Intensity and Size of Strong Tropical Cyclones in 2017 from NASA's SMAP L-Band Radiometer

The Soil Moisture Active Passive Mission SMAP has been providing science data since April 2015. Though originally designed to measure soil moisture, it has been demonstrated that the SMAP radiometer has an excellent capability to measure ocean winds in tropical cyclones (Meissner et al., BAMS, vol. 98(8), August 2017) at a resolution of 40 km and a 1000 km wide swath. The L-band radiometer channels keep very good sensitivity to ocean surface wind speed even at very high wind speeds and they are only minimally impacted by rain. This gives the SMAP sensor a distinctive 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 first briefly discuss the major features of the SMAP sensor and then illustrate methodology and validation results for SMAP ocean surface wind measurements. The most important validation source for the SMAP Tropical Cyclone (TC) winds 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 resampled SFMR winds in hurricanes of the 2015 and 2016 seasons, shows excellent agreement up to 135 kt and no degradation in rain. The main focus of our presentation is the review of SMAP observations of several high intensity TC during the 2017 season. We present SMAP results for maximum wind speeds and the 34-kt, 50-kt and 64-kt radii of super-typhoon Noru, super-hurricane Irma as well as hurricanes Maria, Jose and Harvey. SMAP is able to give realistic estimates of both intensity and storm size of these TC. In particular, the stages of rapid intensification and deintensification of Noru, Irma and Maria are well depicted by SMAP and are in good agreement with the ATCF Best Track Data. Additionally, we compare the results of SMAP with those of other satellite observations. The European C-band scatterometer ASCAT is able to give reliable estimates for the gale force (34-kt) wind radii. However, above 64 kt, the scatterometer signal saturates, which makes intensity estimates of TC above category 1 difficult and unreliable. The signal of other microwave radiometers or scatterometers that operate at higher frequencies (WindSat, QuikSCAT, RapidScat) are generally impacted by rain. The SMAP measurements are valuable for forecasting extreme wind speed events, especially in remote locations where no other observations are available. They can also be used for calibrating high wind measurements from other spaceborne sensors, for example CYGNSS. Remote Sensing Systems produces routinely SMAP winds as well as estimates of intensity and size of TC that are observed by SMAP. They are available at http://www.remss.com/missions/smap.