High-resolution radar structure and growth processes of convective generating cells and precipitation bands

Extremely high-resolution cloud radar measurements of convective generating cells and precipitation banding in cold-season midlatitude cyclones were obtained during the airborne component of the 2009-10 Profiling of Winter Storms (PLOWS) field campaign using the University of Wyoming Cloud Radar (WCR) aboard the NSF/NCAR C-130 aircraft. Using a pulse length of ~37.5 m, the WCR reflectivity and radial velocity profiles provide an exceptional level of detail with respect to the cyclones' structural characteristics. Additionally, a suite of in situ cloud probes supplement the radar observations with direct measurements of hydrometeor size, shape and phase to gain insight into the microphysical processes characteristic of these features.

WCR transects through the cyclones' comma-head regions consistently revealed the presence of kilometer-scale convection atop deeper stratiform layers. The convective vertical velocity structure was confined to this area, with ±3-4 m s^-1 values commonly observed, with weaker and less variable measured velocities on the order of particle fall speeds below. Supercooled water was detected at cloud top using cloud lidar and in situ measurements and was directly observed at temperatures as low as -25°C, aiding the production of ice particles that appeared as fall streaks of enhanced reflectivities coalescing into the deeper stratiform regions below. Fall streaks also merged together into linear structures indicative of locally-enhanced particle concentrations and growth processes. Altitude-averaged hydrometeor properties (particle size, concentration, and mass content) were largely consistent with particle generation in the cloud-top convection with subsequent growth by diffusion and aggregation as they fell through the stratiform layer below; number concentrations were generally largest near cloud top, and particle size and mass content increased with depth in cloud. The rimed appearance of some ice crystals was consistent with the action of accretion in the mixed-phase conditions. Additionally, total particle concentrations were enhanced approximately twofold in the principal merging fall streaks in ice-phase conditions, with the largest increase in concentrations evident for particles with D_max ≥ 1500 µm. The observed structure and microphysical characteristics are thus consistent with a seeder-feeder process producing much of the comma-head precipitation structure in the observed cyclones, with linear bands of enhanced particle concentrations evident embedded within the feeder layer. These findings will be presented in detail to describe the radar-observed structural characteristics of generating cells and precipitation banding, and to further the understanding of the processes that control their development.