Ocean Salinity Campaigns 2015-2016

1) Dielectric constant of sea water at L-band2) Implication of SMOS teams into Spurs 2

Shall we perform New laboratory measurements of the Dielectric Constant of Sea Water at L-band to improve SMOS Level 2 Ocean Salinity Algorithm ?

N. Reul, F. Demontoux (1),J. Boutin, +Discussions with D. Levine

(1) Laboratoire de l'Intégration du Matériau au Système(IMS) CNRS UMR5218Bâtiment CPB pièce NO-1616 av. Pey Berland, 33607 Pessac cedex, France

Objective & context• Objectives: • => to refine the model function for the dielectric constant of sea water at L-band

as a function of salinity and temperature so that remote sensing measurements of salinity from SMOS can be made with better accuracy and to help the coherent synergistic use of SMOS, Aquarius and SMAP data.

Idea=> conduct new series of laboratory measurements of the dielectric constant at 1.4 GHz using a microwave resonant cavity and a conductivity probe operated by the Laboratoire de l'Intégration du Matériau au Système, Bordeaux, france.

Past history: W. Ellison from LIMS (now retired) conducted a very-well recognized sea water dielectric constant measurement campaign for f>3 GHz in 1998 using the facilities from that Lab.

=> Ellison W., A. Balana, G. Delbos, K. Lamkaouchi, L. Eymard, C. Guillou, and C. Prigent (1998), “New permittivity

measurements of sea water,” Radio Sci., vol. 33, no. 3, pp. 639–648, May–June 1998.

Statement of the problem• The dielectric constant of sea water has been shown to be uncertain at L-

band (Dinnat et al., 2003, Wilson et al., 2004, Lang et al., 2010, Levine et al., 2011, Dinnat et al. 2014) The differences between existing models lead to differences in brightness temperature of a perfectly smooth ocean surface of as much as a Kelvin.

The differences vary dominantly with sea surface temperature (SST) and become very significant in cold waters. Such differences are significant for retrieving SSS with a precision of the order of 0.2 psu . An error in Tb of 0.1K translates into an error of 0.2 psu, or more in cold waters.

• SMOS level 2 algorithm uses the dielectric constant model originally proposed by Klein and Swift (1977), (hereafter referred to as KS's model). Aquarius algorithms uses a more recent model developed by Meissner and Wentz (2012) (referred to as MW's model) from Spaceborne radiometric measurements at higher frequency than L-band (>6 GHz).

Previous Sea Water Dielectric constant recent Measurements at L-band:

• Since KS's model publication at the end of the 1970's, several measurements of the dielectric constant of sea water at L-band have been re-conducted for preparing the SMOS and Aquarius missions.

• A large number of techniques: Most widely used =>reflection measurements in a resonant transmission line (dielectric constant obtained from reflection coefficient in a defined reference plane, usually at the interface of the dielectric under test). Another, -less accurate since more indirect method consists in measuring the brightness temperature of a flat water surface with a very stable radiometer.

Main experiments conducted recently include those of: Blanch and Aguasca (2004), Wilson et al. (2004), and Lang et al., (2010) + ongoing work (D. Levine & R. Lang)

We briefly review the main methods and results of these studies here after.

Blanch, S. and A. Aguasca (2004), "Seawater dielectric permittivity model from measurements at L band of seas water at L band, " Proceedings of IGARSS 2004, pp. 1362-1365.

Wilson W. J., Simon H. Yueh, Steve Dinardo, and Fuk Li (2004), "High Stability L-band Radiometer Measurements of Saltwater," IEEE Trans. Geosci. and Remote Sens., Vol. 42, No.9, 1829-1835, September 2004.

Lang, R.H., Cuneyt Utku, Yalcin Turkocin and David Le Vine (2010), “Accurate Measurements of the Dielectric Constant of Seawater at L Band,” NASA Technical Memorandum, NASA TM-2010-215861.

Blanch and Aguasca (2004) Sea water dielectric constant measurements were performed at L-band by Blanch and Aguasca using a resonant cavity with the so called Transmission Method (values extracted from the measurement of the changes in a electromagnetic signal propagating in the sample). The transmission lines =waveguides & coaxial structures. They have measured seawater samples with salinities in the range 0 ≤ S ≤ 40 p.s.u.(around 23 samples). Each sample was cooled near 0º and then slowly heated up to 38º, taking measurements in steps of 0.7º ∼ . At each temperature point they measured the Salinity dependent parameters of the sample in a frequency range 0.5-2.5 GHz and curve-fitted those results to adjust them to a Debye model of the dielectric constant.

