The Aquarius Salinity Retrieval Algorithm

Frank J. Wentz and Thomas Meissner, Remote Sensing Systems Gary S. Lagerloef, Earth and Space Research

David M. Le Vine, NASA Goddard

Presented at 7th Aquarius/SAC-D Science MeetingBuenos Aires, ArgentinaApril 11, 2012

Slopes steeper in warm water and V-pol is steeper than H-pol

Principle:Microwave Emission Decreases with Increasing Salinity

V-pol Curves

H-pol Curves

Meissner-Wentz Model at 40 deg. Incidence Angle

Aquarius

Sun

SolarReflection

QuasiSpecular

SolarBackscatter

Galaxy & Cosmic

Moon

Ocean Surface

Challenge:Many Other Signals Must be Removed

Ionosphere

Atmosphere

Land

Salinity Retrieval Algorithm

Radiometer Calibration AlgorithmRFI flagging

Aquarius Radiometer CountsEarth + Calibration View

Total Antenna Temperature

Remove Space Contributions: Galaxy, Sun, Moon, CS

Earth Antenna Temperature

Remove the Antenna Pattern Effect

Earth Brightness Temperature (TOI)

Correct for Faraday Rotation

Top of the Atmosphere Brightness Temperature (TOA)

Remove Atmospheric Contribution

Sea-Surface Brightness Temperature

Remove Surface Roughness Effects

Specular Brightness Temperature

Find Salinity for which emissivity of Meissner-Wentz dielectric model matches specular TB (v-pol only for now)

Salinity

Required Ancillary Data

Remove Space Contribution 1.4 GHz Daily Solar Flux Measurements (Galaxy Table is Fixed)

Correct Faraday Rotation No ancillary data required; uses Aquarius 3rd Stokes Measurement

Remove Atmospheric Contribution NCEP Profiles of temperature, pressure, vapor, and cloud water

Remove Surface Roughness NCEP Surface Wind Speed and Direction Aquarius Scatterometer Radar Cross Section Measurements

Retrieve salinity from specular emissivity Reynolds OI SST

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Julian Day

Orb

ital A

ngle

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V-pol, Inner Horn

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Julian Day

Orb

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Reflected Galactic Radiation:A Big Signal that is Hard to Compute

Galaxy Emission at 1.4 GHz (up to 15 K) Specular Reflection of Galaxy off the Ocean (up to 6 K)

LeVine and Abraham, IEEE Trans. Geosci & Remote Sens., 42(1), 2004

Green arrows from Galaxy to Ocean: Red arrows from Ocean to Aquarius

Facet integration must be done for every ocean pixel seen by Aquarius Antenna

Computation is a 4-fold integral that must be computed to an accuracy of 0.05 K.

Overall effect is a smoothing of the galaxy map as winds increase.

2 ,, z u c

B u c B

P z zT dz dz T s s i s

s

k P k k nk z n z

Modeling Galactic Radiation:A Tilted Facet Model

Performance of Galaxy Model with Real Data

Rough Oceans Emit More Radiation

Combined NCEP and Scatterometer Roughness Correction

10 K

0 K

10 K

0 K

25 m/s0 m/sNCEP WIND SPEED

Scatt

erom

eter

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ma-

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Land (Sidelobe) Correction

September 2011 October 2011

November 2011 December 2011

Ascending minus Descending Salinity Differences O. Melnichenko, P. Hacker, et al.

Scintillation Map of Ionosphere Two bands around Magnetic Equator: http://www.insidegnss.com/node/1579

Ascending minus Descending Vpol - Hpol TB Differences Orbital Position Maps: Possible Cause: Third Stokes radiation leaking into Second Stokes

Inner Horn Middle Horn Outer Horn

Ascending minus Descending Vpol-Hpol TB Differences Orbital Position Maps: Antenna Pattern Correction modified

Inner Horn Middle Horn Outer Horn

Conclusions

Aquarius and the Salinity Retrieval Algorithm are working well.

Standard deviation of retrieval salinity versus HYCOM Salinity Model = 0.6 psu• Individual measurements (7-sec averages)• In the open ocean well away from land and excluding cold water (SST<5C)

Standard deviation for monthly averages = 0.4 psu.

Many issues/problems need to be resolved to achieve the mission goal of 0.2 psu.• Small radiometer pointing errors • Radiometer calibration time drifts• Ascending versus descending anomalies• Revised Antenna Pattern Correction• Optimum roughness correction• Better land contamination calculations; problem with islands • Influence of longer wave, galaxy, sun, faraday rotation, etc.• TBD