The NOAA airborne C-band SCATterometer (C-SCAT) was recently modified to record not only backscatter from the sea, but also radial velocity of scatterers in the radar beam between the aircraft and sea surface. The objective of this modification was to remotely sense surface winds and 3-D wind structures from the the high-altitude NOAA G-IVSP jet aircraft, and thereby expand the instrument's use from environmental surveillance to tropical cyclone reconnaissance.
As a scatterometer, this system is designed to estimate ocean surface winds from backscatter measurements. However, in heavy rain (which is a frequent occurrence during hurricane research flights), both attenuation and disturbance of the ocean surface by rain drops interfere with wind measurements. C-SCAT was modified by adding a Doppler mode, called Vertically Scanning Doppler Radar (VSDR), to make use of scattering from moderate to heavy rain. When operating as a VSDR, the system provides vertical profiles of the horizontal wind field at up to 24 altitude bins.
The VSDR/CSCAT system was tested for the first time as a dual-mode system during a test flight on 24 Oct, 1997. Data were obtained during multiple flight legs across a squall line in the NW Gulf of Mexico.
In this paper, we will present the data as processed by HRD and Quadrant into vertical profiles of the horizontal wind field. The first step of processing involved implementation of the pulse pair algorithm and Velocity Azimuth Display (VAD) technique for extraction of the horizontal wind vector. The next processing step was a complex correction algorithm to compensate for the aircraft's attitude and velocity. Signal processing techniques were also used to remove undesired clutter return to improve wind velocity estimates, especially for altitude bins in the very important near-surface region.
Profiles were successfully recovered from flight legs at 700 mb (3 km) and from 500 mb (5 km) flight altitude. GPS dropsondes were deployed at several locations. Mean vertical wind profiles over a 25 km box centered on the aircraft were retrieved from the X-band tail Doppler radar. The results were extremely encouraging; all profiles were in good agreement at drop locations.
From this initial test, we conclude: 1) The VSDR concept works, 2) Further work is needed to verify its capability in hurricanes, 3) Flights at multiple incidence angles may be required to eliminate 'range ring' contamination, 4) Verification of CSCAT surface wind retrieval from surface backscatter measurements in scatterometer mode will complete the wind profile retrieval process.
This instrument may usher in a new era for sampling of tropical storm structures, providing a platform suitable for use on either WP-3D or Gulfstream G-IV aircraft for measurement of continuous wind profiles along the path of the aircraft. Future work on the dual mode VSDR/CSCAT radar system will focus on implementing a vertical wind profile algorithm in real time, to provide timely, in-flight profiles of hurricane winds which will be transmitted to the National Hurricane Center for use in their forecasts and warnings.