Comparison between Total Cloud Cover in Four Reanalysis Products and Cloud Measured by Visual Observations at US Weather Stations

A homogeneity-adjusted dataset of total cloud cover from weather stations in the contiguous United States is compared with cloud cover in four state-of-the-art global reanalysis products: the Climate Forecast System Reanalysis from NCEP, the Modern-Era Retrospective Analysis for Research and Applications from NASA, ERA-Interim from ECMWF, and the Japanese 55-year Reanalysis Project from the Japan Meteorological Agency. The reanalysis products examined in this study generally show much lower cloud amount than visual weather station data, and this underestimation appears to be generally consistent with their overestimation of downward surface shortwave fluxes when compared with surface radiation data from the Surface Radiation Network. Nevertheless, the reanalysis products largely succeed in simulating the main aspects of interannual variability of cloudiness for large-scale means, as measured by correlations of 0.81-0.90 for U.S. mean time series. Trends in the reanalysis datasets for the U.S. mean for 1979-2009, ranging from 20.38% to 21.8% decade 21, are in the same direction as the trend in surface data (20.50% decade 21), but further effort is needed to understand the discrepancies in their magnitudes.