CALIBRATION OF A LYMAN-ALPHA SENSOR TO MEASURE IN-CLOUD TEMPERATURE AND CLEAN-AIR DEWPOINT TEMPERATURE

A number of previous studies have described how airborne temperature-s ensor wetting can lead to erroneous temperature measurements under var ious conditions. During the 1985 Joint Hawaii Warm Rain Project (JHWRP ) a large number of trade-wind cumuli were sampled by the University o f Wyoming King Air. It was found that both the reverse flow and the Ro semount sensors showed signs of wetting during the project. Therefore, a post-field calibration of the Lyman-alpha hygrometer was performed to derive the temperature in the cloudy air. The University of Wyoming Lyman-alpha sensor is a simple system with a high-pressure emitter tu be. Both water vapor and oxygen contribute to the total absorption at the altitudes where the sensor is used. Thus, a highly simplified two- gas model is proposed for its function, and the instrument is calibrat ed by comparison with the dewpoint temperatures in clear-air soundings . An accuracy of +/-0.5-degrees-C is estimated for the calibration. Ab sorption by water vapor and oxygen constitute nearly 70% and 30% of th e total absorption, respectively. Even for moderate liquid water conte nts (almost-equal-to 1 g m-3), the difference between the temperatures derived from the Lyman-alpha and from immersion sensors approaches th eoretical predictions for fully wetted sensors. The present study show s a somewhat higher degree of wetting of the reverse-flow sensor than recently published results using an Ophir radiometer.