Calibration of Radiosonde Humidity Sensors Using a Humidity Generator Operating at Low Temperature and Low Pressure

Accurate measurements of essential climate variables (ECV) including temperature and humidity are important because they provide initializations to weather and climate prediction models. The measurement of relative humidity in upper air is frequently conducted by radiosondes having thin-film humidity sensors carried by a balloon up to about 35 km in altitude. One of the difficulties in the humidity measurement in upper air is that sensing characteristics of thin-film humidity sensors are dependent on the surrounding temperature and/or pressure. According to the WMO intercomparison of high-quality radiosonde systems in 2010, measurement values of relative humidity among radiosonde manufactures were significantly deviated from each other (Nash et al., 2011). In order to improve the reliability of humidity measurements in upper air, the calibration of radiosonde humidity sensors under low-temperature low-pressure conditions that mimic upper air environments is needed.

In this presentation, the calibration of radiosonde humidity sensors at low-temperature and low-pressure is demonstrated using a newly developed humidity generator at Korea Research Institute of Standards and Science (KRISS). First, the operation principle of the humidity generator and the validation of the generated humidity at low temperature and low pressure are introduced. The humidity generator is a two-temperature and two-pressure (2T2P) type in which a saturator and a test chamber are immersed in separate baths. The saturation performance of the saturator is evaluated at low temperature at atmospheric pressure. Then, a chilled-mirror hygrometer directly connected to the saturator is calibrated at low-pressures for the compensation of the drop in the frost-point temperature from the reference. A compensation formula for low-pressure effects on the chilled-mirror hygrometer is obtained and is applied for the validation of humidity generation in the test chamber at low temperature and low pressure. A representative humidity generation at low temperature and low pressure and its validation by the hygrometer are demonstrated in the test chamber at a fixed temperature (-70 °C) and at two different pressures (46 hPa and 200 hPa). In these conditions, uncertainty budgets on relative humidity in the test chamber, in which a radiosonde humidity sensor is located, are less than 2 %rh at the coverage factor, k = 2.

Based on the humidity generator, the calibration of a radiosonde humidity sensor is demonstrated by varying relative humidity in the test chamber at the temperature range from -70 °C to 20 °C and the pressure range from 50 hPa to 1000 hPa. The thin-film humidity sensor shows a temperature dependency in the sensing capability in which the sensor indicates significantly lower humidity than the reference humidity by the generator at low temperatures. However, the sensing capability of the humidity sensor is not affected by the surrounding pressure. A calibration formula for the correction of the relative humidity measured by the humidity sensor is obtained. The uncertainty of the humidity sensor is analysed (u_sensor ~ 3.8 %rh at k = 2) by considering the uncertainty due to the humidity generator, sensor stability, hysteresis, reproducibility, pressure effect, and correction formula. The new humidity generator provides the traceability to International System of Units (SI) in the calibration of radiosonde humidity sensors. The humidity measurement by calibrated radiosonde humidity sensors will also be SI-traceable even at low temperature and low pressure upper air environment.

Reference

Nash J, Oakley T, Vomel H, Li W. 2011.WMO intercomparisons of high quality radiosonde systems. Yangjiang, China, 12 July – 3 August 2010, WMO/TD-No. 1580.