Abstract
A diagnostic analysis is made for the rapid development of two subsynoptic scale cyclones that coexisted over Canada in the spring season, using the Level IIIb First GARP Global Experiment dataset assimilated by the European Centre for Medium Range Weather Forecasts and National Weather Service operational data. Both cyclones started developing at 0000 UTC 25 April 1979. When development began, these cyclones wore separated from each other by a distance of only about 1300 km. Nonetheless, the physical processes leading to their initial cyclogenesis are found to be different. One of the cyclones remained a weak, shallow surface low for about 48 h after it formed in a zone of developing upper-level westerly waves. It started developing only when it drifted into a region of the horizontal advection of upper-level potential vorticity anomaly associated with a strong tropopause fold. In contrast, the other cyclone formed in a region of strong surface frontogenesis caused primarily by the velocity deformation. Once formed, it developed rapidly and propagated within a narrow zone of small Richardson number, suggesting that the cyclone developed due to localized baroclinic instability. Eventually the latter cyclone also moved into the region of upper-level potential vorticity advection and absorbed the former to become a major synoptic-scale cyclone. Its deepening rate came close to that of explosive cyclogenesis, while heating by latent heat release was found to be of secondary importance in the rapid development from the heat budget analysis in the core of the cyclone.
