Novel Applications of Remote Sensing
for Improved Quantification of
Sea Spray Source Function
Magdalena D. Anguelova
Michael H. Bettenhausen
William F. Johnston*
Peter W. Gaiser
Remote Sensing Division,
Naval Research Laboratory, Washington, DC
* Computational Physics, Inc., Springfield, VA
Motivation
 Whitecaps
 Gas exchange
 Ocean albedo & roughness
 Geophysical retrievals
Dominate the activation of CCN
 Sea spray aerosols
 Direct effect – cooling
 Indirect effect
 Halogen chemistry
 Reactive Cl and Br
 Tropospheric O3
 Sink of S
 Surface wind
 Ocean color
 Salinity
 Sea spray droplets
 Heat exchange
 Tropical storm intensification
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
Photo courtesy of C. Fairall

 Compete with SO42- aerosols
2
Sea spray source function
Rate of production of sea spray per unit area per increment
of droplet radius, r (s-1 m-2 m-1).
dF (r , a, b,... )
= f(U )  f( r )
dr
Scaling factor Size distribution
Magnitude
Shape
-6
3.41

=

f (U ) W (U10 ) 3.8 10 U10
From photographic
measurements
(Monahan and O’Muircheartaigh, 1980)
f (r80 )
d
f (r ) 
d log10 r80
6 Dec, 2011
From measurements using
various methods
Whitecaps and sea spray, Anguelova et al., NRL
3
Possible improvements
 For the size distribution f(r)
 Extend the size range, large and small ends
1 μm < r80 < 25 μm
de Leeuw et al.
O’Dowd et al.
Keene et al.
Quinn & Bates et al.
Meskhidze & Peters et al.
0.1 μm < r80 < 250 μm
 Recognize and include the effect of organics
 Introduce ambient factors f (r)  f ( r , a , b ...)
(de Leeuw et al., 2011)
Ernie Lewis poster
Sofiev et al., 2011
 For the scaling factor f(U)
 Less uncertainty in measuring W
 Introduce ambient factors W (U )  W (U , a , b ...)
 Remote sensing: method, results, implications
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Objective
 Measure W globally
 Compile database
 Model the high variability of whitecap fraction
W (U )
W (U , T , X , d ,U cur , Ts , S , C)
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
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
5
Whitecaps signature
High
Reflectivity
Vis
6 Dec, 2011
Reflectivity
Emissivity
High
Emissivity
IR
Whitecaps and sea spray, Anguelova et al., NRL
mW
6
Remote sensing of sea foam
 Microwave region
 Advantages
e = (1 - W ) er + W e f
TB = e Ts
 Transparent (almost) atmosphere
 “...4% problem ...at 5 GHz,..., 90% problem at IR” (Swift, 1990)
 Tractable atmospheric correction
 Clouds penetration
 Drawback
 Low resolution
 Smoother geophysical variability
 Trade-off: global coverage
 More data
 Various conditions
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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 Rough sea surface model
 2-scale
 Wave spectrum
e = (1 - W ) er + W e f
Anguelova and Webster, 2006
Anguelova et al., 2006
 Durden/Vesecky/Yueh
 Tuned for roughness only
 Using WindSat code (v. 1.9.6)
© P.R.Hemington
z=0
 Foam emissivity model
 RT model
 Layer with vertically non-uniform
properties
 Distribution of thicknesses
Air, ε0=1
Foam, ε (z)
Courtesy of Prof. Cilliers
6 Dec, 2011
www.pbase.com/petehem/
Models
Whitecaps and sea spray, Anguelova et al., NRL
Water, ε
z = -d
8
Data
 Independent sources





TB from WindSat
V, L from SSM/I or TMI
U10 and  from QuikSCAT or GDAS
Ts from GDAS
S = 34 psu
 Trade-off:
 Sampling issues
 Use long-term data
Sample count
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Estimates of W
 Improvements over the feasibility study (Anguelova and Webster, 2006):
 More physical models
 Independence of the variables
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Compare to photographic data
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
11
Further development
 Models
 Higher resolution
 Improved wave spectrum in 2-scale model
 Comparisons/Validation
 More points for direct comparisons
 Indirect comparisons in terms of other variables
 CO2 fluxes from ship cruises
o and COARE CO2 parameterization
 AOD from AERONET
o and AOD from microphysical aerosol model
 Uncertainty characterization
 Currently uncertainty minimization
 Evaluate the remaining using global geophysical model
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Whitecaps data base
 All available orbits for
 Resolution 50×70 km2
 Time period
 Entire 2006
 Months of 2003, 2007 and 2008
 Gridded data
 With 0.5 x 0.5 grid box
 Any other N x N possible
 Time periods:
 Daily
 Monthly
 Weekly (7 days)
 3-days
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Geographic characteristics of W
March, 2007
0.5 x 0.5
Wind speed formula
W  U103
Satellite, 10.7 GHz, H pol.
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Geographic characteristics of W
March, 2007
0.5 x 0.5
Wind speed formula
Satellite, 18.7 GHz, H pol.
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Develop parameterization(s)
W (U )
W (U , T , X , d ,U cur , Ts , S , C)
 Relative importance of the variables
 Correlation analysis
Wind
Fetch
SST
Stability
Wave height
Wave period
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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New sea spray source function
dF (r , a, b,... )
= f(U )  f( r )
dr
Scaling factor Size distribution
Magnitude
Shape
 Include wave-field characteristics and one
more factor W (U, H )
s
W (U , H s , Ts )
 Improve/choose size distribution
 Include organics
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Sea-salt flux
Annual sea-salt flux (1998)
Haywood et al., Science, 1999
Anguelova and Webster, 2006
2105
 Solar irradiance at TOA
(W/m2)
:
4105
6105
8105
Number flux, dF (s-1 m-2 )
GCM – ERBE
 NO aerosols;
 Max difference over the
oceans.
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Sea foam definition
 Oceanographic definition
 Passive remote sensing
 Whitecaps on the surface
 Radiometers detect only the
 Bubble plumes below
surface foam layers
 Skin depth
 At microwave frequencies
 a few mm to a few cm
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Photographic measurements
 High uncertainty: up to 30% or more
 Intensity threshold
 A and B stages under oblique angle
Stramska and Petelski, 2003
 Patchy coverage
 Open ocean
 Southern ocean!
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Improvements and restrictions
(de Leeuw et al., 2011)
W (U10 )
W (U10 , T )
Whitecap fraction, W (%)
W (U10 , Ts )
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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Modeling whitecap fraction
6 Dec, 2011
Whitecaps and sea spray, Anguelova et al., NRL
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