25 Years (1982-2007) of Polar Winds from AVHRR: Validation and Comparison to the ERA-40 Reanalysis

Recent studies have shown that Arctic climate has changed markedly over the past 30 years. Unfortunately, two important tools for studying recent changes, the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) and the European Center for Medium Range Weather Forecasts (ECMWF, ERA-40) reanalysis products, have relatively large errors in the wind field over the Arctic where there is little radiosonde data available for assimilation. At least ten numerical weather prediction centers worldwide have demonstrated that satellite-derived polar winds have a positive impact on global weather forecasts. The impact on climate reanalyses should be similar. Therefore, a polar wind data set spanning 25 years has been generated using Advanced Very High Resolution Radiometer (AVHRR) data. Comparisons with winds from radiosondes (rawinsondes) show biases in the AVHRR-derived winds of 0.1 – 0.8 m s-1 , depending on the level. In addition, AVHRR has lower speed root-mean-squared errors and speed biases than ERA-40 when compared to rawinsondes not assimilated into the reanalysis. Therefore, it is recommended that the historical AVHRR polar winds be assimilated into future versions of the reanalysis products. We also explore possible kinematic reasons for the disparities between the ERA-40 reanalysis and AVHRR wind fields. A long-range comparison of AVHRR and ERA-40 speed and direction differences with kinematic flow features are investigated. Results show that on average AVHRR winds are faster in jet streams and ridges but slower in troughs and jet exit regions. The results from this study could lead to a better dynamical understanding of why the reanalysis product produces a less accurate wind vector field over regions that are void of radiosonde data.