Rainband seeds

Rainband seeds

Previous field studies have indicated that warm-frontal rainbands form when ice particles from a `seeder’ cloud grow as they fall through a lower-level `feeder’ cloud. In this paper we present results from a parameterized numerical model of the growth processes that can lead to the enhancement of precipitation in a `seeder-feeder’ type situation. The model is applied to two types of warm-frontal rainbands. In the first (Type 1 situation) the vertical air motions are typical of those associated with slow, widespread lifting in the vicinity of warm fronts. In the second (Type 2 situation) the vertical air motions are stronger, and more characteristic of the mesoscale.The model simulations show that in the Type 1 situations the growth of the `seed’ ice crystals within the feeder zone is due to vapor deposition. The feeder zone in this case is slightly sub-saturated with respect to water due to the presence of the seed crystals. In regions where the feeder zone is not `seeded’ from aloft, snow crystals, originating in the feeder zone, grow by deposition and riming and produce a precipitation rate of 1 mm h 1 , compared to 2 mm h 1 for the combined seeder-feeder cloud system. The presence of seed crystals allows for the efficient removal of condensation produced by the feeder cloud. In the Type 2 situation, the strong mesoscale ascent provides liquid water from which the seed crystals grow primarily by riming.For both Type 1 and 2 situations the condensation rates, radar reflectivities and rainfall rates predicted by the model are in reasonable agreement with field observations.

Rainband seeds

Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations.The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between 3-6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was probably responsible for the heavy precipitation on the cold front being organized, on the small mesoscale, into ellipsoidal areas with similar orientations.The precipitation efficiencies in the warm-sector and narrow cold-frontal rainbands were 40-50% and 30-50%, respectively. One of the wide cold-frontal rainbands, in which there was a steady production of ice panicles in the main updraft, had a precipitation efficiency of at least 80%, whereas another wide cold-frontal band, in which some precipitation evaporated before reaching the surface, had a precipitation efficiency of 20%.Ice particles from shallow convective cells aloft played important roles in the production of precipitation in the wide cold-frontal rainbands and in some regions of the warm-sector rainband. These `seed’ ice particles grew by aggregation and by the deposition of vapor as they fell through lower level `feeder’ clouds. About 20% of the mass of the precipitation reaching the ground in the wide cold-frontal rainbands originated in the upper level `seeder’ zones and 80% in the `feeder’ zones.

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