IMPACT OF SATELLITE DATA ON THE CDAS REANALYSIS SYSTEM

In preparation for the execution of the National Meteorological Center and National Center for Atmospheric Research (NMC/NCAR) Reanalysis Pr oject, which will cover the period 1958-93, the impact of satellite da ta on both analyses and forecasts has been assessed. This was done by diagnosing two sets of analyses and forecasts made with and without th e use of satellite data (SAT and NOSAT) within the data assimilation. The analyses and forecasts were performed using a state-of-the-art glo bal data assimilation system and were evaluated for August 1985. The i mpact of satellite data is smaller than that obtained in previous impa ct studies during the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE) that took place in 1979, reflecting the effe ct of improvements that have been implemented in the global analysis s cheme and the model. In the Northern Hemisphere (NH), there are no sig nificant differences between SAT and NOSAT analyses for both primary v ariables and eddy transports. The satellite impact on the forecasts in the NH is positive but very small, reaching about 1% in the 5-day for ecasts, as measured by the average rms errors and anomaly correlations . In the Southern Hemisphere (SH), the difference between the SAT and NOSAT analyses is estimated to be equivalent to the difference between 1.5-day SAT forecasts and the verifying analyses. After 5 days, the S AT forecasts are shown to be superior to the NOSAT forecasts by about 1 day, an advantage apparent whether they are verified against SAT or NOSAT analyses. A comparison of SAT and NOSAT analyses suggests that t he NOSAT captures well over 90% of the variance of monthly mean statio nary waves of the SAT analyses in most of the Tropics and Southern Hem isphere from 20 degrees to 60 degrees S. The daily variability is capt ured at 70%-90% in the Tropics and Southern Hemisphere, except above 2 00 hPa and south of 60 degrees S. In several earlier satellite data im pact studies performed using FGGE (1979) data, it was observed that sa tellite data, which cannot resolve smaller-scale features, have a damp ing effect on the apparent atmospheric circulation. With the improveme nts in data assimilation methods, it is seen that the smoothing effect is much less apparent. A comparison of the SAT and NOSAT monthly trop ical precipitation derived from the 0-6-h forecast cycle shows a gener al agreement with the rain estimates from satellite data. Overall, the se results are very encouraging, indicating that a reanalysis spanning the years before and after satellite data was available should be use ful. In the NH, the analyses are basically unaffected by the satellite data. Even in the SH a large component of both the monthly and the da ily anomalies can be captured in the absence of the satellite data, ex cept in the stratosphere and Antarctic region.