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 covering a wide range of retrieval techniques were compared for environmental parameters that play a key role in the atmospheric hydrological cycle. These were column integrated water vapor (IWV), nearsurface oceanic wind speed (SWS), liquid water path (LWP), and surface rain rate (SRR). The objective was to evaluate the accuracy and limitations of the retrieval schemes on a monthly timescale (October 1993). The retrieval algorithms that were found to be the most accurate were used for the evaluation of an analysis/forecast system whose results are presented 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 were rejected had a non-negligible impact on the monthly statistics. Saturation, which is often present in IWV retrieval algorithms, also influenced 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 algorithms were found in the intertropical convergence zone (ITCZ) (largest difference) and in the midlatitudes. The impact of using different precipitation screens was also analyzed. SWS retrieved with the Goodberlet et al. algorithm was sensitive (in particular in the ITCZ) to the amount 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 and the fact that the beam-filling problem was taken into account empirically. For SRR retrievals, considerable disagreement was demonstrated between different retrieval algorithms. However, several algorithms gave retrievals that were in reasonable agreement with long-term climatological data sets. This agreement was best in the tropics and decreased considerably in the midlatitudes where the retrieval capability of SRR for the SSM/I is not optimal.