A numerical study of midlatitude squall lines with the Canadian regional finite-element model

A research version of the Canadian regional finite-element (RFE) model is used to evaluate the capability of the operational model in reproducing the meso-$ beta$-scale structure and evolution of three different types of midlatitude squall-line systems, and to advance our understanding on the development of these features. In this thesis, we use the well-documented 10-11 June 1985 squall line as a test bed to examine the appropriate incorporation of various physical representations and their coupling with RFE's model components. It is demonstrated through a series of sensitivity studies that the operational prediction of squall lines can be improved if more realistic model physics, reasonable initial conditions, and high resolution are used. It is shown that subgrid-scale moist convection and grid-scale moist physics must be adequately treated in order to reproduce the internal structures of the squall line. Then, the improved version of the RFE model is used to study the role of gravity waves in the development of a prefrontal squall line associated with the 14 July 1987 Montreal flood. It is found that the gravity waves and convection propagate in a "phase-locked" manner and that the wave-CISK mechanism accounts for the maintenance and intensification of the system. It is also found that frontogenetical processes and release of conditional symmetric instability are responsible for the development of a trailing stratiform rainband associated with the July 1987 Montreal flood. Numerous sensitivity experiments are conducted, and they show that the meso-$ beta$-scale structures and the wave-convection system are very sensitive to the interaction of the parameterized convection with grid-scale physical processes. In the last part of the thesis, the along-line variability 26-27 June 1985 squall line during PRE-STORM is examined. It is found that the three-dimensional structures of the squall's circulations are determined by both a large-scale trough and convectively generated disturbances. In particular, it is shown that rear inflows in the stratiform region tend to be more intense to the south of the mesolow and neat the base of the large-scale trough.