A Case Study of Rapid Cyclogenesis over Canada. Part II: Simulations

Abstract

Short-range (36 and 48 h) simulations of a rapid cyclogenesis that occurred over Canada in the spring season are made using a significantly modified version of the hydrostatic, sigma-coordinate, quasi-Lagrangian grid model developed by Mathur. The modifications include the adaptations of a recent variant Kuo cumulus parameterization scheme, a revised computation of the horizontal pressure gradient force, and a high-resolution planetary boundary layer parameterization. Analyzed data are used as an initial field, and to specify the values at the lateral boundaries of the model during integration as well as to verify the results of simulations. The model has several versions in the combinations of dry or moist physics, two different vertical resolutions (18 and 9 layers), and three different horizontal grid spacings (180, 90 and 45 km). The event was diagnostically analyzed in Part I of Ogura and Juang and represents a unique case of two coexisting mesoα-scale surface lows that developed through different physical processes and subsequently merged to form a synoptic-scale cyclone. The model simulates the rapid development of the two surface lows and their subsequent merging reasonably well. The simulated precipitation pattern also agrees with observations. A simulation using a dry version of the model is essentially similar to that from a moist version of the model. This suggests that diabatic heating by condensation is not essential in the rapid cyclogenesis in this case, unlike many explosive cyclogenesis events for winter storms over the ocean. Instead, horizontal advection of potential vorticity and localized baroclinic instability are the essential ingredients in this case. Impacts of reduced vertical and horizontal resolution of the model on the accuracy of the simulations are examined. Also discussed is an effect of boundary layer friction on the prediction of the sea-level central pressure of the surface lows.