Polar Winds From Current and Future Satellite Imagers and Sounders

A major gap in the global observing system exists because very few routine measurements of tropospheric winds are made over the Arctic Ocean and most of the Antarctic continent. Recently, a method has been developed to retrieve tropospheric winds (speed, direction, and height) in the polar regions by tracking cloud and water vapor features in Moderate Resolution Imaging Spectroradiometer (MODIS) data. The product has been shown to have a positive impact on weather forecasts in at least three numerical weather prediction systems. It is now used in two operational forecast systems.

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. What is the consequence of deriving winds from satellite imagers that lack a water vapor channel, such as the Advanced Very High Resolution Radiometer (AVHRR) and, possibly, the future Visible and Infrared Radiometer Suite (VIIRS)?

High latitude winds have also been generated with satellite sounder data, particularly the TIROS Operational Vertical Sounder (TOVS) and the Advanced Microwave Sounding Unit (AMSU), with the Atmosphere Infrared Sounder (AIRS) currently under investigation. While a feature tracking method can be used with sounders, their low spatial and high vertical resolutions have given rise to a very different approach than for imagers, where satellite-derived temperature profiles and an estimate of the surface wind are used to calculate thermal winds at selected tropospheric levels. How do the sounder-based thermal winds differ from imager-based feature-track winds? What is the relative accuracy of feature-track winds from sounders, i.e., what is the effect of spatial resolution on wind retrieval? What capabilities will future operational weather satellites provide for polar wind retrievals? These questions will be addressed and comparisons between winds from different sensors will be presented.