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Validation of satellite-based estimates of whitecap coverage: Approaches and initial results

16th Conference on Air-Sea Interaction 11–15 January 2009, Phoenix, Arizona. Validation of satellite-based estimates of whitecap coverage: Approaches and initial results. Magdalena D. Anguelova, Justin P. Bobak, William E. Asher, David J. Dowgiallo,

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Validation of satellite-based estimates of whitecap coverage: Approaches and initial results

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  1. 16th Conference on Air-Sea Interaction 11–15 January 2009, Phoenix, Arizona Validation of satellite-basedestimates of whitecap coverage: Approaches and initial results Magdalena D. Anguelova, Justin P. Bobak, William E. Asher, David J. Dowgiallo, Ben I. Moat, Robin W. Pascal, Margaret J. Yelland Naval Research Laboratory, Washington, DC Applied Physics Laboratory, University of Washington, Seattle, WA National Oceanography Centre, Southampton, UK

  2. Long-term goal Improve Sea Salt Source Function parameterization by modeling the high variability of whitecap coverage or • u – wind speed (u10 or u*?) • T – atmospheric stability (= Tair – Tsea) • X – wind fetch • d– wind duration • Ucur – water currents • Ts – sea surface temperature • S – salinity • Ck– concentration, type (k) of surface active materials 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  3. Framework Whitecap variability: • Improve existing or develop new models; • Extensive database: W + various factors; • Measurements: W + various factors; • Existing W measurements: • Photographs/video images; • Insufficient for extensive database; • Alternative approach: From satellites to get • global coverage; • wide range of meteo & environ conditions; The first step 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  4. Daily map of W Daily data (swath) for entire 2006, months of 2007 and 2008 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  5. Satellite-based foam fraction W • Independent sources for the input variables: • TB from WindSat; • V, L from SSM/I or TMI; • U10 from QuikSCAT, SSM/I, or GDAS; • Ts from GDAS; • S = 34 psu; • Improvements over the published feasibility study: • More physical models for er, ef, and atm. corr.; • Independence of the variables; • Minimization of errors. • Retrieving W (changes in TB at microwave frequencies): • Using parts of WindSat forward model (v.1.9.6) ; • Rough surface emissivity, er ; • Atmospheric variables  atm. correction; • Foam emissivity model, ef ; 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  6. Validation • Insufficient ground truth values: • Data collection: • Slow and expensive; • Sporadic and non-systematic; • Limited range of conditions; • Fewer in situ-satellite matches in time and space; • Different principles of measurement: • Reflectivity in the Visible (photographic/video); • Emissivity in the Microwave (radiometer); • Various approaches to circumvent difficulties. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  7. Validation approaches • Historical database of in situ values; • Wind speed formula; • Ship-borne measurements; • Air-borne measurements. There are questions and issues with each approach. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  8. In situ historical data by type 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  9. 10 GHz seems good 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  10. All Frequencies, H pol 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  11. Wind speed formula: • Monahan and O’Muircheartaigh, 1980: • U10 from QuikSCAT or GDAS; • Time/space matched with WindSat; Satellite vs Wind formula March, 2007, 0.5 deg x 0.5 deg • Wsat more uniform by latitude; • High lat higher W. Satellite, 18.7GHz, H pol. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  12. Difference maps +W = 0.041% ; - W = 0.44% +W = 0.61% ; - W = 0.63% W = Wsat – Wmod 10H 18H 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  13. Polarfront ship data • In situ data for W; • Two cameras, daylight restrictions (Mar-Oct); • Photographic data processed at 3 intensity thresholds with AWE technique; • Temporally-averaged values in a time window around or close to WindSat pass time; • The effect of time window (minutes to 3 hours) was investigated; • WindSat data for W: • Closest pixel to lat/lon position of each in situ point; • WindSat low resolution (50 km x 71 km); • Three frequencies (10, 18, and 37 GHz), H pol.; • The effect of averaging over NºxNº box (e.g., 1/2ºx1/2º) was investigated; • Experiment HiWASE on Polarfront ship positioned at Station M; • UK colleagues: Margaret Yelland, Ben Moat and Robin Pascal; • Long-term (Sep 2006 to Sep 2009) measurements of W and other variables; • In situ data for W; • Two cameras, daylight restrictions (Mar-Oct); • Photographic data processed at 3 intensity thresholds with AWE technique; • Temporally-averaged values in a time window around or close to WindSat pass time; • The effect of time window (minutes to 3 hours) was investigated; • WindSat data for W: • Closest pixel to lat/lon position of each in situ point; • WindSat low resolution (50 km x 71 km); • Three frequencies (10, 18, and 37 GHz), H pol.; • The effect of averaging over NºxNº box (e.g., 1/2ºx1/2º) was investigated; 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  14. In situ historical data by type 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  15. Polarfront to historical in situ 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  16. WindSat matched to Polarfront 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  17. In situ and satellite winds 180-min time window 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  18. In situ and satellite winds 180-min time window 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  19. RASSI Experiment • RAdiometry and Sea Surface Imagery (2007): • North Atlantic, Gulf of Mexico (Hurricane Dean); • High altitude: 6.7 km (20,000 ft); • Clear sky to partial cloud cover; • Radiometric measurements: • APMIR (Airborne Polarimetric Microwave Imaging Radiometer) • Channels available (GHz): 37VH34, 19VH34, 6.6VH, 6.8VH, 7.2VH (some data at channels at 10.7 and 22.235); • Footprint roughly 1x2 km from on 19 and 37 GHz; • Video measurements: • High resolution video camera; • Field of view of 159 m by 119 m. