Precipitation Development in Midlatitude Cyclones: a Study of Cloud Structures and Processes.

The vertical air motions, microphysical structures and precipitation-growth mechanisms of some of the mesoscale rainbands found in mature Pacific cyclones have been examined through Doppler radar, aircraft, rawinsonde and surface measurements. Warm-frontal and wide cold-frontal rainbands extending above the 0(DEGREES)C level were found to consist of an upper-level seeder zone containing shallow (1-2 km) convective generating cells and a lower-level feeder zone containing stratiform cloud. In the seeder zone, mixed-phase conditions promoted particle growth by riming, deposition and aggregation. In the feeder zone, particle growth occurred primarily through deposition and aggregation. The fractions of total precipitation growth that occurred in the seeder and feeder zones were about 20 and 80%, respectively. Precipitation growth in warm-sector and prefrontal surge rainbands was found to occur through a combination of the seeder-feeder process and mixed-phase growth in low and middle-level convective cells. The warm-sector rainband was found to consist of a zone of deep convection followed by a region of stratiform, dissipating cloud. The prefrontal surge rainband was found to consist of a broad zone of deep, mid-level convection above a low-level layer of stratiform cloud. Both types of rainbands were topped by an upper-level cirrostratus layer. Ice crystal seeding from this layer assured mixed-phase precipitation growth in the convective regions of the bands and promoted aggregation growth in the stratiform regions. In approximately 70% of the snow size spectra measured in rainbands, the concentrations of particles less than 2 mm in diameter deviated significantly below or above the exponential curve that fitted larger diameters (these are termed sub-exponential and super-exponential spectra, respectively). The importance of spectrum deviations from the exponential form lies primarily in the information that they provide concerning the processes of particle growth in clouds. Secondary ice crystal production and depletion of small particles due to aggregation appear to be important in the formation of super-exponential and sub-exponential spectra, respectively. The impact of spectrum deviations on the relationship between radar reflectivity and snowfall rate, though observable, was found to be small with respect to common sources of error in radar estimates of snowfall.