COASTALLY TRAPPED STRATUS EVENTS IN BRITISH-COLUMBIA

Coastally trapped stratus events involve the alongshore propagation of a low-level mesoscale ridge of high pressure, and generally occur onl y during summer in British Columbia. During these events, a body of co ol dense air overlain by stratus cloud propagates northwards in the ma rine layer. This situation results because Coriolis effects trap the d ense air against the coastal mountains and an inversion layer prevents vertical leakage of energy. Detailed analysis of a late summer event and comparison of the observations with available theory are presented . It is shown that the theoretical speeds for the event agree reasonab ly with observations except near major topographic variability. The th eoretical trapping scale (internal Rossby radius) computed from the so unding data matches the observed width of the coastal stratus seen on satellite imagery. It is found that the properties of the event may be accounted for by the propagation in the marine layer of a coastally t rapped intrusion with steep leading edge or shock. Attention is drawn to the role that the complex topography of Vancouver Island plays in i nfluencing the propagation of the event, and some suggestions for fore casting these phenomena are made.