A new sensitive eddy flux instrumentation

Abstract

An instrument for direct measurement of the turbulent water vapour flux has been constructed and used in a field program in southern Sweden. The instrument consists of wet and dry bulb platinum resistance elements of Australian design and of a new fast response wind instrument. The instantaneous wind vector is measured by the aid of strain gauges fixed to elastic plates, exposed to the wind pressure. The signals from the probes are recorded photographically by a mirror galvanometer with free period 0.05 sec. With this galvanometer vertical velocities are recorded with an accuracy of + 0.01 m/s, when the wind speed is not too low. The evaluation of the photographic recordings being highly time consuming, a device has been constructed which at least reduces the time required for the evaluation by a factor of ten. The efficiency of the flux record is controlled by the time constant of the wet bulb element which is of the order 0.5 sec. Then according to DEACON almost full recovery is obtained in lapse conditions but only c. 80 % in inversion conditions. Experimental data is presented from 45 successful 30–60-min measurements. Computed water vapour flux data is presented as well as complementary micrometeo-rological data.