- 200 hPa geopotential heights in the GDAS analysis are

1. September 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 30oN to the South Pole. • 200 hPa geopotential heights are consistently lower for GDAS • compared to CDAS since at least January 2005 over the • global Tropics. • CDAS < observations over the near-equatorial Tropics in the • NH & > obs. over the Atlantic hurricane development region. • - The GFS precipitation forecasts for Day 5 & beyond under- • forecast substantially the precipitation amounts over the US • where Hurricanes Katrina and Rita (and their remnants) • traversed from Louisiana and East Texas northeastward to • New England. • GFS precip. >> than obs. over the Atlantic & eastern • Pacific ITCZs while << over much of the far western tropical • Pacific north of the equator at all forecast projections (1-15 days), • but particularly for the Day-5, -10 and -15 forecasts. • Vertical velocity and upper-level divergence fields are • consistent with the precipitation differences over the Pacific • near-equatorial regions in the GDAS/GFS.

2. 200 hPa geopotential heights in the GDAS analysis are lower than in CDAS over most of the planet (particularly from 30oN to the South Pole. Very small differences are noted over the North American continent and Europe eastward to central Asia. The magnitude 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 January 2005, and that this difference occurs over land as well as ocean, although the difference is largest over the oceans.

4. The precipitation anomalies between CDAS and the satellite estimates agree well over the Indian subcontinent (positive anomalies) and the adjacent oceanic region, although the negative anomalies over the equatorial Indian Ocean are much stronger in the satellite observations than in CDAS. In contrast, CDAS indicates strong negative anomalies over northern Brazil that are not apparent in the observations (rain gauge).

5. Precip. is much too strong in the Atlantic and eastern Pacific ITCZs at all fore- cast projections GFS precip. is much less than the satellite estimates over the much of the extreme western tropical Pacific north of the equator (especially at the 10 & 15-day fcst projections) – similar to the June-August 2005

6. GFS precipitation forecasts averaged over September 2005 are much drier in a swath from Louisiana to New England for the Day-5 forecasts and beyond. This swath covers the area over which Hurricanes Katrina and Rita traversed, and the model underestimated the precipitation amounts substantially over that region.

7. 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 positive almost everywhere.

8. 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, mid-March-May 2005, and July-present. 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.

9. 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 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

10. 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.

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