Remote Sensing Division Naval Research Lab, Washington, DC 20375 Separating Whitecap Fraction of...

Remote Sensing DivisionNaval Research Lab, Washington, DC 20375

Separating Whitecap Fraction of Active Wave Breaking

From Satellite Estimates of Total Whitecap Fraction

Magdalena D. Anguelova and Paul A. Hwang

20 Feb, 2012 Separate active whitecap fraction, Anguelova& Hwang, NRL

Enhanced when waves break Surface expression of breaking

waves Whitecaps Suitable forcing variable!

Air-sea processes and whitecaps

Sea spray flux Gas flux

Sensible heat flux Latent heat flux Enthalpy flux

Momentum flux Dissipation rate

Ambient noise

Mas

sH

eat

Ene

rgy

2

20 Feb, 2012




Ambient noise

Whitecap fraction

Whitecap fraction W The fraction of ocean surface

covered with foam Includes all stages of whitecap

lifetime

Active whitecap fraction WA (zoom

image)

Foam associated with breaking wave crests

Includes only the initial stages of whitecap lifetime

The foam moves along with the wave

Click imageto zoom out

3Separate active whitecap fraction, Anguelova& Hwang, NRL

Mas

sH

eat

Ene

rgy

20 Feb, 2012




Ambient noise

Separate active part from the total

Photographic measurements: Obtain W or WA

Unreliable separation

Radiometric measurements: Obtain W W database W variability

But, we do not have WA

We need to do the next step: New data set WA from W


W

W

W

W

WA

WA

WA

WA

WA

WA

Mas

sH

eat

Ene

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20 Feb, 2012

Physical basis: The concept of breaking crest

Breaking crest length distribution:

Active whitecap coverage:

Energy dissipation:

WA() relationship:

Expression for from the wave spectrum

Integrate over c and obtain:


cdcccdcc

,)(),(

c

A cdcTcW

)(

cdccbgcdc

)()( 51

c

A cdccgTbW

)(41

is breaking fronts velocity

cdc

)(

minmax

4min ln4 cccb

gTW

wA

c

20 Feb, 2012

Evaluate

Determine values of T, b, cmin , and cmax

Obtain active whitecap fraction

minmax

4min ln4 cccb

gTW

wA


20 Feb, 2012

From measured wave spectrum Equilibrium range model

Hanson and Phillips (1999) Gulf of Alaska data

Parametric approach Hwang and Sletten (2008)

Buoy data for Tp and Hs

Validate with measurements

Total dissipation rate


*3.3

*3 2.0, Ua

20 Feb, 2012

Separate swell

Hanson and Phillips (2001) Topographic minima method

Hwang et al. (2012) Spectrum integration method


Hanson and Phillips (1999)

p

Swell

Measured spectrum

Equilibrium range of wind sea

Dimensionless

Dim

ensi

onle

ss S

()

20 Feb, 2012

Parametric approach: Example


*

*3.3

*3 2.0, Ua*

3.3*2.0

Other dataset (Hwang et al, 2011)

Buoy 41001 mixed

Buoy 41001 wind-sea

20 Feb, 2012

Choose values T, b, cmin , and cmax

Breaking parameter

Bubbles persistence , where Tpw from buoy Tp = 0.2

Mueller and Veron, 2009: WA of Monahan, 1993 Tpw influence limited, We would consider other factors: salinity, SST, surfactants

T(0.77 to 2.5) s

Breaker speed cmin = ccpw,

c = 0.3 Gemmrich et al., 2008; fully developed sea Others suggest c 0.8

cmin (1.8 to 5.6) m s-1

cmax/cmin = 10


minmax

4min ln4 cccb

gTW

wA

pwTT

2pw

pw

gTc

b = 0.0157

20 Feb, 2012

Active from total whitecap fraction

Having WA() from buoy data Make match-ups with W from

WindSat 0.5º0.5º around buoy position

Find factor R = WA/W Buoy-satellite match-ups at

different latitudes Parameterize R in terms of

Wind speed or Geography (lat, lon)

Use W database from satellites and R to build WA database


2006

20 Feb, 2012

Active whitecap fraction Phillips-Buoy


20 Feb, 2012



20 Feb, 2012

Sensitivity to value choices


20 Feb, 2012

Sensitivity to choice of cmin


20 Feb, 2012

Sensitivity to value choices


20 Feb, 2012

Sensitivity to choice of T


20 Feb, 2012

Sensitivity to choice of b


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Ratio WA/W


20 Feb, 2012

Summary

Obtain WA from W on a global scale using satellite data Phillips concept to obtain WA from dissipation rate Buoy data for wave spectrum

Isolate swell Parametric approach to obtain Choose values for coefficient of proportionality Calculate WA()

Obtain factor WA/W Future work:

Validate with independent measurements, previous and new

Refine choices for T, b, cmin, and cmax/cmin

Extend WA calculations for more latitudes Parameterize WA/W


20 Feb, 2012 23Separate active whitecap fraction, Anguelova& Hwang, NRL

Click imageto zoom in

Wind direction

WA

W

Photo courtesy of Prof. William M. Drennan, RSMAS, Miami

20 Feb, 2012

Breaking parameter b


b = 0.0157Value # Reference bx103 Notes

1 Duncan (1981) 44 8 Quasi-steady breaking

2 Duncan (1983) 32 to 75 Quasi-steady breaking

3 Melville (1994) 4 to 12 Unsteady, Lowen (1991) data

4 Melville (1994) 3 to 16 Unsteady, scaling arguments

5 Phillips et al. (2001) 0.7 to 1 Radar field measurements

6 Deane&Stokes (2002) 8.6 Unsteady, lab bubble data

7 Melville et al. (2002) 7 Unsteady, lab measurements

8 Banner&Peirson (2007) 0.08 to 1.2 Lab measurements

9 Gemmrich et al. (2008) 0.032 to 0.101 (c ) from video, FLIP

10 Drazen et al. (2008) 3 to 63 Lab data (Fig. 11), b=0.31S2.77

11 Thomson et al. (2009) 17 30 Lake measurements

12 Thomson et al. (2009) 13 50 Indirect estimate

13 Iafrati (2011) 8 to 11 Model (Fig. 7 and eq. 16)Mean value 15.7

Remote Sensing Division Naval Research Lab, Washington, DC 20375 Separating Whitecap Fraction of...

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