Alain Weill

The WISE 2000 and 2001 field experiments in support of the SMOS mission: sea surface L-band brightness temperature observations and their application to sea surface salinity retrieval

Soil Moisture and Ocean Salinity (SMOS) is an Earth Explorer Opportunity Mission from the European Space Agency with a launch date in 2007. Its goal is to produce global maps of soil moisture and ocean salinity variables for climatic studies using a new dual-polarization L-band (1400-1427 MHz) radiometer Microwave Imaging Radiometer by Aperture Synthesis (MIRAS). SMOS will have multiangular observation capability and can be optionally operated in full-polarimetric mode. At this frequency the sensitivity of the brightness temperature (T/sub B/) to the sea surface salinity (SSS) is low: 0.5 K/psu for a sea surface temperature (SST) of 20/spl deg/C, decreasing to 0.25 K/psu for a SST of 0/spl deg/C. Since other variables than SSS influence the T/sub B/ signal (sea surface temperature, surface roughness and foam), the accuracy of the SSS measurement will degrade unless these effects are properly accounted for. The main objective of the ESA-sponsored Wind and Salinity Experiment (WISE) field experiments has been the improvement of our understanding of the sea state effects on T/sub B/ at different incidence angles and polarizations. This understanding will help to develop and improve sea surface emissivity models to be used in the SMOS SSS retrieval algorithms. This paper summarizes the main results of the WISE field experiments on sea surface emissivity at L-band and its application to a performance study of multiangular sea surface salinity retrieval algorithms. The processing of the data reveals a sensitivity of T/sub B/ to wind speed extrapolated at nadir of /spl sim/0.23-0.25 K/(m/s), increasing at horizontal (H) polarization up to /spl sim/0.5 K/(m/s), and decreasing at vertical (V) polarization down to /spl sim/-0.2 K/(m/s) at 65/spl deg/ incidence angle. The sensitivity of T/sub B/ to significant wave height extrapolated to nadir is /spl sim/1 K/m, increasing at H-polarization up to /spl sim/1.5 K/m, and decreasing at V-polarization down to -0.5 K/m at 65/spl deg/. A modulation of the instantaneous brightness temperature T/sub B/(t) is found to be correlated with the measured sea surface slope spectra. Peaks in T/sub B/(t) are due to foam, which has allowed estimates of the foam brightness temperature and, taking into account the fractional foam coverage, the foam impact on the sea surface brightness temperature. It is suspected that a small azimuthal modulation /spl sim/0.2-0.3 K exists for low to moderate wind speeds. However, much larger values (4-5 K peak-to-peak) were registered during a strong storm, which could be due to increased foam. These sensitivities are satisfactorily compared to numerical models, and multiangular T/sub B/ data have been successfully used to retrieve sea surface salinity.

Sea surface emissivity observations at L-band: first results of the Wind and Salinity Experiment WISE 2000

Sea surface salinity can be measured by passive microwave remote sensing at L-band. In May 1999, the European Space Agency (ESA) selected the Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission to provide global coverage of soil moisture and ocean salinity. To determine the effect of wind on the sea surface emissivity, ESA sponsored the Wind and Salinity Experiment (WISE 2000). This paper describes the field campaign, the measurements acquired with emphasis in the radiometric measurements at L-band, their comparison with numerical models, and the implications for the remote sensing of sea salinity.

Comparison of sea surface flux measured by instrumented aircraft and ship during SOFIA and SEMAPHORE experiments

Two major campaigns (Surface of the Oceans, Fluxes and Interactions with the Atmosphere (SOFIA) and Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE)) devoted to the study of ocean-atmosphere interaction were conducted in 1992 and 1993, respectively, in the Azores region. Among the various platforms deployed, instrumented aircraft and ship allowed the measurement of the turbulent flux of sensible heat, latent heat, and momentum. From coordinated missions we can evaluate the sea surface fluxes from (1) bulk relations and mean measurements performed aboard the ship in the atmospheric surface layer and (2) turbulence measurements aboard aircraft, which allowed the flux profiles to be estimated through the whole atmospheric boundary layer and therefore to be extrapolated toward the sea surface level. Continuous ship fluxes were calculated with bulk coefficients deduced from inertial-dissipation measurements in the same experiments, whereas aircraft fluxes were calculated with eddy-correlation technique. We present a comparison between these two estimations. Although momentum flux agrees quite well, aircraft estimations of sensible and latent heat flux are lower than those of the ship. This result is surprising, since aircraft momentum flux estimates are often considered as much less accurate than scalar flux estimates. The various sources of errors on the aircraft and ship flux estimates are discussed. For sensible and latent heat flux, random errors on aircraft estimates, as well as variability of ship flux estimates, are lower than the discrepancy between the two platforms, whereas the momentum flux estimates cannot be considered as significantly different. Furthermore, the consequence of the high-pass filtering of the aircraft signals on the flux values is analyzed; it is weak at the lowest altitudes flown and cannot therefore explain the discrepancies between the two platforms but becomes considerable at upper levels in the boundary layer. From arguments linked to the imbalance of the surface energy budget, established during previous campaigns performed over land surfaces with aircraft, we conclude that aircraft heat fluxes are probably also underestimated over the sea.

