Dynamics of Cloud-Top Generating Cells in Winter Cyclones. Part I: Idealized Simulations in the Context of Field Observations

Abstract

his paper assesses the influence of radiative forcing and latent heating on the development and maintenance of cloud-top generating cells (GCs) in high-resolution idealized Weather Research and Forecasting Model simulations with initial conditions representative of the vertical structure of a cyclone observed during the Profiling of Winter Storms campaign. Simulated GC kinematics, structure, and ice mass are shown to compare well quantitatively with Wyoming Cloud Radar, cloud probe, and other observations. Sensitivity to radiative forcing was assessed in simulations with longwave-only (nighttime), longwave-and-shortwave (daytime), and no-radiation parameterizations. The domain-averaged longwave cooling rate exceeded 0.50 K h?1 near cloud top, with maxima greater than 2.00 K h?1 atop GCs. Shortwave warming was weaker by comparison, with domain-averaged values of 0.10?0.20 K h?1 and maxima of 0.50 K h?1 atop GCs. The stabilizing influence of cloud-top shortwave warming was evident in the daytime simulation?s vertical velocity spectrum, with 1% of the updrafts in the 6.0?8.0-km layer exceeding 1.20 m s?1, compared to 1.80 m s?1 for the nighttime simulation. GCs regenerate in simulations with radiative forcing after the initial instability is released but do not persist when radiation is not parameterized, demonstrating that radiative forcing is critical to GC maintenance under the thermodynamic and vertical wind shear conditions in this cyclone. GCs are characterized by high ice supersaturation (RHice > 150%) and latent heating rates frequently in excess of 2.00 K h?1 collocated with vertical velocity maxima. Ice precipitation mixing ratio maxima of greater than 0.15 g kg?1 were common within GCs in the daytime and nighttime simulations.