Validation of satellite-based estimates 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

16th Conference on Air-Sea 16th Conference on Air-Sea InteractionInteraction

11–15 January 2009, Phoenix, Arizona Validation of satellite-Validation of satellite-

basedbased

estimates of whitecap estimates of whitecap

coverage: Approaches and coverage: Approaches and

initial resultsinitial results

Naval Research Laboratory, Washington, DCApplied Physics Laboratory, University of Washington, Seattle,

WANational Oceanography Centre, Southampton, UK

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

Long-term goalLong-term goalImprove Sea Salt Source Function parameterization by modeling the high variability of whitecap coverage

CSTUdXTuW scur ,,,,,,, uW

STuW s ,,*

or

u – wind speed (u10 or u*?)T – atmospheric stability (= Tair – Tsea)X – wind fetch

d – wind durationUcur – water currentsTs – sea surface temperatureS – salinityCk – concentration, type (k) of surface active materials


The first stepThe first step

FrameworkFramework

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;

CSTUdXTUW scur ,,,,,,,Whitecap variability:Whitecap variability:


Daily map of Daily map of WW

Daily data (swath) for entire 2006, months of 2007 and 2008


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 ;

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.

Satellite-based foam Satellite-based foam fraction fraction WW


ValidationValidation

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.


Validation approachesValidation approaches

Historical database of in situ values; Wind speed formula;Ship-borne measurements;Air-borne measurements.

There are questions and issues

with each approach.


In situIn situ historical data by historical data by typetype


10 GHz seems good10 GHz seems good


All Frequencies, H polAll Frequencies, H pol


Satellite vs Wind formulaSatellite vs Wind formula

Wsat more uniform by latitude;

High lat higher W.

March, 2007, 0.5 deg x 0.5 deg

Satellite, 18.7 GHz, H pol.

Wind speed formula: Monahan and

O’Muircheartaigh, 1980: U10 from QuikSCAT or GDAS; Time/space matched with

WindSat;

41.310

61084.3 UW


Difference mapsDifference mapsWW = = WWsatsat – – WWmodmod

18H10H

+W = 0.041% ; - W = 0.44% +W = 0.61% ; - W = 0.63%


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;

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;

PolarfrontPolarfront ship data ship data


PolarfrontPolarfront to historical to historical in in situsitu


WindSat matched to WindSat matched to PolarfrontPolarfront


In situIn situ and satellite winds and satellite winds

180-min time window


RASSI ExperimentRASSI 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.




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.




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.


Hurricane Dean flightHurricane Dean flight

KSYP

#6

#7

#42003

#42055

3FPQ9

#1#2

#3

#4#5

Buoy

Ship

RASSI


RASSI foam vs historical RASSI foam vs historical in in situ situ


RASSI vs WindSat pairsRASSI vs WindSat pairs

Wind dependent AB factor Wind dependent AB factor 11 to 15 11 to 15 using Monahan & Woolf using Monahan & Woolf (1989) parameterizations for (1989) parameterizations for A+B and AA+B and A


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.

SummarySummary

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.

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

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