Measurements of Humidity and Temperature in the Marine Environment during the HEXOS Main Experiment

Abstract

Accurate measurement of fluctuations in temperature and humidity are needed for determination of the surface evaporation rate and the air-sea sensible heat flux using either the eddy correlation or inertial dissipation method for flux calculations. These measurements are difficult to make over the ocean, and are subject to large errors when sensors are exposed to marine air containing spray droplets. All currently available commercial measurement devices for atmospheric humidity require frequent maintenance. Included in the objectives of the Humidity Exchange over the Sea program were testing and comparison of sensors used for measuring both the fluctuating and mean humidity in the marine atmosphere at high wind speeds and development of techniques for the protection of these sensors against contamination by oceanic aerosols. These sensors and droplet removal techniques are described and comparisons between measurements from several different systems are discussed in this paper. To accomplish these goals, participating groups devised and tested three methods of removing sea spray from the sample airstream. The best performance was given by a rotating screen device, the "spray flinger." Several high-frequency temperature and humidity instruments, based on different physical principles, were used in the collaborative field experiment. Temperature and humidity fluctuations were measured with sufficient accuracy inside the spray removal devices using Lyman-a hygrometers and a fast thermocouple psychrometer. Comparison of several types of psychrometers (using electric thermometers) and a Rotronic MP-100 humidity sensor for measuring the mean humidity illustrated the hysteresis of the Rotronic MP-I 00 device after periods of high relative humidity. Confidence in the readings of the electronic psychrometer was established by in situ calibration with repeated and careful readings of ordinary hand-held Assman psychrometers (based on mercury thermometers). Electronic psychrometers employing platinum resistance thermometers perform very well.

Spray-Mediated Enthalpy Flux to the Atmosphere and Salt Flux to the Ocean in High Winds

“…Briefly, the HEXOS data came directly from tabulations in DeCosmo's (1991) thesis. Smith et al (1992), Katsaros et al (1994), andDeCosmo et al (1996) provided thorough descriptions of the instruments and the measurements. The HEXOS set includes eddy-covariance measurements of the friction velocity (u * ) and the total sensible (H s,T ) and latent (H L,T ) heat fluxes on the Meetpost Noordwijk platform in the North Sea.…”

A Pressure-Sphere Anemometer for Measuring Turbulence and Fluxes in Hurricanes

“…Modern 3D sonic anemometers often contain built-in algorithms to detect and exclude spurious signals produced by particles in the sound path, but these function best in light precipitation (Peters and Fischer 1998). When the sonic transducers become coated with water or are struck by large rain drops, the output usually is severely contaminated (Katsaros et al 1994). A hurricane intercept team at Texas Tech University has been deploying 3D sonic anemometers as part of their instrument package, but the output has generally been too contaminated to be of use (J. L. Schroeder 2004, personal communication).…”

Relative Humidity over the West Florida Continental Shelf

“…They found these sensors to be reliable, except in temperatures of Ϫ20°C, with no hysteresis effects occurring after saturation. Although there has been one report of hysteresis effects at high humidities and very low temperatures for the Rotronics MP-100 based on data collected during the Humidity Exchange over the Sea (HEXOS) experiment in 1986 (Katsaros et al 1994), other studies have not found any evidence of this (e.g., Breaker et al 1998a). Subsequent manufacturing improvements to the MP-100 sensor may have eliminated this problem (Breaker et al 1998b), making these sensors more reliable at high humidities.…”