Complex laboratory modeling of the transfer processes between atmosphere and ocean in the boundary layers
The main quantitative characteristics of atmosphere and ocean turbulent exchange are fluxes of momentum, heat and moisture. These fluxes play an important role in many aspects of meteorological and oceanographic research, including climate modeling, weather forecasting, modeling of boundary-layer processes etc. Turbulent exchange of energy and momentum between the ocean surfaces and the atmosphere to a large extent controls the energy and water cycle and general circulation of the ocean and the atmosphere. The most important characteristics that determine the interaction between the atmosphere and the ocean are flows of momentum, heat and moisture. For their parameterization the dimensionless exchange coefficients (the surface drag coefficient Cd and the heat transfer coefficient or the Stanton number Сh) are used. The purpose of this study is the investigation of the effect of surface waves and foam on the turbulent exchange of momentum and heat within the laboratory experiment, when the wind and wave parameters are maintained and controlled. The effect of spray on turbulent exchange at stormy and hurricane winds is also estimated.
Laboratory modeling provides unique possibilities for investigations these processes for a wide range of wind and roughness parameters.
A series of experiments to study the processes of turbulent exchange of momentum and heat in a stably stratified temperature turbulent boundary layer air flow over the waved water surface was carried out at the High Speed Wind - Wave Stratified flume of IAP RAS (length 10 м, cross section of air channel 0.4 m by 0.4 m). The peculiarity of this experiment was the option to change the surface wave parameters regardless of the speed of the wind flow in the channel. For this purpose a polyethylene net (0.25 mm thick and a cell of 1.6 mm by1.6mm) has been stretched along the channel. The net does not affect the heat exchange, but the characteristics of surface waves depended on the position of the net: the waves were absent when the net was located at the level of the undisturbed water surface, and had maximum amplitude at the maximum depth of the net (33cm). To create a stable temperature stratification of the wind, the air entering the flume was heated to 30-40 degrees. The temperature of the water surface was maintained constant about 15 degrees. The air flow velocity in the flume corresponded to the 10-m wind speed from 10 to 35 m/s. The turbulent fluxes of heat and momentum were retrieved from the velocity and temperature profiles measured with Pitot and hotfilm gauges at the distance 6.5 m from the inlet of the flume and subsequent data processing exploiting the self-similarity of the temperature and velocity profiles. As a result the surface drag and heat exchange coefficients as well as roughness parameters were obtained. Wind wave spectra and integral parameters (significant wave height, mean square slope) were retrieved from measurements by a 3-channel array wave gauge by coherent spectral data processing. To estimate the amount of spray in the air flow, as pray marker was introduced using the effect of a sharp decline in hot film readings in contact with a droplet.
The dependences of the exchange coefficients on the winds speed, the wave parameters and spray marker were obtained. It is shown that the exchange coefficients increase with the wind speed and wave height. It was found, that a sharp increase of the drag and heat exchange coefficients at wind speeds exceeding 25m/s was accompanied by the emergence and increasing concentration of the spray in the air flow over water. The correlation coefficient between the drag coefficient and the spray marker was about 0.75.
The effect of foam presence on the transfer processes and the parameters of the surface roughness within the laboratory simulation of wind-wave interaction was also studied, using a specially designed foam generator. The parameters of air flow profiles and waves elevation were measured with scanning Pitot gauge and wire wave gauges respectively in the range of equivalent wind speed U10 from 12 to 38 m/s (covering strong winds) on the clean water and with foam. It was shown that the foam reduces the amplitudes and slopes of the waves in comparison with the clean water in the hole range of wind speeds investigated, and the peak frequency and wave numbers remain almost constant. The drag coefficient calculating by profiling method demonstrated similar behavior (almost independent on U10 for the strong winds) for case of foam and increased compared with clear water, particularly noticeable for low wind speeds. Simultaneously the investigations of influence of the foam on the peculiarity of the microwave radio back scattering of X-diapason was investigated. These measurements were carried for different sensing angles (30, 40 50 degrees from vertical) and for four polarizations: co-polarized HH and VV, and de-polarized HV and VH. It was shown that foam leads to decrease of specific radar cross section of the wavy surface in comparison with clean water.