With MODIS, both infrared and water vapor channels are used to estimate winds in cloudy areas. A water vapor channel also yields winds in clear areas. In practice, the vast majority of wind vectors come from the water vapor channel retrievals. The AVHRR does not have a water vapor channel, so clear sky winds cannot be retrieved. Nevertheless, for cloudy areas the AVHRR polar winds are similar in quality to the MODIS winds when compared to radiosonde winds.
The MODIS polar winds product has been shown to have a positive impact on weather forecasts at eight numerical weather prediction (NWP) centers in the U.S., Europe, and Asia. It is now used operationally at three centers, and is under consideration for operational use by other NWP centers. While the magnitude and geographic distribution of the impact varies somewhat from one NWP system to another, overall there is a positive impact on forecasts not only in the Arctic and Antarctic, but also in the Northern and Southern Hemisphere extratropics.
An historical record of winds from AVHRR is being generated for climate studies and for potential use in reanalysis systems. The dataset will cover the period 1980 through the present. Differences between the cloud-track AVHRR winds and thermal winds from an atmospheric sounder are under examination.