Evaluation of a wind tunnel designed to investigate the response of evaporation to changes in the incoming longwave radiation at a water surface. I. Thermodynamic characteristics

Abstract. To investigate the sensitivity of evaporation to changing longwave radiation we developed a new experimental facility that locates a shallow water bath at the base of an insulated wind tunnel with evaporation measured using an accurate digital balance. The new facility has the unique ability to impose variations in the incoming longwave radiation at the water surface whilst holding the air temperature, humidity and wind speed in the wind tunnel at fixed values. The underlying scientific aim is to isolate the effect of a change in the incoming longwave radiation on both evaporation and surface temperature. In this initial paper we describe the configuration and operation of the system and outline the experimental design and approach. We then evaluate the thermodynamic properties of the new system and demonstrate that the evaporation, air temperature, humidity and wind speed are measured with sufficient precision to support the scientific aims. We find that the shallow water bath naturally adopts a steady state temperature that closely approximates the thermodynamic wet bulb temperature.