NOAA's Wide Swath Radar Altimeter
Measuring ocean waves and rain rate in hurricanes
The NOAA Wide Swath Radar Altimeter (WSRA) is a unique airborne system providing observations of the sea surface and rain. During flights into hurricanes, the WSRA data are processed on the NOAA hurricane research aircraft and transmitted to the National Hurricane Center through a satellite data link to support the NOAA hurricane operations and research missions. The WSRA provides near-real-time reporting of (1) ocean directional wave spectra, (2) significant wave height, helpful in assessing the radius of 12' seas, (3) rain rate, and (4) the mean square slope of the ocean surface. WSRA data can be used to verify and improve the numerical wave models which forecast the short and long term evolution of a hurricane's wave field. By measuring the absorption of its signal on the round trip to the sea surface, the WSRA can also determine the rain rate. The variation of the radar backscattered power with incidence angle determines the sea surface small scale roughness (mean square slope), which responds rapidly to changes in wind speed. The WSRA was developed by ProSensing through funding from the Department of Commerce Small Business Innovation Research (SBIR) program and the NOAA Joint Hurricane Testbed (JHT).
The black dots on the map indicate the locations of WSRA data acquired during flights on August 24, 25 and 26, 2011 as Hurricane Irene (red line) traveled from the Turks and Caicos Islands, through the Bahamas, and approached the shore of North Carolina. A sample of the sea surface topography measurements of the WSRA is shown with ±12 m gray-scale coding. The aircraft was at 33°N, directly north of the eye and flying north. The directional wave spectrum shown at the right was processed from the sea surface topography and quantifies the wave field components. The nine solid black contours are uniformly spaced from 0.1 to 0.9 relative to the spectrum peak variance. The thick black contour is at the 0.5 level and the dashed black contour is at the 0.05 level. The dominant wave field was swell generated in the vicinity of the eyewall. It had a wave height of 10 m, a wavelength of 360 m, and propagated towards 340°. The secondary wave field of about 200 m wavelength propagating towards 240° was generated by the local wind.