RSS Air-Sea Essential Climate Variables (AS-ECV) from COWVR

The Compact Ocean Wind Vector Radiometer (COWVR) continues and advances the goal of measuring ocean surface wind speed and direction using passive microwave sensors. Designed and built at NASA’s Jet Propulsion Laboratory (JPL) in Southern California, COWVR launched on December 21, 2021 to the International Space Station (ISS), and started recording data on January 8, 2022. The ISS orbit is not sun-synchronous, but rather precesses throughout local solar time, similar to TMI and GMI.

The COWVR retrievals of wind speed, water vapor, cloud water, and rain rate are very similar to those we post for AMSR-2 and GMI. The same retrieval algorithm is used, and the brightness temperatures are all inter-calibrated. AMSR-2 and GMI do have wider swathes and thereby provide more extensive coverage. However, COWVR's inclined orbit gives measurements over the diurnal cycle, and a combination of all three (COWVR, AMSR-2, GMI) should provide a more complete spatial/temporal picture of the AS-ECV dynamics.

COWVR’s unique feature is that it also provides wind direction by virtue of its 2-look design (ocean areas are viewed from both the forward and aft directions) and its fully polarimetric channel set. The only other satellite microwave imager to retrieve wind direction was WindSat, albeit its 2-look capability was limited to a small segment at the edge of the swath. The 2-look capability, first proposed by Wentz [1992], results in a very high skill at selecting the correct wind direction ambiguity. The retrieval algorithm finds wind vector solutions for which the chi-squared between observed and modeled TBs is a local minimum in the 2D wind speed/direction space. There are typically two to four such minima, which are called ambiguities. To select the correct ambiguity, we begin by selecting the ambiguity with the smallest chi-squared value. In some cases, if the wind vector with the smallest chi-squared value disagrees with the surrounding field, a more field-consistent ambiguity is selected. No ancillary data is used by this method. There is the drawback that some real weather features may be smoothed out.

To ensure COWVR’s long-term stability, its brightness temperatures are routinely calibrated to the GMI brightness temperatures. The stability of GMI over the last decade has been recently verified by comparisons with the newly launched WSF-M MWI.

Wentz, FJ, 1992, Measurement of oceanic wind vector using satellite microwave radiometers, IEEE Transactions on Geoscience and Remote Sensing, 30, 960-972.

RSS AS-ECVs from COWVR:

COWVR within the constellation of passive microwave sensors (GHz)

RSS AS-ECV from COWVR Data Citation:

Wentz, F., R. Lindsley, K. Wentz, M. Brewer, M. Densberger, 2024: RSS COWVR V1.0 Air-Sea Essential Climate Variables (AS-ECV) on 0.25 deg grid, Remote Sensing Systems, Santa Rosa, CA, USA. Available at www.remss.com https://doi.org/10.56236/RSS-bu

Acknowledgements:

This research is funded by the NASA ROSES grant 80NSSC23K0987 Evaluation of COWVR as a Cost-Effective Sensor for Providing Climate Data Records of Ocean Vector Winds and Other Air-Sea Variables. In addition, we would like to thank Shannon Brown, Spencer Farrar, and the rest of the COWVR CalVal Team for their support in providing the COWVR TSDR files:

And special thanks to Shannon Brown for taking the concept of the two-look radiometer to fruition.