Improved 20‐ to 32‐GHz atmospheric absorption model

Abstract

An improved model for the absorption of the atmosphere near the 22‐GHz water vapor line is presented. The Van Vleck‐Weisskopf line shape is used with a simple parameterized version of the model from Liebe et al. [1993] for the water vapor absorption spectra and a scaling of the model from Rosenkranz [1993] for the 20 to 32‐GHz oxygen absorption. Radiometrie brightness temperature measurements from two sites of contrasting climatological properties, San Diego, California, and West Palm Beach, Florida, were used as ground truth for comparison with in situ radiosonde‐derived brightness temperatures under clear‐sky conditions. Estimation of the new model's four parameters, related to water vapor line strength, line width and continuum absorption, and far‐wing oxygen absorption, was performed using the Newton‐Raphson inversion method. Improvements to the water vapor line strength and line width parameters are found to be statistically significant. The accuracy of the new absorption model is estimated to be 3% between 20 and 24 GHz, degrading to 8% near 32 GHz. In addition, the Hill line shape asymmetry ratio was evaluated on several currently used models to show the agreement of the data with Van Vleck‐Weisskopf based models and to rule out water vapor absorption models near 22 GHz given by Waters [1976] and Ulaby et al. [1981], which are based on the Gross line shape.