Refraction of microwave signals by water vapor

Abstract

Tropospheric water vapor causes a refractive path length effect which is typically 5–10% of the ‘dry’ tropospheric effect and as large as several meters at elevation angles below 5°. The vertical water vapor profile is quite variable, and measurements of intensive atmospheric parameters such as temperature and humidity limited to the surface do not adequately predict the refractive effect. With the advent of multifrequency microwave radiometry, it has become possible to measure extensive parameters such as total amount of precipitable water, W. Our analysis shows that the refractive effect at the zenith can be written as a series of integrals representing moments of the water vapor density, the zeroth moment being identical to the above‐defined quantity W. We proceed to the hypothesis that a water vapor refraction model that is a function of W alone can be successful at low elevation angles. From an extensive study of numerical ray tracings through radiosonde balloon data, we have constructed such a model in the form Δlw = ƒ(W, E, L, T0) where Δlw is the ‘wet’ refractive effect, E is the elevation angle, L is the latitude, and T0 is the surface temperature. The model predicts the effect at all latitudes and elevation angles between 2° and 10° to an accuracy of better than 4% (11 cm at 3° elevation angle).