L-band Sea Surface Emissivity Radiometric Observations under High Winds: Preliminary Results of the Wind and Salinity Experiment WISE-2001 A. Camps(1), J. Font(2), J. Etcheto(3), A. Weill(4), V. Caselles(5), I. Corbella(1), M. Vall-llossera(1), F. Torres(1), N. Duffo(1), R. Villarino(1), L. Enrique(1), J. Miranda(1), A. Julià(2), C. Gabarró(2), J. Boutin(3), R. Niclós(5), P. Wursteisen(6), M. Berger(6), M. Martín-Neira(6) (1)
Dept. Signal Theory and Communications, Universitat Politècnica de Catalunya (UPC), Campus Nord -D3, Jordi Girona 1-3, 08034 Barcelona, Spain (2) Institut de Ciències del Mar (ICM), CMIMA-CSIC, Passeig Marítim 37-49, 08003 Barcelona, Spain (3) LODYC, UPMC, case 100, Tour 15 2e etage, 4 Place Jussieu, 75252 PARIS CEDEX 05, France (4) CETP, 10-12 av de l'Europe 78140 Vélizy, France (5) Dept. Termodinàmica, Fac. Física, Univ. València (UV), Dr. Moliner 50, 46100 Burjassot, Valencia, Spain (6) European Space Agency, ESTEC, Keplerlaan 1, 2200-AG Noordwijk, The Netherlands E-mail:
[email protected];
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[email protected] I. INTRODUCTION The WISE 2000 and WISE 2001 field campaigns were sponsored by ESA to gather experimental data to improve the knowledge of the L-band brightness temperature dependence with wind speed at different incidence angles and azimuth angles. The goal is to help the development of sea surface salinity retrieval algorithms for SMOS Earth Explorer Mission of the European Space Agency. The Lband AUtomatic RAdiometer (LAURA) plus other sensors to characterize the sea surface state were installed at the Casablanca oil rig, 40 km South East off the coast of Tarragona in Spain. During WISE 2000 wind conditions were low-to-moderate, but during WISE 2001 two strong storms beat the Catalonian coast with sustained winds higher than 100 km/h at the platform meteorological station (69 m height). The first results of the radiometric measurements (azimuth and elevation scans) acquired with LAURA during the WInd and Salinity Experiment (WISE2001) are presented.
incidence angles [3]. The following instrumentation was installed in the oil rig and its surroundings: • LAURA, a fully polarimetric L-band radiometer (Th, Tv, U and V) from UPC (Spain, fig. 1) • Four oceanographic and meteorological buoys from ICM (Spain) and LODYC (France), that measured sea surface salinity, sea surface temperature, wind speed and direction, etc.; • A probe from ICM (Spain) to measure temperature and salinity from the platform at 5 m below sea level; • A portable meteorological station from UPC (Spain) with atmospheric pressure, temperature, relative humidity and rain rate sensors; • A stereo-camera from CETP (France) that provided sea surface topography images and foam coverage; • A video camera from UPC (Spain) mounted on the antenna pedestal pointing to the antenna boresight; • An infrared radiometer from UV (Spain) that provides SST estimates; as well as satellite imagery.
II. DESCRIPTION OF THE WISE CAMPAIGNS The MIRAS instrument [1,2], single payload of the SMOS mission, will be the first two-dimensional aperture synthesis radiometer for Earth observation. It will operate at L-band, where there is a good sensitivity to the soil moisture content and the sea surface salinity, the atmosphere is almost transparent, and there is a frequency band reserved for passive observations. The imaging is performed through a Fourier synthesis process of the crosscorrelations measured between the antenna pairs and provides multi-angular observations of the pixels within a wide field of view from incidence angles from 0° to approximately 65°. The WISE campaigns were designed to obtain sea surface radiometric data under the widest range of
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a) b) Figure 1. UPC LAURA radiometer installed at 32 m above the sea level during: a) azimuth scan and b) pointing to the sky for cold load calibration.
The radiometer was placed in the North-West corner of the platform at about 32 m above the sea level pointing in the direction of the dominant winds and avoiding sun glint effects (except in the afternoon-evening when the
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radiometer was pointed to the North-East). Three types of radiometric measurement sequences were carried out: • Constant incidence and azimuth angles during 1 hour once a day early in the morning at 44° incidence angle and azimuth φ=90° West (field of view common with that of the stereo-camera); • Elevation scan from 25° to 65° incidence angle in 10° steps at fixed azimuth angle (φ=90º West, to minimize RFI from Tarragona city, 20 min). This is the normal mode of operation. During the afternoon-evenings, to avoid sun glint, elevation scans were performed from 25° to 65° incidence angle in 5° steps at azimuth φ=20º East (5 min); • Azimuth scans at fixed incidence angle, starting at an azimuth angle approximately φ=110º West and ending at an azimuth angle approximately φ=20º East in 30º angular steps (20 min). WISE 2001 took place from October 23rd to November 22 . During WISE 2001, the meteorological and oceanographic conditions were the most extreme ones registered on the platform during the last 20 years. Figure 2 shows the 10 m wind speed during the campaign. During more than one third of the campaign winds well exceeded 10 m/s, reaching more than 25 m/s, when the strongest storm happened. Peak waves reached almost 12 m and destroyed the 7 m deck of the oil rig. The ultrasonic anemometer of buoy #1 and the data memory of buoy #2 were also destroyed in this storm. nd
percentile ones. Each point is the result of averaging the instantaneous measurements (τ = 1 s) during 5 minutes. The plot at the lower part of figure 3 shows the slope of each regression line as a function of the incidence angle, which corresponds to the sensitivity to wind speed in K/(m/s) at H- and V-polarizations. A linear fit of these values leads to: ∆Th ≈ 0.23 (1 + θ 70o ) U10 , (1) ∆Tv ≈ 0.23(1 − θ 50o ) U10 .
