Correction of solar irradiation effects on air temperature measurement using a dual-thermistor radiosonde at low temperature and low pressure

The correction of heat-biased temperatures measured by radiosondes during the daytime is affected by other environmental parameters such as ventilation, air temperature and pressure. Among them, the effect of low temperature on temperature sensors under irradiation is not well known. This paper studies the combined effect of low temperature and low pressure on the air-temperature measurement under irradiation to obtain a correction formula using dual thermistors with different emissivities. The temperature variation of both thermistors under irradiation (about 1500Wm(-2)) is increased as temperature and pressure are decreased from 25 to -80 degrees C and from 1000 to 10hPa, respectively. The observed behaviour is explained using heat-transfer equations. In order to calculate the irradiance with no pyranometer, the temperature difference between dual thermistors is formulated as a function of irradiance, temperature and pressure through chamber-based experiments. After the relationship between parameters is established, the temperature difference between thermistors is solely used to calculate the irradiance. The uncertainty of the calculated irradiance using dual thermistors is 6.2% (94Wm(-2)) at the coverage factor k=1. The calculated irradiance is used for the correction of the heat-biased temperatures measured by the thermistors. The combined uncertainty of corrected temperature is 0.79K (k=1), which includes uncertainties owing to the irradiance, pressure and temperature sensors as well as modelling. The dual thermistor-based technique using ground-calibration facilities can provide the traceability to the International System of Units (SI) in the measurement of irradiance and temperature in the upper air.