- 200 hPa geopotential heights in the GDAS analysis are

1. May 2005 Summary of CDAS/GDAS/GFS • - 200 hPa geopotential heights in the GDAS analysis are • lower than in CDAS between 20oN to the South Pole. • - 200 hPa geopotential heights are consistently • lower for GDAS compared to CDAS since at least • November 2004 over the global Tropics. • 200 hPa Errors in the GFS forecasts in excess of 0.5 std. dev. • are observed after Day-5 & grow poleward with forecast projection. • The CDAS monthly precipitation is < observations over the • near-equatorial Tropics in the Northern Hemisphere, the northern • Indian Ocean and “coastal” western Pacific, and > over much • of the Southern Hemisphere near-equatorial Tropics • - GFS precip. >> than obs. over equatorial South America and • the Atlantic ITCZ and < the northern Indian Ocean and “coastal” • western Pacific at all fcst. Projections (1-15 days) • Vertical velocity and upper-level divergence fields are • consistent with the precipitation differences over the Pacific • near-equatorial regions in the GDAS/GFS. • CDAS 2m temperature anomalies are generally within 1K of • observed surface temperature and 1-3K cooler where they differ.

2. 200 hPa geopotential heights in the GDAS analysis are lower than in CDAS between 20oN to the South Pole. The magnitude of the differences over the Tropics are equivalent to about 1 standard deviation in the CDAS monthly means over the 1971-2000 base period.

3. Time series of 200 hPa heights for GDAS and CDAS over the Tropics indicate that heights are consistently lower for GDAS compared to CDAS since at least November 2004, and that this difference occurs over land as well as ocean, although the difference is largest over the oceans.

4. As expected, the zonal 200 hPa hgt errors in the GFS grow with the length of the forecast projection. During May 2005, negative hgt errors are observed at most latitudes with positive errors observed only at about 40oN. Errors in excess of 0.5 std. dev. (right panel) are observed after Day-5 that grow poleward with forecast projection. http://www.cpc.ncep.noaa.gov/products/fcst_eval/html/maps_mrf.html

5. The CDAS monthly precipitation accumulations are less than the satellite-gauge (“CAMS_OPI”) observations over most of the near-equatorial Tropics in the Northern Hemisphere, the SPCZ and over the SACZ. CDAS indicates heavier accumulations than the observations over much of the Southern Hemisphere near-equatorial Tropics except in the region from 70oE to the date line.

6. The difference in the precipitation anomalies between CDAS and the satellite estimates has a very similar pattern to the difference in the monthly accumulation (previous graphic) particularly over the near-equatorial Pacific.

7. GFS precip. much larger than obs. over equatorial South America and the Atlantic ITCZ at all fcst. projections. GFS rainfall is much less over the northern Indian Ocean and coastal SE Asia, and while consistent with what was observed in April 2005, the May rainfall is much less than the satellite estimates. Central Pacific ITCZ is much too strong at all forecast projections, not just at the early projections as was the case in April 2005. .

8. Contrary to what was observed in April 2005, when overestimates of precipitation over the eastern two-thirds of the nation in the “Day-1” forecasts were reduced considerably in the “Day-15” average, these overestimates grew substantially in the longer forecast projections.

9. The CDAS OLR is generally cooler over the continents compared to observed OLR, although this difference may be due to the fact that observed OLR is from measurements at only a few times of day (0200/1400 LST at the equator). GDAS Data not available

10. The evolution of near-equatorial OLR anomalies during Nov 2004 through May 2005 in CDAS shows negative anomalies for the entire period near the date line. That contrasts with the observed OLR which indicates negative anomalies there during Dec 2004 through Feb 2005 only. However, the positive anomalies to the east and west of the date line in CDAS match the observed OLR quite well. Note that GDAS anomalies are not plotted because a reliable climatology is not available from GDAS due to the many model changes that have occurred during the GDAS record.

11. The GDAS upward motion at 500 hPa is considerably stronger than CDAS over the ITCZs in the Atlantic and Pacific which is consistent with the higher rainfall in GDAS over those areas compared to CDAS (see earlier precip. figs – buttons below). This is also consistent with differences in the upper-level Divergence (button below). CDAS precip GDAS precip Divergence

12. GDAS vertical velocity (500 hPa) is consistently higher than CDAS over tropical land regions, generally a bit higher over the SPCZ, and generally a bit lower over the tropical oceans (averaged globally). Both are in good agreement over the NH oceanic storm tracks.

15. CDAS 2m temperatures are generally 1-3K cooler over the land surfaces than is observed; regions where CDAS is up to 1K warmer than the obs. are in relatively small interior sections of North & South America and Eurasia. Note: GDAS data not available for this month GDAS data not available GDAS data not available

16. In general, the CDAS temperature anomalies are within 1K of the observed anomalies; where they differ, the CDAS anomalies aregenerally cooler than the CAMS (station) data.