Wind stress distribution is similar to surface wind except magnitude of differences is greater.

-Some differences exist between models and observations in the northwest Pacific, southeast Pacific, and western equatorial Pacific (left)

An Examination of Oceanic Surface Wind in the NCEP CDAS1, CDAS2, GDAS, CFS, and GFS

Sarah A. Levinson, Pingping Xie, and Wanqiu WangNOAA/NCEP/CPC

Sarah.Levinson@noaa.gov

P 1.22

Observational Wind Comparisons

DataObservations: 2000-2006CDAS1, CDAS2: 2000-2006GDAS: 2002-2006GFS: AMIP runs 1981-2000CFS: 20 yr climatology

Wind Stress ComparisonsPsuedo stress

defined as:τx = w uτy = w v

Introduction

Model Wind Comparisons

Southeast Pacific Comparisons

Wind direction of NCDC is from CDAS2 and does not always agree with Quikscat wind direction.

- Slight variation among JPL, RSS, and NCDC in western equatorial Pacific (right)

- NCDC differs from Quikscat in Southeast Pacific (right) and near South American coast (far right)

Spatial distribution of wind speed among all observations is very similar, but magnitude of JPL Quikscat is smaller. Greatest differences of observed wind speed occur at mid to high latitudes, but there is very little uncertainty at low latitudes (bottom right). Wind speeds vary more with greater speed which is especially apparent in the northern hemisphere. GDAS closely matches JPL Quikscat because it uses JPL Quikscat winds.

In this assessment surface winds over the global oceans in the NCEP reanalyses and models are examined through comparisons with observations. Annual and seasonal differences in surface wind and wind stress among the NCEP products are analyzed with emphasis along the ITCZ and in the Southeast Pacific. Comparisons were first performed among four satellite-based wind data sets: JPL Near-Real-Time Product Quikscatter wind, RSS Version 3a Quikscatter wind, RSS Version 6 SSM/I wind, and NCDC blended winds Version 1.1.

Shaded area (above) indicates maximum and minimum observed wind speed. CDAS1 and GDAS winds are systematically too slow across all latitudes. Between 25S and 25N most models are too slow but at higher latitudes they are mostly within uncertainty values of observations (most of the uncertainty is due to JPL winds).

- Some differences in wind direction near 30-35S in GFS and CFS

- CDAS1 and CDAS2 have incorrect wind direction along South American coast

- CDAS1 is too slow near the equator, and CDAS2 is too fast along 10S

- GFS and CFS are slightly too fast along the equator

- Some variation in wind direction near 30-35S in CDAS1 and CDAS2 although larger differences exist with GFS and CFS

- CDAS1 and CDAS2 have more apparent wind errors near South American coast in ASO than FMA

- CDAS1 is still slow near the equator, but CDAS2 speed differences are more limited to near the coast

- Wind speed bias with GFS along equator has increased in size, and CFS wind speed along and north of equator becomes too slow

- Wind speed is too fast in GFS and CFS near the coast

- CDAS2 and CFS have too strong wind stress near the South American coast

- Wind stress in CDAS1 and CFS is too weak north and south of the equator

-GFS wind stress is too strong in the equatorial Pacific

In both seasons GDAS wind speed and direction agree very well with observations.

RSS Quikscat is chosen as a reference for comparisons. Spatial distribution is produced very well by NCEP products.

DataRSS QS: 2000-2006CDAS1, CDAS2: 2000-2006GDAS: 2002-2006GFS: 1990-1996CFS: 7 yr climatology

- Uncertainties among observations especially for high latitudes, and NCDC wind direction differs from Quikscat

- CDAS1 is too weak in the tropics

- CDAS2 has erroneous wind direction near Southeast Pacific coast which may be responsible for warm bias in GODAS

- GFS is too strong in equatorial Pacific, especially in ASO, which is likely the reason for cold SST bias in CFS forecast (left)

- CFS easterlies are too strong in ASO in far western Pacific which may be contributing to CFS cold bias there

Conclusions

-CDAS2 and GFS wind speeds are most accurate in both seasons (below), GDAS wind is fairly accurate spatially except for its bias against RSS Quikscat

- CDAS1 and CFS winds are too weak along the ITCZ in both seasons, while GFS is too weak in FMA (below)