In situ measurements of exchange processes generally rely on high quality estimates of mean quantities, such as air temperature, humidity, wind speed, radiation and water temperature [e.g., Weller, 1989]. Flux estimates are determined from the bulk transfer coefficients using these variables. At the other extreme, direct measurements of the fluxes, which are independent of the absolute values of the mean quantities, can be obtained from a few specially instrumented ships and fixed platforms, and from aircraft when the precise motion of the platform is known. Application of the so called inertial dissipation technique reduces the sensitivity of the measurements to platform motion and flow distortion by relating the turbulence fluxes to spectra of the horizontal velocity and scalars. The spectral information required is limited to the inertial subrange that, in practice, is at a higher frequency than the platform motion [ Fairall et al., 1990b; Bradley et al. 1991]. While attractive, this method is still beset with assumptions, such as isotropy, empirical constants and so forth that may be easily violated. Uncertainty has persisted in applying land-based measurements to the ocean, however, recent measurements of the turbulent kinetic energy dissipation function by Fairall and Edson [1994] on the R/V Flip and Iselin using eddy correlation techniques indicate very good agreement with earlier land-based measurements.
In situ remote sensing of the air-sea interface from ship- or aircraft-borne instruments is becoming increasing important. As we discover the generally inhomogeneous nature of the marine atmosphere and upper ocean, reliance must be placed on measurements that can resolve the spatial variations in the boundary layers of both the ocean and atmosphere. Scanning lidar techniques to resolve the structure of the boundary layer and derive wind fields are promising for both surface and airborne measurements. Several radar techniques have been used to determine direction wave spectra; surface contour radar (SCR) [ Walsh, 1991], radar ocean wave spectrometer (ROWS) [ Jackson, 1991], C-band synthetic aperture radar (SAR) [ Tilley, 1991], and multimode airborne radar altimeter [ Walsh et al., 1994a]. Data collected from the SCR, ROWS and SAR systems flown on aircraft during the Labrador Sea Extreme Waves Experiment (LEWEX) were compared by Tilley [1991] and are generally in good agreement, demonstrating the utility of these kinds of measurements to obtain surface wave data over large spatial domains.