Detection and characterization of deep water wave breaking using moderate incidence angle microwave backscatter from the sea surface
Jessup, Andrew T.
MetadataShow full item record
The importance of wave breaking in both microwave remote sensing and air-sea interaction has led to this investigation of the utility of a Ku-Band CW Doppler scatterometer to detect and characterize wave breaking in the open ocean. Field and laboratory measurements by previous authors of microwave backscatter from sharp-crested and breaking waves have shown that these events can exhibit characteristic signatures in moderate incidence angle measurements of the radar cross-section (RCS) and Doppler spectrum. Specifically, breaking events have been associated with polarization independent sea spikes in the RCS accompanied by increased mean frequency and bandwidth of the Doppler spectrum. Simultaneous microwave, video, and environmental measurements were made during the SAXON experiment off Chesapeake Bay in the fall of 1988. The scatterometer was pointed upwind with an incidence angle of 45 degrees and an illumination area small compared to the wavelength of the dominant surface waves. An autocovariance estimation technique was used to produced time series of the RCS, mean Doppler frequency, and Doppler spectral bandwidth in real-time. The joint statistics of the microwave quantities indicative of breaking are used to investigate detection schemes for breaking events identified from the video recordings. The most successful scheme is based on thresholds in both the RCS and the Doppler bandwidth determined from joint distributions for breaking and non-breaking waves. Microwave events consisting of a sea spike in the RCS accompanied by a large bandwidth are associated with the steep forward face of waves in the early stages of breaking. The location of the illumination area with respect to the phase of the breaking wave, the stage of breaking development, and the orientation of an individual crest with respect to the antenna look-direction all influence the detect ability of a breaking event occurring in the vicinity of the radar spot. Since sea spikes tend to occur on the forward face of waves in the process of breaking, the whitecap associated with a given sea spike may occur after the crest of the wave responsible for the sea spike has passed the center of the illumination area. Approximately 70% of the waves which produce whitecaps within a distance of 5m of the bore sight location are successfully identified by a threshold-based detection scheme utilizing both RCS and bandwidth information. The sea spike statistics are investigated as functions of wave field parameters and friction velocity u*. For VV and HH polarization, the frequency of sea spike occurrence and the sea spike contribution to the mean RCS show an approximately cubic dependence on u*, which is consistent with theoretical modelling and various measures of whitecap coverage. The data also suggest that the average RCS of an individual sea spike is not dependent on u*. At high friction velocities (u*≈40-50cms-l), the contribution of sea spikes to the mean RCS is in the range of 5-10% for VV and 10-20% for HH. The wind speed dependence of the percentage of crests producing sea spikes is comparable to that of the fraction of breaking crests reported by previous authors. The percentage of wave crests producing sea spikes is found to vary approximately as (Re*)1.5, where Re* is a Reynolds number based on u* and the dominant surface wavelength. This result agrees with measurements of the degree of wave breaking by. previous authors and is shown to be consistent with a cubic dependence on u *. Models for the probability of wave breaking as a function of moments of the wave height spectrum are compared to our results. The Doppler frequency and bandwidth measurements are also used to inquire into the kinematics of the breaking process.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1990
Showing items related by title, author, creator and subject.
The sediments and stratigraphy of the East Coast continental margin : Georges Bank to Norfolk Canyon Stetson, Henry C. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1949-08)The continental shelf off the northeastern coast of the United States was the first of our offshore coastal areas to be charted in detail by the Coast and Geodetic Survey, starting on Georges Bank in 1930. The techniques ...
Wind, sea ice, inertial oscillations and upper ocean mixing in Marguerite Bay, Western Antarctic Peninsula : observations and modeling Hyatt, Jason (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2006-09)Two years of moored oceanographic and automatic weather station data which span the winter ice seasons of 2001-2003 within Marguerite Bay on the western Antarctic Peninsula (wAP) shelf were collected as part of the Southern ...
Ecology of chemical defenses of algae against the herbivorous snail, Littorina littorea, in the New England rocky intertidal community Geiselman, Joy Ann (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1980-02)In the New England rocky intertidal community, space is dominated by two perennial plant types, brown fucoid algae (Ascophyllum nodosum and several species of Fucus) in the mid zones and the red alga Chondrus crispus ...