Scatterometer, Radiometer and GNSS-R observations of Hurricane Dorian

Hurricane Dorian was the first major hurricane of the 2019 Atlantic season. Dorian formed on August 24, from a tropical wave in the Central Atlantic. Over next several days it gradually strengthened and become a hurricane on August 28^th. Dorian rapidly intensified and reached Category 4 status on August 31st. Next day, Dorian reached Category 5 intensity, with maximum sustained winds of 185 mph while making landfall in Elbow Cay, Bahamas first and then another one on Grand Bahamas several hours later. Dorian stalled just north of Grand Bahamas for about a day. It was the strongest known tropical system to impact the Bahamas. A combination of cold water upwelling and an eyewall replacement cycle weakened Dorian to a Category 2 hurricane and it began to move slowly towards the north-northwest. In the early hours of September 6, Dorian weakened to Category 1 intensity as it picked up speed and turned northeast.

 

Hurricane Dorian was exceptionally well sampled by NOAA and Airforce hurricane hunter aircrafts. Large dataset of SFMR surface winds collected during the flights as well as dropsondes documented changes in Dorian’s wind field throughout its duration. AMSR-2 and SMAP radiometer, ASCAT and ScatSat-1 scatterometers as well as CYGNSS GNSS-R wind observations over Dorian were collected at NOAA for analysis. Understanding the differences between remote sending technologies utilized for wind observations is crucial for NOAA operational users. Large and rapid changes in Dorian’s wind field represent an interesting case study where strengths and weaknesses of different wind measurement technologies can be assessed and compared. SFMR wind measurements represent common surface ground truth that can be utilized to bring all these different measurements together.

 

While SMAP L-band radiometer sensitivity to high winds is not affected by rain its low measurement resolution smears wind field variations in the inner core depending on the storm size. AMSR-2 wind retrievals obtained from C-band measurements have somewhat larger measurement resolution and ability to provide more insight of the inner core wind variations although they can be enhanced by rain signature. These measurements are being utilized for 50 and 64kts wind radii. C-band scatterometer measurements such as ASCAT, even though attenuated by heaviest rain, provide both wind speed and direction information making then invaluable for early detection of tropical depression formation. As a matter of fact ASCAT measurements were used by NHC to initiate warnings for Dorian on August 24^th, 2019: “Two ASCAT passes between 1200-1300Z this morning indicated that the system had a closed circulation and surface winds of at least 30 kt, and that is the intensity set for this advisory.” Ku-band scatterometer ScatSat-1 is most impacted by rain and its winds can measurably differ from ASCAT in the areas of heavy rain. Understanding the thresholds when this occurs is imperative for successful utilization of ScatSat-1 winds in NOAA operations. Finally winds from CYGNSS GNSS-R measurements are assessed utilizing NOAA CYGNSS wind product. Examples of all measurements during Dorian duration will be presented and compared with aircraft observations.