A new shipborne microwave refractometer for estimating the evaporation flux at the sea surface

After a brief description of humidity measurement and a short presentation of methods of microwave refractometry for evaporation flux, a new X-band re fractometer system is presented. Based on a new design and a new material f or 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 me an 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) exp eriments under various wind and stability conditions. The inertial subrange is of very high quality. To first order, the evaporation flux and refracti ve index flux are very similar. In extreme meteorological conditions, such as those encountered during CATCH, the sensible heat flux contribution must be determined independently. A great advantage of the system is that the contamination by salt, as is ty pical for other devices at sea, has been found to be negligible for the con ditions encountered.