Real Time Interpretation of SFMR Peak Surface Wind Speeds in Intense Hurricanes as Reported by Reconnaissance Aircraft

During the 2017 Atlantic Hurricane Season there were multiple occasions in which the peak surface winds reported from the Stepped Frequency Microwave Radiometer (SFMR) were questioned during missions flown by both the USAF Reserve WC-130J of the 53rd Weather Reconnaissance Squadron and the WP-3D of the NOAA Aircraft Operations Center.

The SFMR downward looking antenna passively measures brightness temperatures (Tb) below the aircraft in six microwave channels ranging from 4.74 GHz to 7.09 GHz. There are many contributors to the total overall Tb (for example, cloudiness and precipitation between the antenna and the ocean surface). However, by the time a tropical cyclone reaches Category 3, 4 or 5 intensity, the fractional contribution of capillary wave action and foam to Tb, as winds in excess of 50 m/s blow across the surface, overwhelms all other contributors. This greatly reduces any ambiguity as to the relative distribution of contributors to the total Tb even in the extreme rain rates of a hurricane eyewall (75 mm/hr or more). The net result is that accuracy and reliability of SFMR derived surface wind speeds (as a percentage of what is being reported) improves in proportion to increasing wind speed.

While GPS dropwindsondes released at flight level, directly above the surface radius of maximum winds (RMW) along an inbound or outbound azimuth, can often provide some degree of SFMR validation, the fact that they are carried in an arc downwind and splash at a different azimuth from where they were released, prevents them being used for direct comparison. The fact that the most intense hurricanes often attain a great deal of RMW symmetry near peak intesnity helps offset this to some degree. However, a much more daunting task is the Flight Meteorologist picking the exact moment to release the RMW sonde. At peak intensity, Category 5 TCs Patricia (October 2015), Irma (2017) and Maria (2017) were all characterized by extremely narrow surface RMWs. The tendency is for both AOC and 53rd Flight Meteorologists to "miss" the peak surface winds. The sondes tend to be released slightly too soon inbound and too late outbound. The result is that most attempts at RMW sondes splash just slightly outside the surface RMW and do not provide a direct means to validate the peak SFMR surface wind readings for that eyewall penetration.

Analyses of raw SFMR data for multiple missions into Hurricanes Irma and Maria show that the peak surface wind speeds reported on High Density Observations (HDOBs) transmitted in real time every 10 minutes off of 53rd and AOC reconnaissance aircraft were a true representation of what the instrument measured and should have been considered valid in operational intensity estimates.