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  20. RASSI Experiment • RAdiometry and Sea Surface Imagery (2007): • North Atlantic, Gulf of Mexico (Hurricane Dean); • High altitude: 6.7 km (20,000 ft); • Clear sky to partial cloud cover; • Radiometric measurements: • NRL’s APMIR (Airborne Polarimetric Microwave Imaging Radiometer) • Channels available (GHz): 37VH34, 19VH34, 6.6VH, 6.8VH, 7.2VH (some data at channels at 10.7 and 22.235); • Footprint roughly 1x2 km from on 19 and 37 GHz; • Video measurements: • High resolution video camera; • Field of view of 159 m by 119 m. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  21. RASSI Experiment • RAdiometry and Sea Surface Imagery (2007): • North Atlantic, Gulf of Mexico (Hurricane Dean); • High altitude: 6.7 km (20,000 ft); • Clear sky to partial cloud cover; • Radiometric measurements: • NRL’s APMIR (Airborne Polarimetric Microwave Imaging Radiometer) • Channels available (GHz): 37VH34, 19VH34, 6.6VH, 6.8VH, 7.2VH (some data at channels at 10.7 and 22.235); • Footprint roughly 1x2 km from on 19 and 37 GHz; • Video measurements: • UW’s High resolution video camera; • Field of view of 159 m by 119 m. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  22. Buoy Ship RASSI KSYP #42003 #1 #2 #3 #4 #5 3FPQ9 #6 #42055 #7 Hurricane Dean flight 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  23. In situ historical data by type 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  24. RASSI foam vs historical in situ 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  25. RASSI foam vs historical in situ 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  26. RASSI vs WindSat pairs Wind dependent AB factor 11 to 15 using Monahan & Woolf (1989) parameterizations for A+B and A 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  27. RASSI vs WindSat pairs Wind dependent AB factor 11 to 15 using Monahan & Woolf (1989) parameterizations for A+B and A 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  28. Summary • Compensate using different approaches: • Historical in situ data; • Wind speed formula; • Direct validation with ship-borne photographic data; • Direct validation with air-borne video data; • Results: • Ball-park in magnitude compared to in situ data; • More uniform latitudinally than wind formula; • Direct validation shows: • Wsat overestimate at low winds and • Relatively good estimate at high winds; • Future work: • More match-ups of in situ and satellite data; • Indirect validation (with other variables, not directly W); • Tuning of the satellite-based algorithm. • Difficulties in validating satellite-based foam fraction; • Amount of data; • Conditions covered; • Principle of measurements; • Compensate using different approaches: • Historical in situ data; • Wind speed formula; • Direct validation with APMIR/video data; • Direct validation with ship data; • Results: • Ball-park in magnitude compared to in situ data; • More uniform latitudinally than wind formula; • Direct validation shows: • underestimate at low winds and • over estimate at high winds; • How much of this result is correct? • Future work: • More match-ups of in situ and satellite data; • Indirect validation (with other variables, not directly W); • Tuning of the satellite-based algorithm. ? 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  29. Additional slides 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  30. Sample count Match-up issues • Only 4 full swaths: • from GDAS, • Large temporal mismatch for other GDAS match-ups; • Chunks of swaths for most passes: • due to QuikSCAT passes CROSSING the WindSAT passes; • Reflects on the number of samples available for low and high latitudes. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  31. Sample count Samples available for 1 month • High latitudes with high winds are under represented. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  32. Frequency and polarization dependence • Understanding the info each freq and pol gives for W: • Research foam skin depth; • The effect of foam thickness on the skin depth and foam emissivity; • The results could be important for gas exchange (CO2 and other gases); • Combining suitable freqs and pols in one “oceanographically” representative W; 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  33. Compare to in situ A+B data 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  34. Low freqs close to in situ A+B 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  35. 18 GHz • Very similar to 23 GHz • Atmosphere influence? 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  36. All frequencies, V pol 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  37. All Frequencies, H pol 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  38. Biases • Pair binned data; • W = Wsat-Wins; • Plot bins with high count. Freq (GHz), H pol. 6 10 18 23 37 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  39. Satellite vs Wind formula March, 2007 0.5 deg x 0.5 deg Wind speed formula Satellite, 10.7 GHz, H pol. 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  40. Ship cruises • Two ship cruises • in 2006 and 2007 • Ian Brooks and Margaret Yelland, UK: • Data for: • Foam fraction – direct validation; • Sea-salt aerosol flux – indirect val. • Matchups with WindSat data • Spatial and temporal; • Video data availability for about 10 points; • Cruise data still processed 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  41. In situ and satellite winds (raw) 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  42. Binned by wind speed 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  43. Compare in situ – satellite pairs 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  44. Compare in situ – satellite pairs 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  45. RASSI measuring configuration 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  46. Wind speed conditions 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

  47. Wind vector field during the flight 16th Air-Sea Interaction Conference, 14 January, 2009, Anguelova et al.

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