Figure 1: Comparison between K&S (red line) model and Blanch and Aguasca's model (blue line) for a salinity of 37.5 p.s.u. (From Blanch and Aguasca, 2004).

In general, the model is close to the Klein & Swift model for the real part of the permittivity (except at low temperature below 13°C) and show a bit higher absolute values for the imaginary part (more losses) than the K&S model.

This implies values slightly smaller for the brightness temperature in their model in comparison with the one of K&S (i.e higher salinity retrievals).

Wilson et al. (2004)'s pond experiment:L-band radiometer brightness temperature measurements were made by Wilson et al. (2004) from a saltwater pond over a temperature range from 8.5°C to 32°C and a salinity range of 25–40 psu. A precision L-band radiometer with stability better than 0.1 K per day was used for these measurements.

Figure(2) Difference between the horizontally and vertically polarized data and the (a) Klein and Swift, (b)Wentz and Meissner, (c) Blanch and Aguasca, dielectric models for three salinities: 25, 35, and 40 psu. A constant bias was subtracted from the measured data to minimize the rms difference between the measured data and the dielectric model. From Wilson et al. (2004)

Differences between observations and models can reach peak to peak ~0.25 K amplitudeSalinity-temperature dependencies of the dielectric constant predicted by Blanch and Aguasca's model seemed the most different from the V-pol observations, most sensible to SSS

Lang et al. (2010) resonant cavity experiment

Measurements were further made by Lang et al. (2010), using a microwave cavity operated in the transmission configuration. The cavity's temperature was accurately regulated to 0.02°C by immersing it in a temperature controlled bath of distilled water and ethanol glycol. salinities of 30, 35 and 38 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 Klein & Swift Lang

Imag Klein & Swift Lang

Relative accuracy of measurements about 1% !Comparisons with the model function of Klein and Swift showed that the real part of the dielectric constant was about 1.5% below their values while the 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. However, these authors didn't investigated the dielectric constant behavior below 10°C.

Results from by Lang et al show measurements of the real and imaginary parts of the dielectric constant that are smaller than KS predictions. This is in contradiction with results found by Blanch and Aguasca showing measurement amplitudes higher than KS's model.

Implications for SSS remote sensing (1)

Figure 4: Effects of the differences in dielectric constant models (KS minus MW) on the estimation of the vertically polarized brightness temperature of a perfectly smooth (specular) sea water surface observed at 38° incidence angle (Figure from Dinnat et al., 2014).

Differences in the prediction of the specular brightness temperature of the sea surface using either KS or MW's models can reach absolute values higher than 0.2 K, particularly in cold waters less than about 13°C, translating into SSS retrieval uncertainties larger than 0.2 pss.

The horizontal stratification of the contours indicates that the difference is mostly sensitive to SST and much less sensitive to SSS. Over most oceans (i.e. for SST greater than 4°C), the KS model predicts a Tb smaller by a few tenths of a Kelvin than the MW model.

For the coldest waters (less than 10°C), the difference becomes very sensitive to SST.

Tb at vertical polarization (Kelvins) Retrieved SSS (pss)

Implications for SSS remote sensing (2) Illustration over the Gulf stream

SMOS -> Klein & Swift, 1977

Aquarius -> Meissner & Wentz, 2012

Taking the difference in term of 1sr Stokes

Delta TB :• ~0.35K in cold Waters• ~0.1K between 30S-30N

Several dielectric constant models:

S. Guimbard, SMOS Mode Bremen 2013

Implications for SSS remote sensing (3) Biases in High Latitude Seas

Implications for SSS remote sensing (3) Biases in High Latitude Seas

Comparison of flat sea residual at ARGO floatsCo-localization with SMOS=> Clear SST dependent biases

Dielectric constant issues ?Other SST-correlated geophysical effectsErrors (roughness) ?

How to resolve the uncertainties on the sea water dielectric constant at L-band ?

1) Perform new dielectric constant in the lab

2) Implement SST (SSS) –dependent empirical corrections

ÞPossibility to conduct a new experiment to evaluate the sea water dielectric constant at 1.4 GHz using the instrumentations available at the IMS laboratory in Bordeaux.

