For the accurate measurement of temperature in the upper air by using radiosondes, one prerequisite is the compensation of solar radiation effects that cause sensor heating. The development at the Korea Research Institute of Standards and Science (KRISS) of an upper air simulator (UAS) that can simulate radiation effects is reported. The UAS can independently control four environmental parameters: irradiance, temperature, pressure and air speed. An entire radiosonde can be installed in the test chamber and the measurement data transmitted remotely via an antenna. Solar irradiance is mimicked by using a solar simulator that irradiates the radiosonde sensor. The temperature of the test chamber is controlled from −70 to 20°C by placing it inside a climatic chamber. The pressure and ventilation speed in the test chamber are modulated using combinations of sonic nozzles, a mass flow controller and a vacuum pump. A ventilation speed of 5 m·s−1, which mimics the speed of ascent of the radiosondes when lifted using a balloon, is achieved at pressures as low as 7 hPa. The capability of controlling the environmental parameters independently and the stability of each parameter are presented. As a proof of concept, the radiation‐induced bias on the temperature sensor of a commercial radiosonde Vaisala RS41 is measured. The effect of each parameter is investigated by varying it while keeping the other parameters fixed. Radiosonde calibration using the UAS at the KRISS will help improve the traceability of upper air measurements to the International System of Units.

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The temperature measurement in the upper air by radiosondes is affected by various environmental factors such as solar irradiation, ventilation and air pressure. Among them, solar irradiation induces radiative heating of sensors whereas ventilation causes convective cooling. Here, the effect of these opposite factors on air temperature measurement is studied to obtain a correction formula using an experimental set-up consisting of a wind tunnel, a solar simulator and dual thermistors with different emissivities. The relationship between the temperature difference between the dual thermistors, irradiance and ventilation speed is first established in order to calculate irradiance in-situ by solely using the temperature difference. The temperature difference between dual thermistors is linearly proportional to the irradiance up to 1500 W m −2 and the slope of the linear function is decreased as the wind speed is increased up to 10 m s −1 at a fixed pressure level (about 1000 hPa). The uncertainty of the calculated irradiance using dual thermistors is 12.2% at the coverage factor k = 2. The calculated irradiance is then used for the correction of temperature of thermistors relative to the reference temperature in the shade inside the wind tunnel. The combined uncertainty of corrected temperature including uncertainty factors due to the compensation of wind and irradiance as well as the reference temperature is 0.19 K (k = 2). The dual thermistor-based technique can provide the traceability of the temperature measurement in upper air through in-situ compensation of the solar radiation effect.

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ABSTRACT:One of the challenges in temperature measurements in the upper air using a radiosonde is the correction of solar radiation effects. A prerequisite for the correction is in situ measurements of solar radiation. In this study, a new technique for the measurement of solar irradiation followed by the temperature correction was developed using dual temperature sensors with different emissivities. The principle is that the radiation intensity is calculated by using the temperature difference between two thermistors coated with different materials (one with graphite and the other with aluminium) and then the irradiance obtained is used for the correction. As a first stage, the effect of air pressure on the temperature correction process using dual thermistors was examined. The temperature difference between the two sensors was found to be linearly proportional only to the irradiance up to 360 W m −2 , regardless of air pressure from 10 to 1000 hPa at room temperature. This provides a way to calculate the irradiance by simply measuring the temperature difference between the two sensors. The calculated irradiance showed a difference of less than 45 W m −2 (13% of the maximum irradiance) compared to the irradiance measured with a calibrated pyranometer. The extended uncertainty of the calculated irradiance was 53.5 W m −2 (coverage factor k = 2). A reference temperature sensor in the shade was used to obtain a correction formula as a function of air pressure and calculated irradiance. When the temperature was corrected using the calculated irradiance, the extended uncertainty of the corrected temperature was 0.98 K (k = 2).