Extended Abstract (40K)1.1
An analysis of the record-breaking 9–10 November 1996 rainfall in the Saint Lawrence and Champlain Valleys
J. R. Gyakum, McGill University, Montreal, QC, Canada; and D. Durnford
The rainfall of 9-10 November was noteworthy because of its all-time record-breaking 24-h total of 135 mm in Montreal. Our study of this case focuses on 1) a 30-year search for analogues to this case, and 2) a synoptic and mesoscale analysis for this case and that of its best analogue.
Our search for analogues to this case yields an especially similar case, in terms of its sea-level pressure and 1000-500 hPa thickness field, which occurred in November 1977. Although the large-scale mass fields were very similar with anomaly correlation values of approximately 0.80 for much of eastern North America, the 1977 case yielded very different area-averaged precipitation values from that of the 1996 case. The analogue produced only 20.2 mm, versus the Saint-Lawrence valley mean of 75 mm for the 1996 case. A synoptic-scale analyses of these two cases yields the following key differences between the two cases: 1) the surface cyclone tracked slightly north of the Saint Lawrence river Valley in the 1977 case, while the surface low traveled slightly south of Montreal in the 1996 case. 2) Surface frontogenesis and strong associated vertical circulations were more evident in the 1996 case, and 3) Precipitable water values, tropospherically-integrated water-vapor transports, and lower-tropospheric equivalent potential temperatures were each substantially larger in the 1996 case than in the 1977 analogue case. Our mesoscale analysis of each case, derived from a series of high-resolution simulations from the Mesoscale Community model, reveals the presence of a strong mesoscale surface vortex in the 1996 case that develops in the vicinity of favorable coupling index with a strong trough in the potential temperature field of the dynamic tropopause. Such a mesoscale development was absent in the 1977 analogue case.
Session 1, Case Studies I
Monday, 12 August 2002, 10:30 AM-11:45 AM
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