Millimeter-wave Measurement of Frozen Hydrometeors

Snowfall is an important component of the Earth’s precipitation and hydrological cycle. Remote measurements of frozen hydrometeor properties have been limited because coincident measurements of microphysical and electromagnetic properties of snowfall have not been available. Ground-based radars have been widely used to monitor snowfall intensity, but even those measurements are difficult to relate to water equivalent of snow because of the vast diversity of snow habits, density, and size distributions. Moreover, the spatial coverage of radar networks is limited over most regions other than the U.S.A., Europe, and Japan. Snowfall measurement from space has been suggested as a solution to overcome this limitation. If the precipitating clouds contain a sufficient density of ice water equivalent from snow crystals, graupel, or both, then depressed brightness temperatures (Tbs) at 85 GHz is likely [1]. However, if the precipitation is more local or shallow (4 km or less), the Tb depression at 85 GHz may not be as apparent. Interest has focused on frozen hydrometeor measurements at millimeter-wavelengths that are short enough to react with frozen hydrometeors and for which water vapor obscures the variable emission from the underlying snow covered surface. The NOAA Advanced Microwave Sounding Unit (AMSU) has been used to derive snowfall over land using millimeter-wave radiometry ([2], [3], [4], and [5]). This study analyzes the millimeter-wave radiometric measurements of frozen hydrometeors during the field experiment that was held in Wakasa bay of Japan in January 29, 2003. The MM5 cloud simulation is employed to provide temperature and humidity profiles for the radiative transfer calculations. Parameterizations to represent the electromagnetic scattering properties of snow at millimeter-wave frequencies are applied to the hydrometeor profiles derived by airborne radar measurements. Calculated Tbs, radar reflectivity are compared with the millimeter-wave measurements.