middle of summer. Our best estimates for the annual average surface cloud forcings are −10 W m−2 for shortwave, 38 W m−2 for longwave, and −6 W m−2 for turbulent fluxes. Total cloud forcing (the sum of all components) is about 30 W m−2 for the fall, winter, and spring, dipping to a minimum of −4 W m−2 in early July. We compare the results of this study with satellite, model, and drifting station data.