PERFORMANCE AND POTENTIAL FOR IMPROVEMENT OF THE SMOS

DIELECTRIC CONSTANT MODEL

S. Guimbard and the BEC team

Institut de Ciències del Mar CSIC

SMOS Barcelona Expert CentrePg. Marítim de la Barceloneta 37-49, Barcelona SPAINE-mail: jfont@icm.csic.esURL: www.smos-bec.icm.csic.es

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• Introduction

• Dielectric model (DC) behaviors

• Recent results that confirm the use of Klein &

Swift, 1977

• A word on the key areas to improve DC models

• Conclusion

Outline

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DC models are based on few L-Band measurements:

Ho et al., 1974: Samples were collected from the Chesapeake Bay and the Pacific Ocean with the salinity ranging from 4.14 to 35.25%o. Measurements of the dielectric constant at 1.43 GHz of these samples were conducted at temperatures of 5, 10, 20, and 30ºC.

-> ControversyBlanch, S. & Aguasca, 2004: Seawater samples with salinities in the range [0-40] psu in step of 1 psu intervals. Measurements over a range of temperatures, from 0ºC to 40ºC in 0.7ºC intervals.

-> Not well documentedLang et al., 2008-2012: Seawater from Ocean Scientific International Limited (OS1L) at salinities of 30, 35 and 38.27 psu. Measurements over a range of temperatures, from 0ºC to 35ºC in 5ºC intervals.

-> Error bars are provided for the first time

Introduction

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Two Debye formulas for DC models are used:

• Single Debye relaxation:

Klein & Swift, 1977 (KS77)Ellison & al., 1996 (E96)Blanch & Aguasca, 2004 (BA04)

• Double Debye relaxation:

Stogryn, 1995 (S95)Meissner & Wentz, 2004/2012 (MW12)

Only KS77 and MW12 are considered in this study

Introduction

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Characteristics of DC models:

Klein & Swift, 1977 (KS77):-The dielectric constant at infinite frequency ϵ∞ is a constant: ϵ∞ =4.9

- > Seems to depend on temperature.-Static dielectric constant ϵs of sea water exhibits non-linearities

for low (T<5ºC) and high temperatures (T>25ºC)- > Inconsistent with all other models

-“Yield an accuracy of at least 0.3 K for Tb”

Meissner & Wentz, 2004/2012 (MW12) -Double Debye relaxation seems to be unnecessary at L-Band: 12 parameters have to be estimated

-> Introduce unnecessary complexity- The weight of L-band measurements is weak in the derivation of the model-The dielectric constant at infinite frequency ϵ∞ depends on both

temperature and salinity.

Introduction

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KS77 non-linearities

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The differences between the real part of KS77 and MW12 are important, ranging from -1.5 in cold waters (T<5ºC) to 1.2 in warm waters

The differences between the imaginary part of KS77 and MW12 are negligible for 15ºC<T<32ºC.

How errors of the imaginary and real parts influence the retrieval of SSS?

KS77 vs. MW12 behaviour

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A theoretical example

Brightness temperatures of a flat surface is calculated using KS77 and MW12(T and S annual mean fields (WOA2013) were used. The angle of incidence is 40°)

Taking the difference in term of 1sr Stokes gives:

The differences observed are:•Up to 0.35K in cold Waters•~ 0.1K between 30S-30N

0.1 K <-> 0.2 PSU

KS77 vs. MW12 behaviour

9Lang et al.,2010

“The interesting feature is the effect of changing only the imaginary part is much smaller than changing the real part.

This says that the real part must be measured more accurately than the imaginary part. “

This tends to prove that the real part is the major contributor to the differences observed between KS77 and MW12.

This point has been neglected in the past since the imaginary part is more sensitive to temperature and salinity changes.

Lang 2010 error analysis

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• Dinnat et al., IGARSS 2014

• Zhou et al., 2012 (GW)

New measurements of the George Washington University (GW) tend to prove that the Real part of KS77 is more accurate in the range 5< T < 30 C than MW12.

They have reprocessed the Aquarius data using KS77 instead of MW12, they show that

“Aquarius comparison with Argo improves when using KS77 except in very cold and warm waters.”

“Both models show fresher water than Argo in warm water (precipitation/stratification)”

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Key areas for DC improvement

2 regions need to be studied more accurately:

Cold water: SST < 5°C~16% of the ocean surface

(61x106 km2)

Warm water: SST > 25°C~37% of the ocean surface

(130x106 km2)

In building a comprehensive dataset coast, sea ice and precipitations have to be taken into account accurately. Depending on the constraint regarding these 3 parameters, the number of useful data is expected to be small.

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This study confirms the use of Klein & Swift, 1977 as dielectric constant model for SMOS even if there are some issues for low and high temperatures

Empirical adjustments of the KS77 for low and high temperatures should be easy to achieve.

Comprehensive dataset will be needed to assess the new adjustments.

Conclusion

Bibliography• W. Ho, W. Love and M.J. van Melle, Measurements of the dielectric properties of sea water at 1.43 GHz, NASA Contractor

Report CR-2458, 1974.• Klein, L. A. & Swift, C. T. An improved model for the dielectric constant of sea water at microwave frequencies, IEEE Trans.

