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Errors in Polar Clouds and Radiative Forcing in the Reanalysis Models

To assess the performance of numerical models in simulating polar clouds, surface radiative fluxes, and corresponding cloud-radiative interactions, we compare the observed (ARM, Barrow) cloud fractions, downwelling shortwave and longwave fluxes and the cloud radiative forcing against those obtained by four recent reanalysis projects (NCEP/NCAR, ERA40, JRA-25, and NARR) for Arctic locations. For a summer month (June 2001), the ERA40 reasonably simulates cloud fraction timeseries correlating at 0.87 with the corresponding observed ARM data. Simulated cloud fractions for the NCEP/NCAR reanalysis, however, correlate at only 0.29 missing an extended period of low, thin clouds early in the month. The JRA-25 cloud fractions correlate with observations at 0.60. Values of downwelling shortwave flux for all the reanalyses are generally much too high with errors up to 300 Wm -2 during cloudy periods at the daily solar maxima, indicating that the models are not correctly capturing the radiative and/or spatial characteristics of the observed cloud cover. Downwelling longwave radiative fluxes appear to be more correctly simulated than the shortwave flux when the models correctly simulate cloud fractions (i.e., ERA40 – frequently, NCEP/NCAR – infrequently). Periods with discrepancies between observed and simulated cloud fractions can have large errors (~75Wm -2 ) in the downwelling radiation flux. The net effect of clouds on the radiative balance at the polar surface is seen by evaluating the cloud radiative forcing produced by each reanalysis model as a function of season and cloud fraction for each reanalysis’ period of record. The cloud radiative forcing simulated by all the models agree in a seasonal sense with the signs of the observed forcing: positive during winter, spring and autumn and negative in the summer. The magnitudes of the cloud radiative forcing decrease linearly as a function of cloud fraction for observations and the NCEP/NCAR reanalysis, but decrease much more rapidly in the ERA40 and JRA-25 than observations with decreasing cloud fraction. While these models appear to be more successful in correctly replicating the observed cloud fraction than the NCEP/NCAR model, the simulated clouds in these models may be too optically thin and the models too insensitive to radiative effects of cloud covers less than 80-90%.