Microwave properties of frozen precipitation around a North Atlantic cyclone

Microwave brightness temperatures emanating from a North Atlantic cyclone w ere measured by the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological Satellite Program satellite. As other investigators have fou nd before, low 85.5-GHz brightness temperatures (215 +/- 20 K) were observe d from cumulonimbus clouds along the squall line; however, 85.5-GHz microwa ve brightness temperatures observed from the nimbostratus clouds north of t he low center were significantly higher (255 +/- 20 K). In situ measurement s from aircraft during the Canadian Atlantic Storm Program II showed that h eavy snowfall consisting of large tenuous aggregates existed in the nimbost ratus clouds at the time of the SSM/I overpass. Distributions of snow, rain, liquid cloud water, and cloud ice mass were co mputed from a modified version of the fifth-generation Pennsylvania State U niversity-NCAR Mesoscale Model. That model employed a mixed-phase ice micro physics (MPIM) scheme that only considered one type of frozen hydrometeor. The frozen hydrometeor size distributions, density, and mass flux were modi fied to match the in situ observations where they were available and to acc ount for the SSM/I observations using radiative transfer theory. Those revi sed hydrometeor representations were constrained to preserve the vertical h ydrometeor mass flux distributions obtained from the MPIM scheme throughout the analysis. Frozen dense accreted particles were required near the squall line to accou nt for the microwave scattering effect. Snow aggregates, with density that decreased with increasing size, were needed to reproduce the high brightnes s temperatures observed from the nimbostratus clouds.