Millimeter-Wave Radiometric Measurements of Low Amounts of Precipitable Water Vapor

Extremely dry conditions commonly occur in polar regions during the winter months. Accurate measurements of the precipitable water vapor (PWV) during such dry conditions are needed to improve our understanding of the regional radiation energy budgets. The strength associated with the 183-GHz water vapor absorption line makes radiometry in this frequency regime promising for measuring low amounts of PWV. However, retrievals using these frequencies are complicated by the uncertainties in the absorption models and the fact that these frequencies also respond to the vertical temperature distribution as well as liquid and ice clouds. Pacific Northwest National Laboratory and the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program sponsored National Aeronautics and Space Administration (NASA) Goddard Space Flight Center and National Oceanic and Atmospheric Administration (NOAA) Environmental Technology Laboratory to conduct an experiment to investigate the application of millimeter wave radiometry for making ground-based estimates of PWV during the dry conditions. The data set obtained during this experiment is applicable to assessing the potential limitations of millimeter-wave radiometry to retrieving very low amounts of PWV. The data are also valuable for studying discrepancies in the different absorption models for water vapor, oxygen, and super cooled liquid water. In this paper, an overview of the experiment is presented, a summary of the data collected is given, and single-channel clear-air retrievals are shown.