Tropical Cyclones Wind Measurements with the SMAP L-Band Radiometer

The Soil Moisture Active Passive Mission SMAP was launched in January 2015 and has been providing science data since April 2015. Though designed to measure soil moisture, the SMAP radiometer has an excellent capability to measure ocean winds in tropical cyclones at a resolution of 40 km, with a swath width of 1000 km. The L-band radiometer V-pol and H-pol channels keep very good sensitivity to ocean surface wind speed even at very high wind speeds and they are only little impacted by rain. We briefly discuss the major features of the SMAP sensor, the geophysical model function that is used in the ocean vector wind retrieval and the basic steps of the retrieval algorithm. We will then illustrate the capability of this instrument to observe very high surface winds by comparing them to other validation datasets. The most important validation source is NOAA's airborne Step Frequency Microwave Radiometer SFMR, whose wind speeds were collocated with SMAP in space and time and resampled to the SMAP resolution. A comparison between SMAP and SFMR winds in hurricanes of the 2015 season, including Patricia, shows excellent correlation over a wide wind speed range (15 - 70 m/s) and no degradation in rain. This agreement is unique and gives SMAP a distinct advantage over many other space-borne sensors such as C-band or Ku-band scatterometers or radiometers, which either lose sensitivity at very high winds or degrade in rainy conditions. We will analyze the SMAP surface winds during the full evolution of the storms in recent intense tropical cyclones (Patricia, Winston, Fantala, and Nepartak) and compare them with wind measurements from ASCAT, RapidScat, and WindSat, with the NCEP wind fields, and with the best track data from the Joint Typhoon Warning Center.The SMAP wind data are available as twice-daily 0.25 deg gridded maps at www.remss.com.