The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. VIII: A Model for the “Seeder-Feeder” Process in Warm-Frontal Rainbands

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.