Shoaling Waves

Coherent Radar Measurements During Shoaling Waves Plant, Keller, Hayes - APL/UW (Hesany is leaving APL on January 15 to go to Boeing.)

INTRODUCTION

The Shoaling Waves DRI is a program sponsored by the Office of Naval Research to investigate the changes that occur when ocean waves propagate from deep water onto the continental shelf.

ROLE OF APL/UW

The Applied Physics Laboratory of the University of Washington will collaborate with the NOAA Environmental Technology Laboratory and the Cooperative Institute for Research in Environmental Sciences CIRES of Boulder, Colorado to fly a variety of instruments on the NOAA Twin Otter airplane. Bill Plant, Bill Keller, and Ken Hayes of APL/UW are constructing a coherent, real-aperture, imaging/rotating radar (CORAR) to fly on the plane along with several NOAA radiometers. Plans are to image wave systems, determine directional wave spectra, measure wind stress and direction, measure surface currents, and determine the air and sea temperatures. Len Fedor and Vladimir Irisov of CIRES will fly two microwave imagers on the Twin Otter at the same time that APL/UW flys CORAR. Andy Jessup of APL/UW will also fly an infrared imager on the Twin Otter during the pilot experiment.

OBJECTIVES OF CORAR MEASUREMENTS

1) Investigate quadratic wave-wave interactions in deep, intermediate, and shallow waters using area-extensive measurements which are capable of resolving frequency differences between primary and forced waves of similar wavenumbers.

2) Make airborne measurements of the refraction of waves by bottom topography and currents in an attempt to determine bottom topography from area-extensive measurements.

3) Measure the attenuation and reflection of long waves as they propagate into shallow water and compare them with expected effects of different types of bottoms.

4) Investigate the effects of large-scale turning winds on long waves propagating into shallow water.

WORK COMPLETED

Following the initial funding of this project in March 1997, work first focussed on modifying the existing hardware and software to allow CORAR to operate at the speeds of the Twin Otter and to operate in both the rotating and imaging modes simultaneously. The modified system was flown for the first time on the NPS Twin Otter off the coast of Florida in March 1999. After we corrected problems found during this flight series, we flew CORAR in the main experiment in November and December, 1999. The system worked well except that a timing problem that was not discovered during the experiment kept us from properly filtering the signal from the rotating system. This will limit results from this system but should still allow the objectives of the project to be addressed.

Figure 1 below shows CORAR mounted on the Twin Otter as it was used in Florida. The gray cylindrical object beneath the fuselage that can be seen just behind the left wheel is the radome for the rotating antennas. The white sidelooking antennas (four feet long) are seen just above the radome directed to the left of the aircraft. The white pod just behind the door of the aircraft houses NOAA’s 37 GHz polarimetric radiometer. Another pod with a vertical slot in it can be seen on the end of the wing in the upper photograph. This houses NOAA’s scanning 60 GHz radiometer whose purpose is to measure the air/sea temperature difference.

Figure 1. CORAR and NOAA’s radiometers mounted on the NPS Twin Otter in North Carolina in November, 1999.

RESULTS

Figure 2 shows neutral wind speeds and directions at a 10 m height obtained from CORAR’s rotating mode during flights on December 3, 1999. These results were obtained using standard scatterometer techniques by observing the angular dependence and wind speed dependence of the normalized radar cross section of the sea. The model function used to extract winds agreed well with that used to retrieve winds from NASA’s NSCAT satellite-borne scatterometer for winds above about 5 m/s. Below that speed, we found that better wind speeds were obtained using measurements we made several years ago on an airship as the model function (Plant et.al., 1998). The reason for the difference between the spaceborne scatterometer model function and the airship data is the greater variability of the wind over the large footprint of the satellite scatterometer than over the small footprint of the airborne system. Details of this phenomenon are given in Plant (2000).

Figure 2. Neutral winds at 10 m heights obtained via scatterometry from CORAR’s rotating mode during SHOWEX.

Investigation of wave spectra is the real objective of SHOWEX, however, and distorted wave spectra have been obtained to date from CORAR’s imaging mode. A pair of these spectra are shown in Figure 3. The spectrum on the left is the spectrum of the wave-induced modulation of the radar cross section which accentuates the low frequency part of the wave spectrum. Wave height spectra cannot be obtained from it without knowledge of the frequency behavior of the modulation transfer function. The spectrum on the right is that of the wave-induced modulation of line-of-sight velocity. This spectrum emphasizes the high-frequency part of the wave spectrum. Wave height spectra can be obtained from it using the well-known relationship between wave height and wave orbital velocity. Obviously a multiplicity of wave trains are visible in these two spectra and should yield interesting results on wave/wave interactions in the near future.

The distortions in these spectra (note the "encounter wavenumber" on the bottom axis) are a result of the low speed of the Twin Otter, about 50 m/s. A low speed was deliberately chosen so that spectra from the imaging mode would yield information about the intrinsic speed of the waves. When compared with spectra from the rotating mode, which have not been produced yet, these spectra will yield information about the dispersion relationships of the various wave trains.