Comparison of NCEP/NCAR and ERA-40 total cloud cover with surface observations over the Tibetan Plateau

The annual and seasonal total cloud cover (TCC) variations in the eastern and central Tibetan Plateau (TP) during 1961-2005 are analysed using 71 surface observational stations. The mean TCC decreases from the southeastern to the northwestern TP, consistent with the patterns of atmospheric moisture in the region. The annual mean TCC shows a significant decreasing trend of -0.09 percentdecade-1, mainly contributed by winter. About 65% of the stations show significant downward trends on the annual basis with large trend magnitudes occurring in the central TP. The seasonal patterns confirm the annual patterns in most cases. Compared with the surface observations, both National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis (NCEP/NCAR) (1961-2005) and ERA-40 (1961-2001) can reproduce the decreasing TCC trends. The shift of TCC before and after the mid-1980s is obvious in observations and both reanalyses, reflecting the changes of large-scale atmospheric circulation. However, NCEP/NCAR underestimates and ERA-40 overestimates observations on the annual and seasonal basis, presumably caused by the different cloud parameterization schemes. A Taylor diagram diagnose summarizes the discrepancies between observations and reanalyses.