Radiometric Measurements of Whitecaps and Surface Fluxes Magdalena D. Anguelova Remote Sensing Division Naval Research Laboratory Washington, DC, USA In memory of Ed Andreas

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., 2011 Potter et al., 2015 VIS IR

Radiometric Satellite Measurements Freq (GHz) Gaiser et al., 2004 Salisbury et al., 2013, 2014 October GHz WindSat Spatial resolution  25 km suitable for open ocean Temporal resolution twice a day suitable for long-term monitoring

Need: High Spatial Resolution in Margins Littoral zone Polar Regions Northern Sea Trans-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 d ~ H / (FD) Antenna aperture D Sensor frequency F Flight 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 ( GHz) GPS frequencies penetrate rain and provide high temporal sampling

Radiative Transfer Equation Measure T B at some height Model T B contributions Atmospheric model ( t etc.) Existing mature tool Dielectric constant model ( e 0 ) Fresnel equation Roughness model ( e r ) Wave spectrum Foam model ( e f ) tetT s + 2 rt 2 T CB + T BU + rtT BD e = (1-W)(e 0 + e r ) + We f r = 1 - e TB =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 Attenuation k a, k s, k e Size parameter a / Scattering in foam Scattering theory F > 40 GHz a /  1 Scattering increases F  40 GHz a /  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 T B Compare to aircraft T B data Scattering Multi-particle Mie code Bubble size distribution Vertical profile Add spray layer WindSat freqs Total atmospheric transmissivity Humidity

Low Frequency for Salinity L-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1 22 nn f1f1 f2f2 fnfn REFLECTION & SCATTERING Ruf et al., m s -1 7 m s m s -1 Delay Doppler Maps f 

Foam Reflectivity at High Winds

Conclusions Low 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 GHz High spatial resolution Compact sensors on UAVs Measurements Coastal zone Arctic ocean Foam modeling Identify atmospheric conditions With multi-particle Mie theory

? Acknowledge Colleagues Collaborators Chawn Harlow group Exeter, UK Jeff Piepmeier group GSFC Shannon Brown group JPL Mike Bettenhausen Ian Adams Justin Bobak Peter Gaiser Derek Burrage Paul Hwang