ANALYSIS OF SURF ZONE RADAR BACKSCATTER MEASUREMENTS

Transcription

1 ANALYSIS OF SURF ZONE RADAR BACKSCATTER MEASUREMENTS LONG-TERM GOAL David R. Lyzenga Naval Achitectue and Maine Engineeing Dept. Univesity of Michigan Ann Abo, MI phone: (734) fax: (734) Awad # N To undestand the elationship between ada backscatte and wave beaking, especially in shallow wate, and to develop emote sensing methods fo measuing and monitoing pocesses elated to wave beaking in littoal egions. OBJECTIVES The shot-tem objectives of this effot ae to establish empiical o semi-empiical elationships between the ada backscatte and beaking wave paametes such as the wave height and enegy dissipation ate, by compaing backscatte measuements made unde a ange of conditions with in situ wave measuements. Ultimately, a moe fundamental undestanding will be sought in tems of the hydodynamic and electomagnetic inteactions involved. APPROACH I have begun by examining the backscatte data collected by an aibone ada (the ERIM DCS) ove the Field Reseach Facility at Duck, Noth Caolina, duing a seies of flights in May, I have also begun to examine ada data collected by the ERS satellite, access to which was ganted by the Euopean Space Agency in esponse to a poposal submitted as pat of this effot. Finally, I plan to analyze maine navigational ada data collected by Dennis Tizna at Duck, NC, in ode to investigate the backscatte at low gazing angles. WORK COMPLETED Data fom two of the aibone synthetic apetue ada (SAR) images collected on May 7 and 14, 1996 have been analyzed and compaed with wave measuements made at the Field Reseach Facility. Two ERS SAR fames of the Duck aea have been acquied and one has been examined to date. I plan to ode ERS images fo some West Coast locations also, and have eviewed the wave data fom seveal NOAA buoys on the days coesponding to the available ERS images in ode to select an optimal data set fo this analysis. RESULTS Two unexpected esults wee obtained duing the fist yea of this poject. The fist is that the ada backscatte is not popotional to the ate of enegy dissipation due to wave beaking, as was initially

2 Repot Documentation Page Fom Appoved OMB No Public epoting buden fo the collection of infomation is estimated to aveage 1 hou pe esponse, including the time fo eviewing instuctions, seaching existing data souces, gatheing and maintaining the data needed, and completing and eviewing the collection of infomation. Send comments egading this buden estimate o any othe aspect of this collection of infomation, including suggestions fo educing this buden, to Washington Headquates Sevices, Diectoate fo Infomation Opeations and Repots, 1215 Jeffeson Davis Highway, Suite 1204, Alington VA Respondents should be awae that notwithstanding any othe povision of law, no peson shall be subject to a penalty fo failing to comply with a collection of infomation if it does not display a cuently valid OMB contol numbe. 1. REPORT DATE REPORT TYPE 3. DATES COVERED to TITLE AND SUBTITLE Analysis of Suf Zone Rada Backscatte Measuements 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Univesity of Michigan,Naval Achitectue and Maine Engineeing Depatment,Ann Abo,MI, PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR S ACRONYM(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Appoved fo public elease; distibution unlimited 13. SUPPLEMENTARY NOTES See also ADM ABSTRACT 11. SPONSOR/MONITOR S REPORT NUMBER(S) 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT a. REPORT b. ABSTRACT c. THIS PAGE Same as Repot (SAR) 18. NUMBER OF PAGES 4 19a. NAME OF RESPONSIBLE PERSON Standad Fom 298 (Rev. 8-98) Pescibed by ANSI Std Z39-18

3 supposed. Rathe, it was found that the ada coss section pe unit aea, integated ove the entie extent of the suf zone in the coss-shoe diection, is popotional to the incident wave height o, oughly, the squae oot of the integated enegy dissipation ate. This esult was obtained fom an analysis of the two aibone SAR images shown in Figue 1. The incident wave height was appoximately 1.5 m on May 7 and 0.5 m on May 14. The incident enegy flux, and thus the ate of enegy dissipation pe unit distance in the along-shoe diection, was theefoe about a facto of nine lage on the fist day than the second; but the integated ada coss section is only about a facto of thee lage. Figue 1(a). X-band, HH-polaization aibone SAR image collected ove Duck, NC on May 7, Figue 1(b). X-band, HH-polaization aibone SAR image collected ove Duck, NC on May 14, This esult led to the development of a new model fo the beaking wave backscatte, based on the assumption that the backscatte coss section is popotional to the suface aea of the beaking zone as defined by Duncan (1981) and as obseved by Walke et al. (1996). Accoding to this model the ate 2 of enegy dissipation pe unit cest length is given by H 0.1U gd L c 0.01U gl c whee L is the

