Post on 19-Jan-2016
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
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20 Feb, 2012
Sea spray flux Gas flux
Sensible heat flux Latent heat flux Enthalpy flux
Momentum flux Dissipation rate
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
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eat
Ene
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20 Feb, 2012
Sea spray flux Gas flux
Sensible heat flux Latent heat flux Enthalpy flux
Momentum flux Dissipation rate
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
4Separate active whitecap fraction, Anguelova& Hwang, NRL
W
W
W
W
WA
WA
WA
WA
WA
WA
Mas
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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:
5Separate active whitecap fraction, Anguelova& Hwang, NRL
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
6Separate active whitecap fraction, Anguelova& Hwang, NRL
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
7Separate active whitecap fraction, Anguelova& Hwang, NRL
*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
8Separate active whitecap fraction, Anguelova& Hwang, NRL
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
9Separate active whitecap fraction, Anguelova& Hwang, NRL
*
*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
10Separate active whitecap fraction, Anguelova& Hwang, NRL
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
11Separate active whitecap fraction, Anguelova& Hwang, NRL
2006
20 Feb, 2012
Active whitecap fraction Phillips-Buoy
12Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Active whitecap fraction Phillips-Buoy
13Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Active whitecap fraction Phillips-Buoy
14Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Active whitecap fraction Phillips-Buoy
15Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Sensitivity to value choices
16Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Sensitivity to choice of cmin
17Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Sensitivity to value choices
18Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Sensitivity to choice of T
19Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Sensitivity to choice of b
20Separate active whitecap fraction, Anguelova& Hwang, NRL
20 Feb, 2012
Ratio WA/W
21Separate active whitecap fraction, Anguelova& Hwang, NRL
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
22Separate active whitecap fraction, Anguelova& Hwang, NRL
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
24Separate active whitecap fraction, Anguelova& Hwang, NRL
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