EVALUATION OF GLOBAL NUMERICAL WEATHER PREDICTION ANALYSES AND FORECASTS USING DMSP SPECIAL SENSOR MICROWAVE IMAGER RETRIEVALS .1. SATELLITE RETRIEVAL ALGORITHM INTERCOMPARISON STUDY

Special sensor microwave/imager (SMM/I) satellite retrieval schemes co vering a wide range of retrieval techniques were compared for environm ental parameters that play a key role in the atmospheric hydrological cycle. These were column integrated water vapor (IWV), nearsurface oce anic wind speed (SWS), liquid water path (LWP), and surface rain rate (SRR). The objective was to evaluate the accuracy and limitations of t he retrieval schemes on a monthly timescale (October 1993). The retrie val algorithms that were found to be the most accurate were used for t he evaluation of an analysis/forecast system whose results are present ed in part 2. The agreement between IWV retrieval algorithms was found to be good, in concordance with prior studies. However, depending on the range of IWV values considered, considerable biases were shown to exist. The choice of precipitation screen for which IWV retrievals wer e rejected had a non-negligible impact on the monthly statistics. Satu ration, which is often present in IWV retrieval algorithms, also influ enced these statistics. The agreement between different SWS retrieval algorithms was not as good as for IWV. The largest differences between the Wentz [1994] and the Goodberlet et al. [1989] retrieval algorithm s were found in the intertropical convergence zone (ITCZ) (largest dif ference) and in the midlatitudes. The impact of using different precip itation screens was also analyzed. SWS retrieved with the Goodberlet e t al. algorithm was sensitive (in particular in the ITCZ) to the amoun t of data rejected. Among the LWP retrieval algorithms considered, two algorithms gave acceptable and similar results except for a constant factor (1.75). It is argued that the discrepancy of a constant factor was in part due to the lack of in situ measurements for calibration an d the fact that the beam-filling problem was taken into account empiri cally. For SRR retrievals, considerable disagreement was demonstrated between different retrieval algorithms. However, several algorithms ga ve retrievals that were in reasonable agreement with long-term climato logical data sets. This agreement was best in the tropics and decrease d considerably in the midlatitudes where the retrieval capability of S RR for the SSM/I is not optimal.