Cloud Forcing of Turbulent and Radiative Surface Energy Budgets on the Arctic Ice Cap: One Year of Data from the SHEBA Experiment (INVITED)

From November 1997 to October 1998 the Surface Heat Budget of the Arctic (SHEBA) engaged in a variety of atmospheric, ice, and oceanic measurements on the Arctic icecap as part of ice station SHEBA. The ice station was launched at 143 W and 75 N and ended at 166 W and 80 N. We believe this is the first set of eddy-correlation heat flux, radiative fluxes, and comprehensive cloud data obtained over an entire annual cycle on the ice cap. In this paper we will present an analysis of various aspects of forcing of surface fluxes by clouds during SHEBA. Cloud forcing is the difference in the mean flux and what it would be if clouds were removed from the system. We present a unique calculation of Arctic surface cloud forcing over an annual cycle exclusively utilizing ground-based remote and in situ sensors deployed as part of the SHEBA project. The measurements incorporated as part of this study included optical radiometer sets, turbulent fluxes, a depolarization lidar, ceilometer, a microwave water vapor/liquid radiometer, and radiosondes. The lidar and ceilometer are used to define cloud fraction and cloud base height. The lidar and microwave radiometer allow us to differentiate clouds with water from pure ice clouds. Modern clear-sky radiative flux models tuned to the observations are found to give estimates of cloud radiative forcing that are better than direct observation of fluxes in cloud-free conditions. This is because the cloud fractions are very high in summer so the statistical sampling errors are too large, particularly for shortwave flux. Using these data, we have processed the time series in 20-day blocks to give the annual evolution of the components of the surface cloud forcing: upward, downward, and net longwave (LW) and shortwave (SW) radiative fluxes and the sensible and latent turbulent heat fluxes. Total cloud forcing (the sum of all components) is about +20 Wm^-2 for most of fall, winter, and spring and dips to a minimum of -20 Wm^-2 around July 1. The duration of the negative period is about one month, depending on the choice of surface albedo.