This method was already used by Demontoux et al. (2008) to measure the effective permittivity of the soil and litter media at L-band in the context of SMOS mission preparation.

The sea water samples will be inserted inside a section of the wave guide which will be used as a support containing the sample.

The electromagnetic parameters of the samples will be determined using the Nicolson, Ross and Weir method (NRW) [12] for rectangular waveguides.

This calculation process is based on reflection and transmission measurements and the network analyzer is well suited for this type of measurement.

The principle of the calculation is based on the fact that introduction of the sample into the guide produces a change of impedance. The waveguide is a two ports system (each port is an input and an output of the system) in which multiple reflections within the material are taken into account by the coefficients S11 and S21.These latter are the coefficients of the scattering matrix which represents the various exchange coefficients between all the inputs and outputs of the system. This makes it possible to calculate Γ (|Γ| ≤1), the coefficient of the first reflection and T , the coefficient of transmission:

Because it concerns a non magnetic sample, its electromagnetic characteristics are given by:

coefficient of the first reflection coefficient of transmission

Principle of Measurements

From the above equations, the average values of the real and imaginary parts of the dielectric constant, ε' and ε'' respectively, are computable from measurements of the coefficients S11 and S21. These computations will be made in the 1.3–1.5 GHz measurement domain considered in the experimental setup.

1) Shall we use real sea water samples and representative of different oceans orSynthetic sea water would do it?

2) Most uncertainties in ε=function(f,sss,sst) comes from SST dependenciesÞ Need to focus mainly on that SST dependencies but small SSS uncertainties remain too

3) A key requirement of any Lab or Pond-like campaign for sea water dielectric constant Characterizzation at L-band is measurement Accuracy !We need much better than

<1% in real and imaginary parts<0.1 K in Tbs

Real Klein & Swift Lang

1%

Campaign Requirements:

Q1: Q1a: Do we need to use realSea water samples ?

Q1b: If yes, do we need samplesfrom different oceans of the world?

In Ellison et al., (1998), the conductivity of synthetic seawater was compared to the ones of either an aqueous solution of NaCl or to the one of a natural sea water sample of the same salinity.

At low microwave frequency (they used 3 GHz, see Figure), the conductivity of synthetic sea water and NaCl solution is systematically higher than that of natural seawater by 1.5% and 2-3%, respectively at all temperatures.

Between 3 and 20 GHz, the permittivity difference between natural and articifical sea water is within 1%, which was the experimental uncertainty of their measuring system.

A1a: They concluded that to obtain more precise permittivity & conductivity measurements for sea water, especially for frequencies less than 3 GHz, use of natural seawater samples would be highly recommended.

In principle (pers discussion with G. Reverdin from Locean laboratory): the chemical composition (at least in terms of ionic constituents) of sea water is very similar between surface waters in the south Pacific, and North Atlantic, including the waters surrounding Brest in France (small changes for carbonates/hydrocarbonates or for Ba+, but this is very small change).

There are more silicates in south ocean water as well and organic constituent may change. However, as shown in Elisson et al (1998), the influence of the organic content, in quantities which occur in natural conditions was not detectable by their measuring system, designed to have an experimental accuracy of about 1%.

Q1b: Do we need representative water samples from different oceans of the world?

A1b: use of water samples taken along N.Atlantic coasts in clear waters (e.g. in bay of Brest) would be sufficient and representative for the dielectric constant characterisation.(Provide the water samples are properly cleaned for biological activity=> poison)

Then temperature can be varied using a

F. Demontoux met William Ellison (retired) who worked on that problem & discussed with him. He proposed 2 work options with differing accuracies & time investments:

Option 1: 2-3 month work using currently available material & equipement (wave guide centered at 1.3 GHz and not 1.4 GHz) Investment => engineer or post doc for 2-3 months.

Option 2 :Taking maximum care of accuracy constraints =>investment on material : control-loop mechanism in T° of an accurate measurement cell in réflexion/transmission + Nitrogen circuit to avoid any condensation) and cost for 1 one year work activity for a dedicated person (campaign set-up & measurements).

