A new generation of extreme wind speed measurements from space

Observing the dynamics and effects of Tropical and Extra Tropical storms is essential with obvious societal benefits. Yet, it remains a particular challenge as in-situ observations are scarce and opportunities to apply and validate remote-sensing techniques for surface wind speeds above hurricane force and for phenomenal sea states are rare. Often hampered by heavy precipitation, current and historical active and passive microwave measurements can be limited. A new generation of L-band (~1.4 GHz) satellite sensors (SMOS, AQUARIUS and SMAP) and dual C-band (~7.0 GHz, AMSR-2) are now able to retrieve estimates of extreme surface wind speed over the ocean with wide swath (>1000km) coverage, a spatial resolution of ~40 km and revisit of ~3 days for an individual satellite. Much less affected by rain and atmospheric effects, foam coverage and thickness (especially at wind speed >10 m/s) modify the L-band microwave surface emissivity, and thus the retrieved brightness temperature (Tb) measurements over the sea surface. These changes in brightness temperature signal can be used to retrieve surface wind speed modulus (Reul et al. 2012). Five years of SMOS, almost one year of SMAP L-band, and 4 years of AMSR-2 C-band brightness temperature data intercepting a large number of Tropical Cyclones (TC) have been analysed. New geophysical model functions have been derived (and validated using aircraft SFMR measurements) to retrieve reliable ocean surface wind speeds from Tb over Tropical Cyclones (Zabolotskikh et al, 2015a,b; Reul et al., 2015). Merged high wind speed products between SMOS, SMAP and AMSR-2 data have been produced for an ensemble of Tropical Cyclones during the very active 2015 hurricane season in the Pacific. These examples illustrate the capability of the combined low microwave frequency passive sensor data to provide regular sampling and accurate estimates for the evolution of the 2D surface wind field structure in TCs, including estimates for maximum sustained wind speed up to ~100 knots, radii of maximum wind, radii at 34, 50 and 64 knots. L-band satellite instruments are now acting as satellite ocean wind sensors with a capability to provide quantitative surface wind information in high-extreme conditions that are complementary to that available from contemporary satellite scatterometer and higher frequency passive microwave measurements.

Reul N., Tenerelli J., Chapron B., Vandemark D., Quilfen Y., Kerr Y. (2012). SMOS satellite L-band radiometer: A new capability for ocean surface remote sensing in hurricanes. Journal Of Geophysical Research-oceans, 117,

Zabolotskikh E., Mitnik L.M., Reul N., Chapron B. (2015a). New Possibilities for Geophysical Parameter Retrievals Opened by GCOM-W1 AMSR2. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, PP(99), 1-14.

Zabolotskikh E., Reul N., Chapron B. (2015b). Geophysical model function for the wind excess emissivity at AMSR2 C- band at high winds. IEEE Geoscience and Remote Sensing Letters (GRSL), in press.

Reul N., B. Chapron,Y.Quilfen, E.Zabolotskikh, S. Guimbard, J.F Piolle and C.Donlon, A revised L-band radio-brightness sensitivity to extreme winds under Tropical Cyclones: the 5 years SMOS-storm database, in revision, Remote Sensing of Environment, (2015).