Search Results

You are looking at 1 - 2 of 2 items for

  • Author or Editor: G. Kraan x
  • Refine by Access: All Content x
Clear All Modify Search
G. Kraan
,
W. A. Oost
, and
P. A. E. M. Janssen

Abstract

With video equipment and image processing software the authors have measured the whitecap coverage of the sea surface near the Dutch coast. Reality proved the initial idea about the simplicity of this way of whitecap measurement to be overly optimistic. A model was developed that produces a theoretical estimate of the whitecap percentage as a function of the wave age, which in turn depends on the wave peak frequency and the friction velocity in the air. The comparison between the measured whitecap coverage and the model results shows an order of magnitude correspondence for most data. A group of outliers appears to be connected to the strength of the current.

Full access
K.B. Katsaros
,
J. DeCosmo
,
R.J. Lind
,
R.J. Anderson
,
S.D. Smith
,
R. Kraan
,
W. Oost
,
K. Uhlig
,
P.G. Mestayer
,
S.E. Larsen
,
M.H. Smith
, and
G. De Leeuw

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 semen device, the “spray Ringer.” 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-α 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-100 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 psychrometer employing platinum resistance thermometers perform very well.

Full access