Atmospheric Profiling of Water Vapor Density with a 20.5–23.5 GHz Autocorrelation Radiometer

Abstract

A tropospheric water vapor density profiling system is presented. The hardware consists of an autocorrelation radiometer (CORRAD) operating over a frequency range from 20.5 to 23.5 GHz. The CORRAD directly measures the autocorrelation of downwelling thermal emission from the atmosphere. The 3 GHz predetection bandwidth of each measurement provides for extremely rapid decorrelation of the noise inherent in all radiometer measurements. This, in turn, allows for high temporal resolution of the water vapor dynamics. Fourier transformation of the raw data produces a brightness temperature spectrum with 100 MHz resolution across the frequency range. Inversion of the radiative transfer integral equation to solve for the water vapor distribution is constrained by the 31 equivalent frequency channels. Previous microwave profilers of the troposphere, with 2 to 5 frequency channels, were much less constrained and the inversion process was accordingly more sensitive to measurement noise. Water vapor profiles estimated by the inversion are in good agreement with coincident radiosonde measurements made by the National Weather Service.