11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Friday, 7 June 2002
Understanding precipitation mechanisms in snowfall using polarization radar and in situ techniques
Sabine Goeke, NCAR, Boulder, CO
The purpose of this work is to study the evolution of hydrometeors in winter precipitation. The full complexity of ice particle growth to precipitation size particles remains elusive because of difficulties in predicting and observing composition and conditions of clouds and precipitation at adequate spatial and temporal scales. In situ probes mounted on an aircraft have sufficient resolution to provide detailed information for many purposes, but such observations have limited spatial and temporal coverage. Observation by remote sensing provides much better sampling, but the utility of these measurements critically depends on the correct interpretation of the data in terms of the fundamental quantities of interest.

Possibilities and limitations using polarimetric observables to detect precipitation regions governed either by riming or aggregation or detect regions of rather pristine crystals were addressed. For this purpose data of airborne in situ probes were compared to measurements of a ground-based dual-polarization S-band radar. Evidence that polarimetric measurements are capable to provide information regarding prevailing growth mechanisms could be found. The ability to detect remotely cloud volumes dominated by different growth mechanisms is of interest for microphysical studies as well as of practical relevance. The riming process indicates indirectly the presence of super-cooled liquid water droplets coexisting with ice particles. The presence of super-cooled liquid water droplets may suggest icing conditions and therefore a threat for aviation safety.

Future research will address open questions about orographic precipitation, whether the riming process might increase it's efficiency. The Mesoscale Alpine Program (MAP) - with its field phase taken place in fall 1999 - was partly designed to examine the orographic control of precipitation. Microphysical data were obtained by a polarization radar and airborne in situ probes during heavy precipitation events.

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