During the MC3E field campaign, the scientists have at their disposal many different instruments to study clouds and precipitation. But what happens if the clouds are too thin or not present at all? The ARM facilities also have at their disposal an array of instruments that can bridge this gap. They are called radiometers. Radiometers are passive remote sensing instruments that measure the integrated amounts of water vapor and liquid water in the atmosphere. ARM has many of these instruments at the site, but during this experiment an additional one-of-a-kind radiometer was added to their arsenal. This radiometer comes all the way from the University of Bonn, Germany, and is called the Advanced Microwave Radiometer for Rain Identification or ADMIRARI.

So what makes this instrument so unique among radiometers? The ADMIRARI has three frequencies (10.65 GHz, 21.0 GHz, 36.5 GHz). Each of these frequencies has both horizontal and vertical polarization (i.e., meaning the orientation of the EM waves that are measured), and the radiometer is fully steerable and can scan the whole sky, which the other radiometers at the site cannot do. This permits the radiometer to differentiate between cloud liquid water and rain water. This is done by looking at the difference of measured power between the two polarizations at a single frequency. This difference is caused because rain drops, unlike cloud drops, are not round, but have an oblate spheroid shape.

Submitted by Véronique Meunier, graduate student, McGill University.