Radiometric Measurements of Whitecaps and Surface Fluxes Magdalena D. Anguelova Remote Sensing...
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Transcript of Radiometric Measurements of Whitecaps and Surface Fluxes Magdalena D. Anguelova Remote Sensing...
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RadiometricMeasurements
of Whitecaps
and Surface Fluxes Magdalena D. Anguelova
Remote Sensing DivisionNaval Research LaboratoryWashington, DC, USA
In memory of Ed Andreas
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WindSat frequencies (6–40 GHz)• What we can do now?• What are we missing?
High frequencies (40-200 GHz)• Marginal areas• Use of unmanned aerial vehicles (UAVs)
Low frequencies (L-band, GPS)• GNSS-R (Global Navigation Satellite System Reflectometry)• Salinity and Tropical Cyclones (TCs)
Radiometric Measurement at:
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Surface Fluxes
Courtesy: Jayne Doucette, WHOI
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Surface Fluxes and Whitecaps
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Whitecap Fraction• Photographic data
• Digital cameras• Improved algorithms
• IR data• Lifetime separation• Independent method
• Radiometric data• From satellites• All weather conditions• Independent method
de Leeuw et al., 2011Potter et al., 2015
VIS
IR
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Radiometric Satellite MeasurementsFreq (GHz)
6.810.718.723.837.0
Gaiser et al., 2004
Salisbury et al., 2013, 2014
October 200637 GHz
WindSat
Spatial resolution 25 km
suitable for open ocean
Temporal resolution twice a daysuitable for
long-term monitoring
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Need: High Spatial Resolution in Margins
• Littoral zone• Polar Regions
Northern SeaTrans-Polar
NW Passage
Arctic New Routes by 2025
Wind speed (m s-1)WindSat
Res 25 km
Coastal zone
High frequency compact
radiometer deployed on
UAVs
2012202
020252030
d ~ H / (FD)Antenna aperture DSensor frequency FFlight altitude H
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Need: High Temporal Resolution in All Weather
Rain bands limit surface observations needed
for wind retrievals.
…but, these frequencies
cannot penetrate rain to measure wind
vectors under TCs Wind Speed (kn)
Rain Rate (mm/hr)
Current microwave sensors operating at med-high frequencies can measure rain…
WindSat (6.8-23.8 GHz)
GPS frequencies penetrate rain and
provide high temporal sampling
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Radiative Transfer Equation • Measure TB at some height• Model TB contributions
• Atmospheric model (t etc.)• Existing mature tool
• Dielectric constant model (e0)• Fresnel equation
• Roughness model (er)• Wave spectrum
• Foam model (ef)
TBRadiometer
t
Flight altitude
eTs
TCB
TBU TBD
OceanTs
tetTs +2rt2TCB+ TBU + rtTBD
e = (1-W)(e0 + er) + Wef
r = 1 - e
TB =
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Modeling Foam Emissivity• Foam structure
• Air-water mixture• Closely packed bubbles• Bubble sizes and shape varying
• Bubble diameters• a << 1 mm• to a few mm
• Vertical profile• Void fraction• Bubble size distribution
• Foam layer thicknesses• A few mm• To 20 cm and more
air
water
Emissivity =
absorptivity
Attenuationka, ks, ke
Size parameter a/ Scattering in foam
Scattering theory
F > 40 GHza / 1
Scattering increases
F 40 GHza / 1
Scattering negligible Effective medium Anguelova and Gaiser, 2013
Photo: Bill Asher, APL (UW)
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Transition to High Frequencies• Atmospheric attenuation
• At 1-40 GHz favorable for surface observations• At > 40 GHz increasing
• However• Imaging windows• Surface contributes• Tabulate suitable conditions
• Model TB
• Compare to aircraft TB data
• Scattering• Multi-particle Mie code• Bubble size distribution• Vertical profile• Add spray layer
WindSat freqs
Total atmospheric transmissivityHumidity
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Low Frequency for SalinityL-band (1.4 GHz)• Aquarius mission• SMOS mission
Foam effects• Only very thick foam layers• Accuracy of salinity retrievals
at high winds• Detect haline wake after storm
passage
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Low Frequency for TC--GPS measurements
GPS Receiver
L1
Axelspace.com
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GPS Receiver
L1
Axelspace.com
GPS Receiver
L1
GNSS-R Remote Sensing
1 2 n f1f2
fn
REFLECTION & SCATTERING
Ruf et al., 2015
2 m s-1
7 m s-1
10 m s-1
Delay Doppler Maps
f
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Foam Reflectivity at High Winds
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ConclusionsLow Frequency
L-band and GPS L1• Higher temporal resolution (GPS)• All weather measurements• Measurements
• TC core• Salinity
• Foam modeling:• With effective medium theory• Low reflectivity limits GNSS-R
retrievals
High frequency 40-200 GHz
• High spatial resolution• Compact sensors on UAVs• Measurements
• Coastal zone • Arctic ocean
• Foam modeling• Identify atmospheric
conditions• With multi-particle Mie theory
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Thank you
Thank you!
?Acknowledge
Colleagues CollaboratorsChawn Harlow group
Exeter, UKJeff Piepmeier group
GSFCShannon Brown group
JPL
Mike BettenhausenIan AdamsJustin BobakPeter GaiserDerek BurragePaul Hwang