Hurricane Imaging Radiometer (HIRAD) Wind Speed Retrievals and Validation Using Dropsondes.

Surface wind speed retrievals have been generated and evaluated using Hurricane Imaging Radiometer (HIRAD) measurements from flights over Hurricane Joaquin, Hurricane Patricia, Hurricane Marty, and the remnants of Tropical Storm Erika, all in 2015. Procedures are described here for producing maps of brightness temperature, which are subsequently used for retrievals of surface wind speed and rain rate across a ~50 km wide swath for each flight leg. An iterative retrieval approach has been developed to take advantage of HIRAD's measurement characteristics. Validation of the wind speed retrievals has been conducted, using 636 dropsondes released from the same WB-57 high altitude aircraft carrying HIRAD during the Tropical Cyclone Intensity (TCI) experiment. The HIRAD wind speed retrievals exhibit very small bias relative to the dropsondes, for winds tropical storm strength (17.5 m s-1 ) or greater. HIRAD has reduced sensitivity to winds weaker than tropical storm strength, and a small positive bias (~2 m s-1 ) there. Two flights with predominantly weak winds according to the dropsondes have abnormally large errors from HIRAD, and large positive biases. From the other flights, root mean square differences between HIRAD and the dropsonde winds are 4.1 m s-1 (33%) for winds below tropical storm strength, 5.6 m s-1 (25%) for tropical storm strength winds, and 6.3 m s-1 (16%) for hurricane strength winds. Mean absolute differences for those categories are 3.2 m s-1 (25%), 4.3 m s-1 (19%), and 4.8 m s-1 (12%), with bias near zero for tropical storm and hurricane strength winds.