Dynamics of cloud-top generating cells in winter cyclones

Recent field observations suggest that cloud-top precipitation generating cells (GCs) are ubiquitous in the warm-frontal and comma-head regions of midlatitude winter cyclones. The presence of fall streaks emanating from the GCs, and their persistence either to the surface or until merging into precipitation bands suggests that GCs are a critical component of the precipitation process in these cyclones. This dissertation assesses the influence of radiative forcing and a range of cloud-top stability profiles on the development and maintenance of cloud top generating cells (GCs) in high-resolution (100 m grid spacing) idealized Weather Research and Forecasting model (WRF) simulations with initial conditions representative of the vertical structure of a cyclone observed during the Profiling of Winter Storms (PLOWS) campaign.;First, sensitivity to radiative forcing under stability and shear representative of the 14-15 February 2010 cyclone observed during PLOWS was assessed in three simulations with: longwave only (night), longwave and shortwave (day), and no radiation parameterization. Simulated GC kinematics, structure, and ice mass for the nighttime simulation are shown to compare well with Wyoming Cloud Radar, cloud probe, and other PLOWS observations from overnight on 14-15 February 2010. The domain-averaged longwave cooling rate in the day and night simulations were both in excess of 0.5 K h-1 near cloud top, with maxima > 2 K h-1 commonly observed atop GCs. Shortwave warming was weaker by comparison, with domain-averaged values 0.1--0.2 K h -1 and maxima of 0.5 K h-1 atop GCs. The stabilizing influence of cloud-top shortwave warming was evident in the daytime simulation's vertical velocity spectrum, with the 99th percentile of vertical velocity in the 6--8 km layer of 1.2 m s-1, compared to 1.8 m s-1 for the nighttime simulation. GCs regenerate in simulations with radiative forcing after the initial stability is released, but do not persist in simulations when radiation is not parameterized, demonstrating that radiative forcing is critical to GC maintenance under the thermodynamic and vertical wind shear conditions present in this cyclone. When present, GCs are characterized by high ice supersaturation (RHice > 150%) and latent heating rates frequently in excess of 2 K h-1 collocated with vertical velocity maxima. Precipitation mixing ratio maxima of > 0.15 g kg-1 were common within GCs in both the daytime and nighttime simulations.;Second, the influence of cloud-top instability paired with nighttime, daytime, or no radiative forcing on the development and maintenance (or lack) of GCs is assessed. Under initially unstable conditions, GCs develop regardless of radiative forcing, but only persist clearly with radiative forcing. Cloud-top destabilization due to longwave cooling leads to development of GCs even under initially neutral and stable conditions, supporting the hypothesis that GCs are ubiquitous atop winter cyclones because of radiative forcing. GCs do not develop in initially stable simulations with no radiation. Decreased range in vertical velocity spectra for daytime radiative forcing simulations is consistent with offset of cloud-top destabilization through longwave cooling by shortwave heating.