An Effective, Economic, Aspirated Radiation Shield for Air Temperature Observations and Its Spatial Gradients

Abstract

Abstract This paper presents the design and evaluates the performance of a double-walled electrically aspirated radiation shield for thermometers measuring air temperature and its gradients in the atmospheric surface layer. Tests were performed to quantify its solar radiation error and wake production, and to characterize the observer effect of the forced aspiration on vertical temperature gradients in the calm and stable boundary layer. Construction requirements were to design a unit that uses inexpensive off-the-shelf components, to assemble easily, to facilitate reconfiguration to accommodate various sensors, and to reduce power consumption with the goal of reducing costs and enabling use in sensor networks in remote locations. The custom-aspirated shield was evaluated in reference to a triple-walled commercially available model and subjected to rigorous testing in a wind tunnel and field experiments. The relative radiation error of air temperature measurements in the custom-aspirated shield was equal to or smaller than that in the reference shield within ±0.08 K for solar irradiances ≥1000 W m−2 and calm winds. At night, thermal imagery revealed no significant differences in surface temperatures of both shields and the air temperature. Both shields produced significant wake within a ±30° sector of incident flow. Even for weak flows ≤0.7 m s−1, higher-order moments were increased by a factor of 3, while the mean airflow speed was reduced by up to 30% compared to uncontaminated directions. Careful inspection of the spatiotemporal dynamics of air temperatures in a vertical profile showed negligible impact of the forced aspiration on the finescale structure of the observations for the nocturnal and transitional calm surface layers.