EVALUATION OF THE GLOBAL ATMOSPHERIC MOISTURE BUDGET AS SEEN FROM ANALYSES

For the period 1987 to 1993, quantities central to the global moisture budget from the global analyses of the European Centre for Medium-Ran ge Weather Forecasts (ECMWF), the U.S. National Meteorological Center (NMC), and NASA/Goddard have been computed and compared. The precipita ble water is computed and compared with satellite data from the Specia l Sensor Microwave Imager (SSM/I). Fluxes of moisture and their diverg ence have been used to estimate the vertically integrated moisture bud get and thus evaporation minus precipitation (E - P) as residuals. Res ults of several test computations show that small biases exist in prec ipitable water as vertical resolution and methods of computing vertica l integrals are changed, but the impact is small on the moisture budge t. In the Tropics and subtropics the moisture budget is dominated by t he divergence field rather than the moisture amounts, and consequently initialization of the analyses has an impact on the perceived moistur e budget. The diurnal cycle is shown to be important especially for E - P. For the vertically integrated moisture budget, the use of pressur e coordinates instead of the model coordinates on which the data are a nalyzed produces acceptably small differences for the most part, altho ugh adequate resolution at low levels and proper treatment of the surf ace is important. Computations at different horizontal resolutions rev eal the importance of adequate resolution in the vicinity of steep oro graphy but also that resolution is not a big factor on large scales ev en where steep gradients in precipitation exist. The implication is th at it is the veracity bf the large-scale divergence and moisture field s themselves that contribute to problems, and these arise from the moi st physics of the assimilating model used in four-dimensional data ass imilation, which dominates the character of the analyses. However, in the subtropics large positive biases are present in precipitable water in the ECMWF and, to a lesser extent, NMC analyses and are partly due to the assimilation of biased TOVS retrievals. The effects of the lar ge and spurious changes in analysis systems at ECMWF and NMC are manif ested in the results. Differences between ECMWF and NMC E - P locally are 60%-75% of the values themselves in the Tropics at T31 resolution and about one-half as much at T15 resolution, and they are not diminis hing with time. By making use of estimates off and P from the NASA/God dard reanalysis, both spatial and temporal variability of E - P are fo und to be dominated by the P field. Accordingly, for part of 1987 and 1988, estimates of precipitation from the Global Precipitation Climato logy Project are used to qualitatively assess the E - P estimates. The ECMWF estimates appear to be best at that time, but since then the NM C values have become less intense while the ECMWF estimates have great ly intensified. The large differences that still exist in such estimat es are apt to be carried over to the NMC and ECMWF reanalysis products .