- 200 hPa geopotential heights in the GDAS analysis are

1. August 2005 Summary of CDAS/GDAS/GFS • - 200 hPa geopotential heights in the GDAS analysis are • lower than in CDAS over most of the planet except the North • American continent, particularly from 20oN to the South Pole. • 200 hPa geopotential heights are consistently lower for GDAS • compared to CDAS since at least February 2005 over the • global Tropics. • Error growth with forecast projection in the 200 hPa hgt. field is • much less during August 2005 compared to previous months • The CDAS monthly precipitation differences from CAMS_OPI • are similar to June & July 2005: • CDAS < observations over the near-equatorial Tropics in the • NH & > obs. over the Atlantic hurricane development region. The • Area of relative dryness in the NH tropical western Pacific • expanded this month. • GFS precip. >> than obs. over the Atlantic & Pacific ITCZs • and the Arabian Sea, and < over parts of SE Asia and Indonesia • at all forecast 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. • GDAS sfc. temps much closer to obs. than CDAS.

2. 200 hPa geopotential heights in the GDAS analysis are lower than in CDAS over most of the planet (particularly from 20oN to the South Pole. Very small differences are noted over the North American continent and eastern Europe. The magn- itude of the differences over the Tropics is equivalent to about 1 standard deviation of the CDAS monthly means over the 1971-2000 base period. This is the same basic pattern since March 2005.

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 February 2005, and that this difference occurs over land as well as ocean, although the difference is largest over the oceans.

4. The zonal 200 hPa hgt errors in the GFS grow with the length of the forecast projection. During Aug 2005, negative hgt errors in excess of 10m are generally observed for the day-4 forecasts and beyond. Errors in excess of 0.5 std. dev. (right panel) are observed after Day-5 that grow (slightly) with forecast projection in the Topics. The errors are notably less compared to the previous few months. http://www.cpc.ncep.noaa.gov/products/fcst_eval/html/maps_mrf.html

5. Similar to the past 3 months, 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. Note thecomparative dryness in CDAS in the NH between SE Asia & the date line (also seen in the GFS fcsts). As seen in July 2005, CDAS indicates heavier accumulations than the observations over the hurricane development region in the tropical Atlantic west of the Caribbean in August 2005. June 2005 July 2005

6. The precipitation anomalies between CDAS and the satellite estimates agree well over the Indian subcontinent (positive anomalies) and the adjacent oceanic region. Anomalies of opposite sign are observed in the tropical western Pacific.

7. Precip. is much too strong in the Atlantic and Pacific ITCZs at all forecast proj- ections – same as all previous months. GFS precip; is much less than the satellite estimates over the much of the extreme western Tropical Pacific north of the equator.

8. GFS precipitation forecasts generally too wet (by ~ 100 mm) east of the Mississippi for the Day 1 forecasts. For the Day 5 , -10 & -15 day forecasts, the model precipitation forecasts are generally Wetter than the gauge observations over the eastern third of the nation with a widespread pattern of underforecast amounts beteween the Rockies and Ohio Valley.

9. The CDAS OLR is cooler over the extratropical portion of the NH continents compared to observed OLR and warmer over much Of the tropics north of the equator. GDAS OLR is substantially closer to the observed OLR compared to CDAS, and the differences with observed OLR are almost completely positive.

10. The evolution of near-equatorial OLR anomalies during Feb 2005 through August 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 Feb 2005 and mid-March-May 2005. The relatively dry period during Feb-Mar 2005 on either side of the date line agrees well with the observed OLR as do the weak negative anomalies near the date line during July 2005. 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 (particularly so in August 2005) and Pacific which is consistent with the higher rainfall in the GFS over those areas compared to CDAS (see precip. figs – buttons below). This is also consistent with differences in the upper-level divergence (button below). CDAS precip GFS precip Divergence

12. GDAS vertical velocity (500 hPa) is consistently higher than • CDAS over tropical land regions, in good agreement over the • tropical oceans, although consistently lower there since April • 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 generally in areas with high terrain. In contrast, GDAS 2m temps. are 1-3K warmer than CDAS over the majority of the continental regions.

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

17. Global Mean precip from GFS is about 25% higher than GPCP.