An Airborne Multifrequency Microwave Analysis of Precipitation within Two Winter Cyclones

Abstract

NASA’s Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign gathered data using “satellite-simulating” (albeit with higher-resolution data than satellites currently provide) and in situ aircraft to study snow storms, with an emphasis on banding. This study used three IMPACTS microwave instruments, two passive and one active, chosen for their sensitivity to precipitation microphysics. The 10-GHz through 37-GHz passive frequencies were well suited for detecting light precipitation and differentiating rain intensities over water. The 85-GHz through 183-GHz frequencies were more sensitive to cloud ice, with higher cloud tops manifesting as lower brightness temperatures, but this did not necessarily correspond well to near-surface precipitation. Over land, retrieving precipitation information from radiometer data is more difficult, requiring increased reliance on radar to assess storm structure. A dual-frequency ratio (DFR) derived from the radar’s Ku- and Ka-band frequencies provided greater insight into storm microphysics than reflectivity alone. Areas likely to contain mixed-phase precipitation (often the melting layer/bright band) generally had the highest DFR, and high-altitude regions likely to contain ice usually had the lowest DFR. The DFR of rain columns increased toward the ground, and snow bands appeared as high-DFR anomalies.