Laurence Eymard

The Fronts and Atlantic Storm-Track Experiment (FASTEX): Scientific Objectives and Experimental Design

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as ...

Microwave land emissivity calculations using AMSU measurements

Atmospheric parameter retrievals over land from Advanced Microwave Sounding Unit (AMSU) measurements, such as atmospheric temperature and moisture profiles, could be possible using a reliable estimate of the land emissivity. The land surface emissivities have been calculated using six months of data, for 30 beam positions (observation zenith angles from -58/spl deg/ to +58/spl deg/) and the 23.8-, 31.4-, 50.3-, 89-, and 150-GHz channels. The emissivity calculation covers a large area including Africa, Eurasia, and Eastern South America. The day-to-day variability of the emissivity is less than 2% in these channels. The angular and spectral dependence of the emissivity is studied. The obtained AMSU emissivities are in good agreement with the previously derived SSMI ones. The scan asymmetry problem has been evidenced for AMSU-A channels. And possible extrapolation of the emissivity from window channels to sounding ones has been successfully tested.

Potential of Advanced Microwave Sounding Unit-A (AMSU-A) and AMSU-B measurements for atmospheric temperature and humidity profiling over land

A neural network retrieval method has been applied to investigate AMSU‐A/AMSU‐B atmospheric temperature and humidity profiling capabilities over land. The retrieval method benefits from a reliable estimate of the land emissivity and skin temperature as well as first guess information regarding the temperature‐humidity profiles. It has been applied on a large geographic area (60°W–60°E, 60°S–60°N) and atmospheric situations (winter and summer). The retrieved RMS errors are within 2 K and 9% in temperature and relative humidity, respectively. Regardless of scanning conditions, vegetation types, and atmospheric situations, the algorithm retrieval results are satisfactory for both temperature and relative humidity. The retrieval approach has been evaluated by comparison with available in situ measurements.

On the Wet Tropospheric Correction for Altimetry in Coastal Regions

In order to correct the altimeter range from tropospheric humidity, a microwave radiometer is added to altimetry missions (ENVISAT/MWR, Jason/JMR, TOPEX-Poseidon/TMR). Over open ocean, the combination altimeter/radiometer is satisfactory. This is not the case in coastal zones, where the signal coming from the surrounding land surfaces contaminates the measurement and makes the humidity retrieval method unsuitable. In this paper, a radiometer simulator is built, using data from the FETCH experiment (high resolution meteorological model, in-situ measurements, and TMR measurements). This simulator is used to perform sensitivity tests and to evaluate the current methods used up to now to retrieve the wet tropospheric correction in transition areas. Refinements are then proposed. Finally, a method using the proportion of land in the mixed pixel to decontaminate measured brightness temperatures is developed

FIRST THREE YEARS OF THE MICROWAVE RADIOMETER ABOARD ENVISAT : IN-FLIGHT CALIBRATION, PROCESSING AND VALIDATION OF THE GEOPHYSICAL PRODUCTS

Abstract The Envisat microwave radiometer is designed to correct the satellite altimeter data for the excess path delay resulting from tropospheric humidity. Neural networks have been used to formulate the inversion algorithm to retrieve this quantity from the measured brightness temperatures. The learning database has been built with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and simulated brightness temperatures by a radiative transfer model. The in-flight calibration has been performed in a consistent way by adjusting measurements on simulated brightness temperatures. Finally, coincident radiosonde measurements are used to validate the Envisat wet-tropospheric correction, and this comparison shows the good performances of the method.

On Wet Tropospheric Correction for Altimetry in Coastal Regions

In order to correct the altimeter range for tropospheric humidity, a microwave radiometer is added to altimetry missions [Envisat/microwave radiometer, Jason/Jason Microwave Radiometer, and TOPEX-Poseidon/TOPEX Microwave Radiometer (TMR)]. Over open ocean, the combination altimeter/radiometer is satisfactory. This is not the case in coastal zones, where the signal coming from the surrounding land surfaces contaminates the radiometer measurement and makes the humidity-retrieval method unsuitable. In this paper, a radiometer simulator is built, using data from a field experiment (in situ measurements and collocated TMR measurements) and analyses from a mesoscale-forecast model. This simulator is used to perform sensitivity tests and to evaluate the current methods to retrieve the wet tropospheric correction in transition areas. The purpose of this paper is to analyze and compare the performances of these methods. After examining simple correction methods (extension of the open-sea wet tropospheric correction and use of the meteorological model value), we evaluated the feasibility and performances of two methods, which propose to take into account the land-surface effect in the brightness-temperature estimation. The latter was found to give significantly better results.

An Assessment of Jason-1 Microwave Radiometer Measurements and Products

In the context of the sea level survey at the mm level, it is necessary all along the lifetime of the altimeter mission to survey the quality of the products from the microwave radiometer. The calibration of the brightness temperatures has been validated using reference brightness temperatures over selected continental areas as well as simulations for a wide range of oceanic and atmospheric situations. The validation of the wet path delay is performed by comparison with radiosonde measurements and pointed out that both the JMR and the TMR estimate wet path delay around 5 mm higher than the one measured by radiosondes. Furthermore, it appeared that the correction of the TMR drift degrades the product with respect to radiosonde measurements. The monitoring of the brightness temperatures since launch shows a mean drift around +0.1 K/year for the 18.7 GHz, −0.6 K/year for the 23.8 GHz channel, and around −0.4 K/year for the 34 GHz channel.

Evaluation of latent heat flux fields from satellites and models during SEMAPHORE

Latent heat fluxes were derived from satellite observations in the region of Structure des Echanges Mer- Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE), which was conducted near the Azores islands in the North Atlantic Ocean in autumn of 1993. The satellite fluxes were compared with output fields of two atmospheric circulation models and in situ measurements. The rms error of the instantaneous satellite fluxes is between 35 and 40 W m 22 and the bias is 60-85 W m22. The large bias is mainly attributed to a bias in satellite-derived atmospheric humidity and is related to the particular shape of the vertical humidity profiles during SEMAPHORE. The bias in humidity implies that the range of estimated fluxes is smaller than the range of ship fluxes, by 34%-38%. The rms errors for fluxes from models are 30-35 W m22, and the biases are smaller than the biases in satellite fluxes (14-18 W m 22). Two case studies suggest that the satellites detect horizontal gradients of wind speed and specific humidity if the magnitude of the gradients exceeds a detection threshold, which is 1.27 g kg 21 (100 km)21 for specific humidity and between 0.35 and 0.82 m s21 (30 km)21 for wind speed. In contrast, the accuracy of the spatial gradients of bulk variables from models always varies as a function of the location and number of assimilated observations. A comparison between monthly fluxes from satellites and models reveals that satellite-derived flux anomaly fields are consistent with reanalyzed fields, whereas operational model products lack part of the mesoscale structures present in the satellite fields.

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...

The Megha-Tropiques mission

The Megha-Tropiques satellite is devoted to the study of the atmospheric water cycle in the tropics and its relation to the radiative budget. It is aiming to study both the energy and water budget of the intertropical band and the life cycle of the convective complexes in the Tropics. The orbit of the satellite allows it to sample several times per day the zone from 23°N to 23°S, where most of the precipitation of the planet and large energy exchanges occur. The three instruments of the mission are a microwave imager, a microwave water vapor sounder and a radiative budget instrument. The launch of this mission by an Indian Rocket is foreseen in 2006-2007. It will hopefully coincide with the time frame of the Global Precipitation Mission, allowing to improve its tropical coverage.