Field Evaluation of a Dual-Channel Microwave Radiometer Designed for Measurements of Integrated Water Vapor and Cloud Liquid Water in the Atmosphere

Abstract

The dual-channel microwave radiometer is evaluated in regard to the measurement of integrated water vapor and supercooled liquid water. The study includes comparisons of integrated vapor content measured by the radiometer and rawinsondes as part of the Sierra Cooperative Pilot Project in the central Sierra Nevada. In addition, two radiometers with virtually identical characteristics were brought together on the Colorado Orographic Seeding Experiment at Steamboat Springs, Colorado, in order to study the stability and comparability of integrated vapor and liquid measurements. Comparison of vapor measurements by the radiometer and by rawinsonde yielded a correlation coefficient of 0.94, and a rms difference of 0.08 cm. There were no significant differences between results for paired data gathered in storms in the presence of supercooled liquid water and for paired data gathered on clear days under more ideal conditions. The collocated-radiometer experiment showed slightly closer agreement than did the comparisons of radiometer data with rawinsondes. Comparisons of paired data from the vapor channel yielded a correlation coefficient of 0.95 with a rms difference of 0.05 cm, while the liquid water channel data yielded a correlation coefficient of 0.99 with a rms difference of 0.02 mm. The study lends further credence to other theoretical estimations of the accuracy of the radiometer measurement, in that the measured values of integrated vapor are probably within 15% of truth. Measurements of supercooled liquid water are reproducible and very stable.