No abstract

A low-temperature low-pressure humidity chamber has been developed at the Korea Research Institute of Standards and Science for the calibration of radiosonde humidity sensors. The humidity generator operates in a two-temperature and two-pressure mode in which a saturator and a test chamber are separately immersed in two baths. The saturation performance of the saturator is first evaluated at a low temperature (−75 °C) and at atmospheric pressure. In this condition, the uncertainty of the frost-point temperature by the saturator is 53 mK with the coverage factor k = 2. Then, a chilled mirror hygrometer is calibrated for the compensation of low-pressure effects such as a pressure drop along the tubing line inside the hygrometer. Significant differences between the frost-point temperature at the hygrometer inlet and that measured by the hygrometer are observed at a pressure lower than 20 kPa. An empirical formula for compensating the low-pressure effects on the hygrometer is obtained and, subsequently, is used for the validation of the frost-point temperature generated at a low temperature and low pressure in the test chamber. The corresponding uncertainty due to the calibration of the hygrometer including the compensation of low-pressure effects is 306 mK at k = 2. A representative humidity generation and its validation by the hygrometer is demonstrated at a temperature (−70 °C) and two pressures (4.6 kPa and 20 kPa) in the test chamber. Uncertainty budgets on relative humidity in the test chamber are presented from 10 %rh to 40 %rh with a 10 %rh interval at both pressures. Consequently, the combined uncertainty is less than 2%rh (k = 2) which includes uncertainty factors such as the saturation efficiency, the adsorption/desorption hysteresis, the validation by the hygrometer measurement at low pressures, and so on. The newly developed humidity chamber enables the calibration of radiosonde humidity sensors at low temperature and low pressure conditions that mimic upper air environments in a traceable manner to the International System of Units (SI).

Abstract. An upper-air simulator (UAS) has been developed at the Korea Research Institute of Standards and Science (KRISS) to study the effects of solar irradiation of commercial radiosondes. In this study, the uncertainty in the radiation correction of a Vaisala RS41 temperature sensor is evaluated using the UAS at KRISS. First, the effects of environmental parameters including the temperature (T), pressure (P), ventilation speed (v), and irradiance (S) are formulated in the context of the radiation correction. The considered ranges of T, P, and v are −67 to 20 ∘C, 5–500 hPa, and 4–7 m s−1, respectively, with a fixed S0=980 W m−2. Second, the uncertainties in the environmental parameters determined using the UAS are evaluated to calculate their contribution to the uncertainty in the radiation correction. In addition, the effects of rotation and tilting of the sensor boom with respect to the irradiation direction are investigated. The uncertainty in the radiation correction is obtained by combining the contributions of all uncertainty factors. The expanded uncertainty associated with the radiation-corrected temperature of the RS41 is 0.17 ∘C at the coverage factor k=2 (approximately 95 % confidence level). The findings obtained by reproducing the environment of the upper air by using the ground-based facility can provide a basis to increase the measurement accuracy of radiosondes within the framework of traceability to the International System of Units.

Relative humidity in upper air is frequently measured using radiosondes at an altitude of up to roughly 35 km. In this study, the calibration of a radiosonde humidity sensor was demonstrated using a humidity generator that can mimic the humidity, temperature, and pressure conditions in upper air. The calibration of the radiosonde humidity sensor was conducted in terms of relative humidity at a temperature range of −70 °C to 20 °C, and at a pressure range of 5 kPa to 101 kPa. Consequently, it was revealed that the radiosonde humidity sensor presents a significant temperature dependency in such a way that indicates lower humidity values than the reference values measured by the generator as temperature is lowered. The deviation from the reference humidity was measured at about 30%rh at −70 °C. However, the effect of low pressure (5 kPa) on the sensor measurement was less than 1%rh. The uncertainty budget on the calibration of the radiosonde humidity sensor, including factors due to humidity generation, correction of temperature effect, pressure effect, and sensor reproducibility, are presented. Consequently, the calibration uncertainty of the humidity sensor was 3.8%rh at the coverage factor k = 2. The calibration capability of the radiosonde humidity sensor demonstrated in this study will provide traceability to the International System of Units (SI) in the humidity measurement in upper air.