The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. X: Wavelike Rainbands in an Occlusion

Abstract

The dynamical and microphysical structures of trains of wavelike rainbands in an occlusion are described. The rainbands were located in a region of potential instability behind the cold front aloft They were 3?5 km wide, ?80 km long, spaced 5?10 km apart, and arranged nearly parallel to the winds at their upper levels. Temperatures fluctuated in a wavelike manner in and out of the rainbands the temperatures inside the rainbands being ?1?2°C higher than outside. The vertical air velocities also fluctuated. Near the top of the rainbands upward velocities of 1 m s?1, relative to the ambient air, were present inside the rainbands, but outside of the rainbands the relative velocities were ?1 m s?1 downward. At lower levels the fluctuations in vertical velocities were about one-half those at the upper levels, and they showed fewer systematic variations with respect to the location of the wavelike rainbands. The convergence/divergence and airflow patterns had wavelike structures in the vicinity of the rainbands, with stronger convergence at low levels. Near the top of the rainbands the liquid water contents ranged from 0.4?1.0 g m?3 total precipitation particle concentrations from ?4?8 L?1 and ice particle concentrations from ?10?60 L?1, all three decreasing with decreasing height. The sizes of the ice panicles and precipitation-size waterdrops followed exponential distributions. Horizontal roll vortices in the boundary layer, reinforced by motions produced by the release of latent beat, me discussed as one possible mechanism for this type of wavelike rainband.