A 27-year record of AVHRR-derived polar winds for retrospective analyses

A major gap has historically existed in the global observing system because very few routine measurements of winds are made over the Arctic Ocean and most of the Antarctic continent. This gap has been somewhat reduced since the early 2000s with the routine retrieval of tropospheric winds in the polar regions by tracking cloud and water vapor features in Moderate Resolution Imaging Spectroradiometer (MODIS) data from the Terra and Aqua satellites. Model impact studies from numerical weather prediction centers worldwide have demonstrated a positive impact on forecasts in the Arctic, Antarctic, and extending into the middle latitudes with the assimilation of the MODIS winds.

Unfortunately, the era of MODIS polar winds will be limited to approximately the current decade as the Terra and Aqua satellites are already beyond their designed lifetimes. Winds derived from NOAA's Advanced Very High Resolution Radiometer (AVHRR) instrument complement the current MODIS winds dataset, though the AVHRR is less robust in terms of spectral channels and spatial resolution. Future polar satellite sensors, for example the Visible/Infrared Imager/Radiometer Suite (VIIRS) on the NPOESS satellites, are similar to the MODIS, but lack a water vapor channel. Therefore, a long-term record of satellite-derived polar winds will only be available using AVHRR-like instruments.

We have begun reprocessing 27 years of AVHRR data to generate a polar winds dataset that can be used in reanalysis efforts. Short time periods, on the order of several months, were identified to run limited experiments using time-coincident MODIS and AVHRR winds. The forecast impact of these short-term datasets will be evaluated, in collaboration with the NASA GMAO, using the Goddard Earth Observing System Data Assimilation System Version 5 (GEOS-5). This is in preparation for the entire AVHRR winds dataset to be assimilated into the next GMAO reanalysis that will follow on from the Modern Era Retrospective-analysis for Research and Applications (MERRA).

A status of the data processing, preliminary results of initial experiments in the GEOS-5, and future plans will be presented.