A New Shipborne Microwave Refractometer for Estimating the Evaporation Flux at the Sea Surface

Abstract After a brief description of humidity measurement and a short presentation of methods of microwave refractometry for evaporation flux, a new X-band refractometer system is presented. Based on a new design and a new material for the microwave cavity, it does not need calibration for refractive index variations because of its reduced thermal time constant. The new device has been combined with a sonic anemometer and traditional mean meteorological measurements on a 12-m shipborne mast. It has been found to be very efficient for obtaining humidity fluctuations and fluxes in the CATCH 97 (Couplage avec l’ATmosphere en Conditions Hivernales) and FETCH 98 (Flux, Etat de la mer et Teledetection en condition de fetCH variable) experiments under various wind and stability conditions. The inertial subrange is of very high quality. To first order, the evaporation flux and refractive index flux are very similar. In extreme meteorological conditions, such as those encountered during CATCH, the sensible heat f...

Dual Doppler radar investigation of the tropical convective boundary layer

Abstract Radar observations of the convective boundary layer wore made during the “COPT81” experiment in May–June 1981 over western Africa. The lower atmosphere was characterized by the interaction between the southerly monsoon and the African easterly jet above it. The mean wind shear between both Bows is found to organize convective elements along preferred directions and to transfer energy to the perturbed field (velocity variances, momentum fluxes). However, the convective organization is also the cause of increasing horizontal variance and in some cases of a negative shear stress production. The global behavior of the convective boundary layer in the four observed situations appeared similar to those observed in middle latitudes, but the strong wind shear is an important feature of the layer.

Sea surface emissivity observations at L-band: first preliminary results of the WInd and Salinity Experiment WISE-2000

This paper presents the first measurements processed from the data acquired with the L-band AUtomatic RAdiometer (LAURA) during the WInd and Salinity Experiment (WISE-2000). Experimental results are compared to a sea surface emissivity model developed by the Polytechnic University of Catalonia (UPC). The sensitivity of the brightness temperatures at vertical and horizontal polarizations to wind speed are discussed, as well as the weak azimuthal signature found.

L-band sea surface emissivity radiometric observations under high winds: Preliminary results of the Wind and Salinity Experiment WISE-2001

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 L-band 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 (WISE-2001) are presented.

Combined airborne radio-instruments for ocean and land studies (CAROLS)

The CAROLS, L band radiometer, is built and designed as a copy of EMIRAD II radiometer of DTU team. It is a Correlation radiometer with direct sampling and fully polarimetric (i.e 4 Stockes). It will be used in conjunction with other airborne instruments (in particular the C-Band scatterometer (STORM) and IEEC GPS system, Infrared CIMEL radiometer, one visible camera), in coordination with in situ field campaigns for SMOS CAL/VAL. The instruments are implemented on board the French research airplane ATR42. A validation campaign with four flights was made over south west of France, Hourtin Lake and Bay of Biscay (Atlantic Ocean) in September 2007. In order to qualify the radiometer data, different types of aircraft movements were realized: circle flights, wing and nose wags. Simultaneously to flights, different ground measurements were made over continental surfaces and ocean. First results show a good quality of data over ocean surfaces. For continental surfaces, important Radio-Frequency Interferences (RFI) were observed over a large part of the studied region.

Millimeter-Wave Radar Sensor for Snow Height Measurements

A small and lightweight frequency-modulated continuous-wave (FMCW) radar system is used for the determination of snow height by measuring the distance to the snow surface from a platform. The measurements have been performed at the Centre des Études de la Neige (Col de Porte), which is located near Grenoble in the French Alps. It is shown that the FMCW radar at millimeter-wave frequencies is an extremely promising approach for distance measurements to snow surfaces, e.g., in the mountains or in an Arctic environment. The characteristics of the radar sensor are described in detail. The relevant accuracy to measure the distance to a snow layer is shown at different heights and over an extended time duration. A dedicated laser snow telemeter is used as reference. In addition, the reflection from different types of snow is shown.

A New Platform for the Determination of Air–Sea Fluxes (OCARINA): Overview and First Results

The present paper describes a new type of floating platform that was specifically designed for estimating air-sea fluxes, investigating turbulence characteristics in the atmospheric surface boundary layer, and studying wind-wave interactions. With its design, it can be deployed in the open ocean or in shallow water areas. The system is designed to be used from a research vessel. It can operate for ~10 hours as a drifting wave rider and three hours under power. Turbulence and meteorological instrument packages are placed at a low altitude (1-1.5 m). It was deployed for validation purposes during the FROMVAR 2011 experiment off the west coast of Brittany (France). Wind friction velocity and surface turbulent buoyancy flux were estimated using eddy-covariance, spectral, bulk and profile methods. The comparisons of the four methods show a reasonable agreement except for the spectral buoyancy flux. This suggests that the platform design is correct. Also the wind measured at a fixed height above the sea shows spectral coherence with wave heights, such that wind and swell are in phase, with largest wind values on top of swell crests. This result in qualitative agreement with current model predictions supports the capability of OCARINA to investigate wind-swell interactions.