Comparison of Infrared Atmospheric Brightness Temperatures Measured by a Fourier Transform Spectrometer and a Filter Radiometer

Abstract

Increased interest in using atmospheric brightness temperature measurements from simple infrared radiometers combined with radars and lidars has prompted the investigation of their accuracy for various sky conditions. In comparisons of atmospheric brightness temperatures (Tb) measured by a Fourier transform infrared (FTIR) spectrometer and a single-band filter radiometer (PRT5), the authors establish that the PRT5 measurements agree with those from the more sophisticated and accurate FTIR within 1.5 K rms over the range where both instruments are calibrated. Below the PRT5's cold calibration cutoff of 205 K, the PRT5 measures too warm. The FTIR, which is calibrated over the entire measurement range, provides a calibration for the erroneous PRT5 measurements, enabling quantitative use of the simple and inexpensive PRT5 over a larger, more useful range. The corrected data agree within 1.5 K rms, with over 90% differing by less than one temporal standard deviation. The calibration correction technique is applicable to a variety of radiometers and most importantly shows that PRT5-type radiometers are indeed capable of accurately measuring clear-sky and cirrus emission.