RadiometricMeasurements

of Whitecaps

and Surface Fluxes Magdalena D. Anguelova

Remote Sensing DivisionNaval Research LaboratoryWashington, DC, USA

In memory of Ed Andreas

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:

Surface Fluxes

Courtesy: Jayne Doucette, WHOI

Surface Fluxes and Whitecaps

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

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

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

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

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 =

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)

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

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

Low Frequency for TC--GPS measurements

GPS Receiver

L1

Axelspace.com

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

Foam Reflectivity at High Winds

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

Thank you

Thank you!

?Acknowledge

Colleagues CollaboratorsChawn Harlow group

Exeter, UKJeff Piepmeier group

GSFCShannon Brown group

JPL

Mike BettenhausenIan AdamsJustin BobakPeter GaiserDerek BurragePaul Hwang