Sixty years of hurricane surface wind observation using techniques ranging from hand-held sea state imagery to the Stepped Frequency Microwave Radiometer

Surface wind estimation in hurricanes has been a requirement for tropical cyclone reconnaissance for over 60 years. From 1950 to 1980 surface wind estimation was accomplished mainly by matching appearance of the sea state to photographs that had been ‘calibrated' according to wind speed, accomplished mainly by Mr Charles Neumann using a large format camera leaning out the window of a Navy PB4Y2 aircraft flying at 200 ft. These images were calibrated with the use of ‘double-drift' wind estimates. Later spot observations from dropsondes were also used over the years for surface wind estimation and to calibrate the sea state photographs. Surface wind estimates were also derived using Doppler navigational radar, Loran, omega, inertial and GPS navigation techniques to estimate flight level winds from which surface winds were derived using the famous 0.8 (now 0.9) factor. Satellite scatterometry first came into use in 1978 with the advent of SEASAT and was able to measure winds in the gale force wind region, but not the inner core due to rainfall attenuation effects. Various methods were attempted to remove rain effects, but to no avail in the hurricane inner core. This methodology became a mainstay during the QuikSCAT and ASCAT observational periods that followed. In 1980, the first airborne C-band multi-frequency microwave radiometer was flown in Hurricane Allen on board a NOAA WC-130H aircraft. With the advent of GPS dropsondes in 1997, this instrument, dubbed the Stepped Frequency Microwave Radiometer (SFMR) was calibrated with high confidence once a multi-year data set was obtained using coincident observations. Several technological upgrades resulted in a drastically reduced noise level over the early experimental versions so that accurate wind estimates to within +/- 5 kt became possible. In 2004, Congressional funding provided the support to equip all 10 Air Force WC-130J aircraft with the SFMR, and in 2006 this instrument was approved for operational use. Since then, it has become the standard for surface wind measurement that had proved so elusive for so many years (decades). A scanning version of the SFMR, called the Hurricane Imaging RADiometer (HIRAD), using a similar suite of four C-band frequencies like the SFMR, has now been developed for use on the NASA Global Hawk UAV aircraft, leading to a swath of surface wind observations mapping the entire hurricane eyewall in one flight leg, nearly instantaneously in time. This instrument was flown experimentally for the first time during the NASA GRIP experiment in 2010 and is scheduled to fly during the NASA Hurricane and Severe Storms Sentinel (HS3) program from 2012-2014.