Using radars as radiometers: promises and pitfalls

Many meteorologists and scientists studying the atmosphere use radars to obtain information on precipitation and winds from the scattering of microwaves on targets. Others use radiometers to measure the integrated vapor and integrated liquid water content of the atmosphere from the emission of microwaves in the atmosphere. Although both types of instruments use microwaves to make their measurements, they obtain different and sometimes complementary information by different methods. Yet the hardware present in the two instruments has many similarities, and radars have essentially all the equipment needed to make radiometric measurements, albeit at different frequencies than radiometers typically function.

Measurements of atmospheric emission have been made with S-band and X-band radars. While considerable qualitative information is available, quantitative interpretation is hindered by the generally poorer receiver calibration of radars compared to radiometers, especially in the context of the small signal to be observed. Because in many cases, one can compute the radiometric emission of the atmosphere with much better accuracy than one can measure it, radiometry offers tremendous potential as a method for constantly monitoring the performance of the radar receiver system, especially at longer wavelengths (S- and C-band). At shorter wavelengths, with minimum modification to the receiver system to better monitor its performance, it is possible to obtain moisture and/or integrated liquid water information by measuring the emissions from the atmosphere blended with the receiver noise. Applications of such measurements span from building a radar that can measure the attenuation it suffers from to use of the humidity and liquid water measurements in research and data assimilation.