Modeling clouds and radiation for the November 1997 period of SHEBA using a column climate model

A column version of the Arctic regional climate system model (ARCSYM) has b een developed for testing general circulation model parameterizations in th e Arctic. The ARCSYM column model has been employed for a 23-day period in November to simulate conditions over a multiyear ice flee that has been the site of intensive observations as part of the Surface Heat Budget of the A rctic (SHEBA) project. The large-scale tendencies of temperature, moisture, and wind are specified with values obtained from a special column data set obtained from the European Centre for Medium-Range Weather Forecasting. Co mparisons between the ARCSYM column simulations and SHEBA data reveal that modeled temperature profiles are too cold aloft and generally too warm in t he boundary layer. The occurrence of low clouds is severely underpredicted while the high cloud fraction is over predicted. The modeled longwave radia tive cooling at the surface is 1.5-3 times as large as that observed. Much of this bias is related to problems with the treatment of clear-sky radiati ve transfer and in the simulated cloud optical properties. At the same time , the magnitude of modeled downward sensible heat flux at the surface is mu ch too large. This has been related, in part, to the method for scaling tem perature at the lowest modeled level to its surface air value under conditi ons of strong static stability. The importance of properly treating longwav e radiative transfer under extremely cold, clear-sky conditions is evident in the sensitivity studies. The best simulation of cloud properties was ach ieved by assuming liquid cloud processes and properties at temperatures abo ve 255 K. This temperature is significantly colder than that used in many c limate models. The occurrence of supercooled clouds in the simulation drama tically reduced longwave cooling at the surface due to increases in the opt ical depth and fractional coverage of clouds. Results from a coupled sea ic e-atmosphere simulation reveal that improvements in the atmospheric paramet erizations are enhanced when the system is coupled.