Q2: Can we reach the desired accuracy ? ÞOn going discussion with F. Demontoux & D. Levine

Progresses were also made on the Aquarius side with a on-going effort from R. Lang & D. Levine since their original work (2010)

David will send me his report as soon as available

Aquarius team efforts

SMOS & SPURS 2

Diagnosing the physics of a rainfall–dominated salinity minimum

Aims of SPURS2(statuts from Report of a workshop in Pasadena, April 16-18, 2014 April 16-18, 2014)

•Where does the freshwater go? In other words, how does the ocean integrate thefreshwater forcing and destroy the salinity variance created at the surface?

•By what oceanic processes is the freshwater transformed from shallow, patchy “puddles” of freshwater into the large-scale mean distribution of salinity that makes up the Eastern Pacific Fresh Pool?

• What impact will this horizontal and vertical variability have on the performance of satellite-based measurements of SSS?

• What are the local and non-local effects of the freshwater flux on the ocean?

• How does the ocean SSS impact the feedbacks on the atmosphere?

a. Site: Eastern Tropical Pacific ITCZ in the region of 10°N 125°W.

b. Scheduling: The field campaign would take place over one full year, with the ship time arranged in three cruises about six months apart and an anticipated start date in the summer of 2016. Since interest is highest in the dynamics of rainfall events, it will be best to have the initial deployment cruise and final asset recovery cruise in the summer/fall rainy period. As such, these cruises could be also used for specific process studies, which would be an attractive option to encourage partnership between funding agencies.

c. Intensive Surveys: These should include shipboard surveys of basic T, S and velocity structure at a variety of scales, including the use of CTD sections, thermosalinographs, and special techniques to sample the surface layer. Microstructure measurements from ship andmobile assets are also encouraged. Flux measurements, especially of rainfall, would be valuable.

d. Moored Assets: We anticipate the use of one or more moorings, potentially arranged meridionally along 125°W, with priority on air-sea flux measurements (freshwater, heat, and momentum) and upper-ocean temperature, salinity, and velocity structure. The concentration of instruments can be more surface-focused than was required in SPURS-1.

e. Lagrangian Assets: Drifting or Lagrangian instruments such as floats and surface drifters must consider the stronger current regime of this site compared to SPURS-1. The potential for upstream seeding of drifters or active selection of the parking depth of floats should beconsidered.

f. Mobile Assets: Wavegliders, Seagliders or other mobile platforms (such as Sail-drones, if available) may be particularly useful in this high-current regime. Monitoring of the meridional thermohaline structure will be of most interest.

g. Ongoing regional measurements: Maintenance of the 125°W line of the TAO/TRITON array is highly desired for this project, as well as any enhancements such as improved flux measurements or higher resolution monitoring of T and S structure. Similarly, regional enhancements of VOS deployed XBTs or drifters would be most welcome.

h. Regional and local modeling: As in SPURS-1, we anticipate the use of ocean and coupledocean-atmosphere models on a variety of scales to help understand and interpret our data and plan shipboard operations during the cruises.

i. Remote Sensing: Aquarius data was particularly useful during SPURS-1, and access toaltimetry products and SST products also proved useful. Similar contributions would be beneficial to SPURS-2.

j. Data Management Plan: A data management project that includes real-time support of the cruises, data quality control and data archiving is expected.

k. Novel assets: Novel, mature technology and platforms would be welcome contributions toSPURS-2. For example, drifting buoys might capture some unique information of the spatial variability of the surface forcing. Use of non-conventional platforms such as FLIP and ASIS, and instruments such as ship-mounted rain radars would be valuable, though funding foractivities that fall outside of the main focus of SPURS-2 or that would consume the funds needed to pursue the main goals of SPURS-2 must be sought from agencies other than NASA.

SMOS team implications:Þ A Key campaign for SMOS with respect 1) The vertical stratification problem2) The understanding of Rain-induced signal in SMOS SSS & freshwater cycle studies

3) STSE post-doc S. Guimbard will work on physical processes specifically associated to that area

4) J. Boutin and G. Reverdin will send drifting surface buoys to be deployed by ourUS colleagues

Participation to SPURS 2 experiment: 10 drifting buoys, shipping of instruments and participation to meetings CNES has agreed to support for SPURS 2 but the budget is quite limited and a participation of ESA would be very helpful.

SURPLAS tied to a SVP-BS drifter (CAROLS2010 cruise, Gulf of Biscay)

SVP

Surplas

Measurements at 5 cm & 45 cm depths