Satellite observations of polar precipitation using Aqua

Polar precipitation (snowfall) has been observed in 13-km resolution multispectral passive microwave imagery from the Advanced Microwave Sounding Unit (AMSU) and Humidity Sounder for Brazil (HSB) aboard the Aqua satellite. For example, on 20 July 2002, Aqua observed at ~100-minute intervals a precipitation event approximately 200 x 1000 km in size moving from near the North Pole toward northern Canada with features moving as fast as 100 km/h. Several smaller, rapidly moving precipitation events have also been observed between June and September. Sensors similar to AMSU/HSB, i.e. AMSU-A and AMSU-B, are currently operating on the NOAA-15, NOAA-16, and NOAA-17 satellites. Each of these four polar-orbiting satellites repeatedly observes suitably located polar precipitation events at intervals of ~100 minutes using sensors that image the earth at 19-20 frequencies between 23 and 191 GHz. The polar precipitation images are produced by an algorithm developed for mid-latitude regions; it utilizes microwave channels in the opaque oxygen and water vapor absorption bands near 54 and 183 GHz, respectively. Because these bands are opaque near the surface, scattering of cold cosmic radiation by large ice particles aloft has a unique signature related to particle size and indirectly related to surface precipitation rate. This signature is more stable for convective precipitation than for stratiform events.