Measurements of the Marine Boundary Layer from an Airship

Abstract

In 1992 and 1993, the authors made measurements of the marine boundary layer off the coast of Oregon from an airship. In 1992, these measurements consisted of coherent microwave backscatter measurements at Ku band taken from the gondola of the airship and micrometeorological and wave height measurements made from an airborne platform suspended by a cable 65 m below the gondola so that it was between 5 and 20 m above the sea surface. In 1993, an infrared imaging system was added to the suite of instruments operated in the gondola and two narrowbeam infrared thermometers were mounted in the suspended platform. In both years, a sonic anemometer and a fast humidity sensor were carried on the suspended platform and used to measure surface layer fluxes in the atmosphere above the ocean. A laser altimeter gave both the altitude of the suspended platform and a point measurement of wave height. By operating all these instruments together from the slow-moving airship, the authors were able to measure atmospheric fluxes, microwave cross sections and Doppler characteristics, air and sea surface temperatures, and wave heights simultaneously and coincidentally at much higher spatial resolutions than had been possible before. Here the authors document the methods and present observations of the neutral drag coefficient between wind speeds of 2 and 10 m s?1, the relationship between the wind vector and the microwave cross section, and the effect of a sharp sea surface temperature front on both the wind vector and the microwave cross section. The drag coefficients first decrease with increasing wind speed, then reach a minimum and begin to increase with further increases in the wind speed. The values of the drag coefficient at very low wind speeds are higher than those given by Smith, however, and the minimum drag coefficient seems to occur somewhat above the wind speed he indicates. The authors show that their measured azimuthally averaged cross sections fall somewhat below the SASS II model function of Wentz et al. at low wind speeds but are rather close to that model at higher wind speeds. Coefficients describing the dependence of the cross section on azimuth angle are generally close to those of SASS II. The azimuthally averaged cross sections generally fall within the 90% confidence interval of the model function based on friction velocity recently proposed by Weissman et al. but are often near the upper limit of this interval. Somewhat surprisingly, a residual dependence on atmospheric stratification is found in the neutral drag coefficients and in the microwave cross sections when plotted against a neutral wind speed obtained using the Businger?Dyer stability corrections. This indicates that these corrections are not adequate over the ocean for stable conditions and the authors suggest that wave-induced shear near the surface may be the reason. Finally, it is shown that winds around a sea surface temperature front can rapidly change direction and that the microwave cross section follows this change except very near the front where it becomes more isotropic than usual.