4 length and d is the thickness of the beaking egion o olle. The ada coss section pe unit cest length is then popotional to 10 H U gc, and the ada coss section pe unit aea is given by L o V D f B U gco whee D H f B beaking faction. O is the ate of enegy dissipation pe unit aea, O is the wave length and f B is the The second esult obtained duing this investigation is that wave beaking in the suf zone can cause eithe an incease o a decease in the ada backscatte, depending (appaently) on the wind speed as well as ada paametes such as the wavelength, polaization, and incidence angle. This is illustated by the two ERS sub-images shown in Figue 2, both of which wee collected on Novembe 20, 1996, with an incident wave height of slightly ove 1 m. The image on the left epesents an aea appoximately 60 km noth of the Duck pie, whee the wind speed was appaently quite low (pobably less than 3 m/s), as evidenced by the dak backgound in the offshoe egion. The bight band nea the shoeline is intepeted to be the esult of wave beaking in the suf zone, and is consistent with the bight etuns obseved in the aicaft images shown in Figue 1. The image shown in Figue 2(b) includes the egion suounding the Duck pie, whee the wind speed was ove 3.5 m/s accoding to measuements at the FRF facility. In this egion, thee is a dak band nea the shoeline instead of a bight line. It is hypothesized that this dak band is due to a suppession of the shot wind-geneated waves that ae esponsible fo the highe etun in the offshoe egion. (a) 60 km noth of Duck, NC (b) nea Duck pie (FRF facility) Figue x4.5 km subsets of C-band, VV-pol ERS-2 image collected on Novembe 20, 1996.

5 The decease in backscatte nea the shoeline in Figue 2(b) is elated to the elative magnitudes of two effects that may depend also on the incidence angle and the ada wavelength. The incidence angle fo the ERS images is 23 o, o about half that fo the aicaft images in Figue 1, and the ada wavelength is oughly twice as lage. These dependencies will be examined futhe though the analysis of additional data sets. IMPACT/APPLICATION The esults of this ongoing investigation ae expected to impact the development of emote sensing methods fo monitoing coastal pocesses. TRANSITIONS Some esults of this investigation have been used in the ONR Littoal Remote Sensing poject. RELATED PROJECTS In addition to the afoementioned connections with the LRS poject, this investigation is elated to numeical electomagnetic scatteing studies by Eic Eicson at ERIM Intenational and to maine navigational ada obsevations by Dennis Tizna at NRL. REFERENCES J.H. Duncan, An expeimental investigation of beaking waves poduced by a towed hydofoil, Poc. R. Soc. Lond. A 377, , D.T. Walke, D.R. Lyzenga, E.A. Eicson, and D.E. Lund, Rada backscatte and suface oughness measuements fo stationay beaking waves, Poc. R. Soc. Lond. A 452, , PUBLICATIONS D.R. Lyzenga and D.B. Tizna, Compaison of model pedictions of wave enegy dissipation with ada backscatte measuements in the suf zone, AGU Sping Meeting, Boston, MA, May 26, D.R. Lyzenga, Effects of intemediate-scale waves on ada signatues of ocean fonts and intenal waves, J. Geophys. Res. 103, , D.R. Lyzenga and G.O. Mamoino, Measuement of suface cuents using sequential synthetic apetue ada images of slick pattens nea the edge of the Gulf Steam, J. Geophys. Res. 103, , D.R. Lyzenga and E.A. Eicson, Numeical Calculations of Rada Scatteing fom Shaply Peaked Ocean Waves, IEEE Tans. Geoscience and Remote Sensing 36, , E.A. Eicson and D.R. Lyzenga, Pefomance of a Numeical Iteative Solution of the Suface Cuent Integal Equation fo Sufaces Containing Small Radii of Cuvatue, Radio Science 33, , 1998.