Further work is required to re-analyze the radiometric data to account for atmospheric instability when estimating the 10 m wind speed from 69 or 2.6 m height wind speed measurements [4] and to estimate the errors in the sensitivity to wind speed associated to measurement errors. Azimuth scans Figure 4a shows a time series of consecutive measurements (approximately one sample per second) at vertical and horizontal polarizations for various azimuth angles at 45° incidence angle. Average 10 m wind speed is 11.0 m/s, and the significant wave height (H1/3) increased from 3.04 m at 18 h to 5.25 m at 24 h. It can be noticed that the measurements exhibit a standard deviation much larger than instruments’ radiometric sensitivity (∆T ≈ 0.5-0.6 K). However the average value at each azimuth angle is correlated, and the average value is approximately the same when the radiometer points back to the same azimuth angle 20 minutes later (samples around 1000 and 2000).
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Figure 2. Composite wind speed at 10 m height during the WISE 2001 campaign derived from three different sensors: 3-cup anemometer of the oil rig meteorological station (69 m height), ultrasonic anemometer of buoy #1 (2.6 m height) and 3-cup anemometer of buoy #2 (2.6 m height).
The measured sea surface salinity was very stable during the whole campaign, around 38 psu, except on November 18th due to an intense rain event. The sea surface temperature showed the start of the cooling from the warm summer value 22°C down to 16°C. At the beginning of the campaign, the atmosphere was stable, but quickly changed to unstable conditions (Tair-Tsea ≈ -6°C… -10°C). III. PRELIMINARY RESULTS OF WISE 2001 Scans in incidence angle Figure 3 shows the plots of the brightness temperatures at horizontal (upper row) and vertical (center row) polarizations versus the wind speed at 10 m, for incidence angles from 25° to 65°. The solid line in each plot represents the regression line and the dashed ones the 50%
Figure 4b shows the average value (crosses) and the average value plus minus one standard deviation (triangles) of the values shown in fig. 4a. There is a clear azimuthal signature. However, its amplitude is larger than model predictions. Further studies are required about its origin. Finally, a closer look to these signals reveals that the variations of Th(t) and Tv(t) are roughly out-of-phase, and their period is approximately 12 s. This value is in agreement with the peak wave period registered by the wave buoy at the same time (14 s). IV. CONCLUSIONS This paper has presented the preliminary results of the WISE 2001 campaign. During this experiment, the two strongest storms ever seen in the Casablanca oil rig happened and the radiometer kept recording up to 27 m/s at 69 m height. The preliminary analysis of the radiometric data collected shows a sensitivity to wind speed at H- and V-polarizations of 0.23 K/(m/s) extrapolated at nadir, which increases up to 0.47 K/(m/s) at H-polarization and decreases down to -0.17 K/(m/s) at V-polarization at 65° incidence angle. On November 10th, during the first storm, an azimuthal peak-to-peak variation of 4 K was recorded both in H- and V-polarizations. This large variation was also seen in the same conditions at other incidence angles, but further data analysis is required to discover its origin (swell, different masses of water…).
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Figure 3. Scatter plots of brightness temperature at H- and V-polarizations versus wind speed at 10 m height for different incidence angles and derived sensitivities to wind speed. s01111011
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[1] Martín-Neira, M., and J.M. Goutoule, ”MIRAS- A Two-Dimensional Aperture-Synthesis Radiometer for SoilMoisture and Ocean Salinity Observations'', ESA bulletin, No 92, pp 95-104, November 1997. [2] P. Sivestrin, M. Berger, Y. Kerr, J. Font, “ESA’s Second Earth Explorer Opportunity Mission: The Soil Moisture and Ocean Salinity Mission – SMOS”, IEEE Geoscience and Remote Sensing Newsletter, No 118, pp 11-14, March 2001. [3] A. Camps, J. Font, J. Etcheto, V. Caselles, A. Weill, I. Corbella, M. Vall-llossera, N. Duffo, F. Torres, R. Villarino, L. Enrique, A. Julià, C. Gabarró, J. Boutin, E. Rubio, S.C. Reising, P. Wursteisen, M. Berger, M. MartínNeira, “Sea Surface Emissivity Observations at L-band: First Results of the Wind and Salinity Experiment WISE2000”, IEEE Transactions on Geoscience and Remote Sensing, in press. [4] A. Guissard, “Atmospheric Instability above the ocean and implications for scatterometry”, submitted for publication at the IEEE Transcations on Geoscience and Remote Sensing, Dec. 2001.
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b) Figure 4. a)Series of samples acquired during an azimuth scan at 45° incidence angle. November 10th, 2001, 19 h. WS10 m = 11.0 m/s. b) Average values and average values plus minus one standard deviation for each azimuth angle corresponding to fig. 4.
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ACKNOWLEDGMENTS The WISE campaigns were performed with the financial support of ESA (ESTEC Contract No 14188/00/NL/DC). The design and implementation of the L-band radiometer were supported by Spanish Government (TIC1999-1050C03-01). We thank Repsol Investigaciones Petrolíferas Base Tarragona for their support during the organization and performance of the WISE campaigns.
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