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Stuart D. Smith, Kristina B. Katsaros, Wiebe A. Oost, and Patrice G. Mestayer

Humidity Exchange over the Sea (HEXOS) is an international program for the study of evaporation and spray droplet flux from sea to air. The program includes measurements in the field, simulation studies in wind tunnels, interpretive studies such as flow distortion modeling, boundary-layer modeling and development of parameterization for use in synoptic, and climatic models of the atmosphere and ocean.

The HEXOS Main Experiment (HEXMAX) was carried out in October and November of 1986 at the Dutch offshore research platform Meetpost Noordwijk (MPN) and from ship, aircraft, and shore stations in the vicinity. Evaporation, wind stress, and heat flux were determined at all stations using combinations of eddy correlation, dissipation, and profile methods. Concurrent measurements of spray and aerosol distributions and other relevant parameters, and the regular occurence of favorable winds and weather make the HEXMAX dataset unique in its completeness and in the range of conditions covered. Some preliminary results are reported.

The series of the French Couche Limite Unidimensionelle Stationnaire d'Embruns (CLUSE) experiments is designed to study in a simulation tunnel the surface flux of droplets generated by bursting bubbles and the interaction of these droplets with the turbulent fields of humidity, temperature, and velocity in the boundary layer. Grand CLUSE, in the spring of 1988, simulated aerosol generation and interactions of the aerosols with turbulent fields of temperature, humidity and wind. A series of four smaller Petit CLUSE experiments addressed specific techniques and the simulation of specific processes.

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Kristina B. Katsaros, Stuart D. Smith, and Wiebe A. Oost

HEXOS is an international program for the study of evaporation and spray-droplet flux from sea to air. The program includes measurements in the field at moderate-to-high wind speeds, wind-tunnel studies, instrument development, boundary-layer modeling, and subsequent development of parameterization for use in synoptic and climatic models of the atmosphere and the ocean. Present accomplishments of the program are 1) a wind-tunnel study of the flow distortion around the Dutch research platform, Meetpost Noordwijk, 2) a pilot experiment at this platform in November 1984, and 3) an investigation of processes near the air-sea interface in a wind-wave simulation tunnel. The main field experiment, taking place in the autumn of 1986 at and around the Noordwijk platform, includes measurements of the fluxes of water vapor, spray droplets, sensible heat, and momentum, as well as the structure of the planetary boundary layer and the state of the sea. This multidisciplinary effort involves direct measurements from the platform, a mast, a ship, a tethered balloon, moorings, and an aircraft, plus measurements obtained remotely by laser scintillometer, lidar, and radar.

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Wiebe A. Oost, Christopher W. Fairall, James B. Edson, Stuart D. Smith, Robert J. Anderson, John A.B. Wills, Kristina B. Katsaros, and Janice DeCosmo

Abstract

Several methods are examined for correction of turbulence and eddy fluxes in the atmospheric boundary layer, two of them based on a potential-flow approach initiated by Wyngaard. If the distorting object is cylindrical or if the distance to the sensor is much greater than the size of the body, the undisturbed wind stress can be calculated solely from measurements made by the sensor itself; no auxiliary measurements or lengthy model calculations are needed. A more general potential-flow correction has been developed in which distorting objects of complex shape are represented as a number of ellipsoidal elements.

These models are applied to data from three turbulence anemometers with differing amounts of flow distortion, operated simultaneously in the Humidity Exchange over the Sea (HEXOS) Main Experiment. The results are compared with wind-stress estimates by the inertial-dissipation technique; these are much less sensitive to local flow distortion and are consistent with the corrected eddy correlation results. From these comparisons it is concluded that the commonly used “tilt correction” is not sufficient to correct eddy wind stress for distortion by nearby objects, such as probe supports and neighboring sensors.

Neither potential-flow method is applicable to distortion by larger bodies of a scale comparable to the measuring height, such as the superstructure of the Meetpost Noordwijk (MPN) platform used in HEXOS. Flow distortion has been measured around a model of MPN in a wind tunnel study. The results were used to correct mean winds, but simulation of distortion effects on turbulence levels and wind stress turned out not to be feasible.

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