Abstract
Satellite and synoptic data are used to establish the environments in which two comma cloud systems occurred over the Pacific Ocean; serial rawinsonde, aircraft, and single- and dual-Doppler radar data provide information on the mesoscale and microscale structures of the systems.
The disturbances formed within polar air masses in regions of moderately strong cyclonic vorticity. A surface low-pressure center was associated with the comma cloud, and a surface-pressure trough was situated under the tail of the comma cloud. In both cases, there was a wind maximum near 850 mb, located on the southeast flank of the comma cloud, just ahead of the short-wave trough.
Well-defined rainbands were present in both comma cloud systems. The average width of the rainbands was ∼20 km and their average separation ∼30 km. The rainbands were aligned along the direction of the mean wind and perpendicular to the thermal wind over the depth of the rainbands. Precipitation cores, produced by embedded convection, within the rainbands had an average spacing along the length of the rainbands of ∼17 km. The precipitation cores contained updraft speeds of several meters per second and relatively high liquid water contents; they retained their identities over periods of several hours. Wind shifts, lines of convergence and associated updrafts occurred at low levels toward the rear of the rainbands. At higher levels, cloud particles moved from the rear toward the front of the rainbands, where they fell out as precipitation through a low-level flow of moist air. The precipitation was augmented by convective elements in an unstable layer near the top of the rainband, which produced ice crystals that grew by riming and aggregation as they fell through the low-level, moist inflow.
The spacing and orientation of the rainbands can be explained by the theory for mixed dynamic/convective instability developed by Sun. The precipitation cores embedded in the rainbands may have been the result of enhanced updrafts at the points where infection-point instability rolls, oriented nearly perpendicular to the length of the rainbands, intersected the rainbands.
