Passive microwave signatures of Arctic snowstorms observed from satellites

Arctic precipitation has been imaged by the passive Advanced Microwave Sounding Unit (AMSU) and Humidity Sounder for Brazil (HSB) aboard the Aqua satellite, and by nearly identical instruments (AMSU-A and AMSU-B) aboard the National Oceanographic and Atmospheric Administration NOAA-15, -16, and -17 satellites. Events were considered to be precipitation if they exhibited appropriate radiometric signatures and morphological evolution over consecutive satellite overpasses at 100-minute intervals. For such Arctic events, the observed cold perturbations near 183±7 GHz were as much as 20 K, and the cold perturbations near 52.8 GHz were on the order of 1 K. These observed clumped cold perturbations imply the presence of scattering hydrometeors of diameter > 1 mm, which will precipitate. A comparison of these images with those of brightness temperatures from window channels that are sensitive to surface emissivity shows that such events are not likely to be the result of surface variations. Arctic precipitation rate estimates have been calculated using a precipitation-rate algorithm developed previously for mid-latitude climates. For example, on July 20, 2002 a precipitation event ∼200×1000 km in size was observed by Aqua AMSU/HSB moving ∼100 km/h from ∼80° N toward northern Canada, and is consistent morphologically with numerical weather predictions. These estimates require substantial scaling to be accurate for polar atmospheres. Without careful modeling, areas of dry air can sometimes be mistakenly flagged as precipitation.