Capability of the SMAP Mission to Measure Ocean Surface Winds in Storms

Abstract

he NASA 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 storms at a resolution of 40 km with a swath width of 1000 km. SMAP radiometer channels operate at a very low micro-wave frequency (L-band, 1.41 GHz, 21.4 cm), which has good sensitivity to ocean surface wind speed even in very high winds and is very little impacted by rain. This gives SMAP a distinct advantage over many spaceborne ocean wind sensors such as C-band (ASCAT) or Ku-band (RapidScat) scatterometers as well as radiometers operating at higher frequencies (SSMI, TMI, Wind-Sat, AMSR, GMI), which either lose sensitivity at very high winds or degrade in rainy conditions. This article discusses the major features of a new ocean wind vector retrieval algorithm designed for SMAP. We compare SMAP wind fields in recent intense tropical cyclones with wind measurements from current scatterometer missions as well as WindSat. The most important validation source in hurricanes is the airborne Stepped Frequency Microwave Radiometer SFMR, whose wind speeds are matched with SMAP in space and time. A comparison between SMAP and SFMR winds for 8 storms in 2015, including Patricia, one of the strongest hurricanes ever recorded, shows excellent agreement up to 65 m s-1 without degradation in rain.