Modeling Clouds and Radiation for the November 1997 SHEBA Using a Column Climate Model.
Pinto, J.O., Curry, J.A., Lynch, A.H., and Persson, P.O.G.
Abstract
A column version of the Arctic Region Climate System Model (ARCSyM) has been developed for testing GCM parameterizations in the Arctic. The ARCSyM column model has been employed for a 23 day period in November to simulate conditions over a multi-year ice floe that has been the site of intensive observation as part of the SHEBA (Surface HEat Budget of the Arctic Ocean) project. The large-scale tendencies of temperature, moisture and wind are specified with values obtained from a special ECMWF column data set. The surface temperature is either specified from observations or simulated using an interactive sea ice model. The ARCSyM column simulations are compared with SHEBA observations to assess model biases. The simulated boundary layer is generally too warm and moist. Cloud fraction is fairly well simulated while precipitation amounts are generally too low. The modeled net longwave radiation at the surface shows a negative bias (surface cooling) which has been related to problems with the treatment of clear-sky radiative transfer and in the simulated cloud optical properties. The longwave surface cooling bias is partially offset by a warming bias in the modeled sensible heat flux at the surface. The importance of properly treating longwave radiative transfer under extreme cold, clear-sky conditions is evident in the sensitivity studies. The sensitivity studies also revealed that the importance of allowing supercooled liquid water clouds to occur. The occurrence of supercooled clouds in the simulation dramatically reduced longwave cooling at the surface due to increases in the optical depth and fractional coverage of clouds. Results from a coupled sea ice/ocean/atmosphere version of column ARCSyM are also discussed.