The NVAP global water vapor data set: independent cross-comparison and multiyear variability

Space-time variability in the global distribution of atmospheric total colu mn water vapor (tcwp) greatly impacts the hydrologic cycle. NASA's Water Va por Project (NVAP) produced a global 1 degrees x1 degrees tcwp data set for use as a tool to investigate, among other things, atmospheric variability. An independent cross-comparison of the NVAP tcwp product was performed usi ng the TOPEX/POSEIDON (TIP) TOPEX microwave radiometer (TMR) data and the E uropean Centre for Medium-Range Weather Forecasts (ECMWF)-based range delay data set produced by Meteo-France (MF) and distributed with T/P data. When these T/P range delay data are converted to tcwp, they show that NVAP is b iased dry and ECMWF/MF is biased wet relative to the independent TMR measur ement. Although the absolute accuracy of the NVAP tcwp product is uncertain , results indicate its relative accuracy is sufficient for variability stud ies. Empirical orthogonal function (EOF) analysis and spectral analysis app lied to this data set show that seasonal variability over the annual cycle accounts for about 20% of the variance (EOF1). An El Nino-southern oscillat ion (ENSO) signal is found in the annually demeaned data; the magnitude of the crosscorrelation between the temporal amplitude (TA) of EOF1 and the Ni no 3.3 (SST) time series is .9. Comparisons also were made between the NVAP patterns of variability in tcwp and independent reanalysis and interpretat ion of numerical model generated atmospheric fields. In general, there is g ood agreement between the NVAP data and the reanalysis fields. Finally, spe cific recommendations are made for: (1) improvement of the NVAP data set up on reanalysis and (2) use of the NVAP data, in place of ECMWF/MF-based rang e delay data, for T/P retrievals when TMR data are not available if and whe n T/P data are reanalyzed. This latter recommendation is especially importa nt for regions of the tropical Indo-Pacific (e.g., Indonesia) where islands can interfere with valid TMR retrievals. (C) 2001 Elsevier Science Inc. Al l rights reserved.