A study of the extratropical reintensification of former Hurricane Earl using Canadian Meteorological Centre regional analyses and ensemble forecasts

Abstract

Former Hurricane Earl reintensified rapidly while traveling through Canadian waters in September 1998. Its central pressure decreased 40 hPa over a 36-h period, and it produced heavy rain on Cape Breton Island. Nova Scotia, and over Newfoundland. A diagnostic study is conducted from a potential vorticity (PV) perspective using Canadian Meteorological Centre (CMC) regional analysis data. Former Hurricane Earl's rapid redevelopment was related to the interaction between two preexisting positive PV anomalies: a diabatically generated low-level anomaly and an upper-level anomaly. This process was accompanied by a cold air intrusion and warm air "wrapping up" process. As well, the behavior of the operational CMC numerical weather prediction models is examined using output from the ensemble forecast system (giving 10-day forecasts, with eight members and one control run) integrated from three different initial times (0000 UTC on each of 3, 4, and 5 September 1998). Some members failed to maintain former Hurricane Earl's observed closed cyclonic circulation during the weakening period, and subsequently developed only a weak low pressure system. Others maintained the identity of former Hurricane Earl throughout both the weakening and reintensifying periods. Static PV inversions suggest that the more successful forecasts of Earl's reintensification were associated with preferentially strong lower- tropospheric cyclonic circulations induced by the upstream upper-tropospheric PV maximum. This induced lower- level flow also produced the very large-amplitude low-level thermal perturbations characteristics of a deepening baroclinic low.