Antennas Propag., 1977, 25, 104-111 , doi:10.1109/TAP.1977.1141539• Stogryn, A.; Bull, H.; Rubayi, K. & Iravanchy, S. The microwave permittivity of sea and fresh water GenCorp Aerojet, Azusa,

CA, 1995• Ellison, W.; Balana, A.; Delbos, G.; Lamkaouchi, K.; Eymard, L.; Guillou, C. & Prigent, C. Study and measurements of the

dielectric properties of sea water. ESTEC/ESA contract 11197/94/NL/CN, 1996 • Meissner, T. & Wentz, F. J. The complex dielectric constant of pure and sea water from microwave satellite observations,

IEEE Trans. Geosci. Remote Sens., 2004, 42, 1836-1849 , doi:10.1109/TGRS.2004.831888 • Blanch, S. & Aguasca, A. Seawater dielectric permittivity model from measurements at L-band. Proceedings of IGARSS 2004.,

2004, 1362-1365 , doi:10.1109/IGARSS.2004.1368671 • Somaraju, R. & Trumpf, J. Frequency, Temperature and Salinity Variation of the Permittivity of Seawater. IEEE Trans.

Antennas Propag., 2006, 54, 3441-3448 , doi:10.1109/TAP.2006.884290• Meissner, T. & Wentz, F. The Emissivity of the Ocean Surface Between 6 and 90 GHz Over a Large Range of Wind Speeds and

Earth Incidence Angles IEEE Trans. Geosci. Remote Sens., 2012, 50, 3004-3026 , doi:10.1109/TGRS.2011.2179662 • R. Lang, C. Utku, Y. Tarkocin and D. Le Vine, Accurate Measurements of the Dielectric Constant of Seawater at L Band, NASA

Technical Memo NASA/TM-2010-215861, Oct. 2010.• Zhou, Y.; Lang, R.; Drego, C.; Utku, C. & Le Vine, D. Improved measurements of seawater dielectric constant at L-band

Microwave Radiometry and Remote Sensing of the Environment (MicroRad), 2012 12th Specialist Meeting on, 2012, 1 -4 , doi:10.1109/MicroRad.2012.6185228

Backup slides

Ho et al., 1973 measurements controversy

• More recently Stogryn et al. (1995) made measurements of seawater from 7 to 14 GHz by using a temperature controlled transmission waveguide of variable length. They measured seawater with a salinity range from 0 to 38 psu with temperatures from 0°C to 30°C. They proposed a Debye permittivity model with a double relaxation time to better fit the data at higher frequencies. In the process of determining the Debye parameters they combined their data (7 to 14 GHz) with the Ho and Hall (1973) data at 2.65 GHz and with the Ho et al (1974) data at 1.43 GHz. They found the deviation from the Debye model for this data set was several percent. When they eliminated the Ho data, the error reduced to approximately 0.8%. They conjectured that the Ho data contained systematic errors and speculated that these errors could be due to microwave heating of the sample within the quartz tube. They pointed out that this was a possibility because of the extremely small diameter of the tube.

DC at infinite frequency• “The parameter ϵ∞(T, S) is, in reality, a "fudge factor." It is determined so

that the experimental data fit a Debye function “as best as possible.” If the data followed exactly a Debye relaxation, then there is a physical interpretation as the "high-frequency“ permittivity, where high frequency is way beyond the ultraviolet! As real liquids rarely follow a Debye relaxation over such a large frequency range, the ϵ∞( T, S) obtained by a curve fitting procedure on data measured at frequencies less than 20 GHz has no physical significance” [ Ellison et al., 1996 ]

» Small impact on the real part (<0.05)

Impact on the imaginary part up to 0.2 for low temperatures

Klein & Swift, 1977 (KS)

• Empirical fit to a single Debye formula

Stogryn's Model (1995)

• Empirical fit to a double Debye formula:

• 13 parameters are determined using experimental data

Meissner & Wentz, 2012 (MW)

• Double Debye Relaxation Fits

• 12 parameters are determined using an ensemble of weighted data from laboratory measurements and SSM/I observations in order to fit the Debye relaxation parameters by minimizing the total error between observations and model. A minor update of this fit for ε was done in Meissner & Wentz, 2012 by incorporating results from WindSat measurements at C-band and X-band.

Liebe et al. [12] state that a second Debye relaxation frequency is needed to fi t the experimental data for pure water above 100 GHz

George Washington University (GW)Lang et al., 2010

• Measurements of the complex dielectric constant of seawater at 1.413 GHz using a resonant cavity technique.

• Seawater had been purchased from Ocean Scientific International Limited (OS1L) at salinities of 30, 35 and 38.27 psu.

• Measurements of these seawater samples were then made over a range of temperatures, from 10°C to 35°C in 5°C intervals.

• Real and imaginary parts of the dielectric constant had a relative accuracy of about 1%.

• Comparisons with the model function of Klein and Swift showed – real part of the dielectric constant was about 1.5% below their values

– imaginary part, on the average, was about 2% below the Klein and Swift results. The differences in the imaginary parts between the two results increased

with rising temperature and salinity.

George Washington University (GW)Zhou et al., 2012

• Measurements of the complex dielectric constant of seawater at 1.413 GHz using a resonant cavity technique.

• Seawater had been purchased from Ocean Scientific International Limited (OS1L) at salinities of 30, 35 and 38.27 psu.

• Measurements of these seawater samples were then made over a range of temperatures, from 0°C to 35°C in 5°C intervals.

• The comparison of the GW data with the other model function results shows that the GW data matches KS model function from 5° C to 30° C. At low and high temperatures, however, GW data has a 1-2% difference and is generally more linear than KS curve. When compared with Meissner-Wentz model function, GW results are approximately 1 % bigger than the MW results except for high salinities at high temperatures where the GW and MW results approach one another.