THE SYNOPTIC-SCALE AND PLANETARY-SCALE SIGNATURES OF PRECIPITATING SYSTEMS OVER THE MACKENZIE RIVER BASIN

The synoptic- and planetary-scale signatures of precipitating systems over the Mackenzie River Basin (MRB) are elucidated using composites b ased on a 28-year sample of widespread precipitation events. These wet events are defined as days on which 5 or more of 12 surface stations in the MRB receive at least 2.5 mm of precipitation. Seasonal composit es based on a total of 600 wet events reveal a sequence of statistical ly significant flow anomalies. Examination of individual wet events mo tivates stratification of the seasonal samples according to sea-level pressure distribution. One evolution that is particularly common durin g fall, winter and spring involves lee cyclogenesis over the southern MRB in association with a strong cyclone over the Gulf of Alaska; such events are dubbed Gulf Redevelopment (GR) cases. A composite based on 59 wintertime GR events indicates upslope flow north of the lee cyclo ne and warm advection along an east-west oriented warm front during th e precipitation event. Composites of the Q-vector and the divergence o f this field confirm the presence of quasigeostrophic (QG) forcing for ascent over the MRB during this period. A thermally indirect ''topogr aphic tilting'' mechanism, involving downslope warming over the southe rn MRB and upslope cooling to the north, is hypothesized to increase w arm-frontal baroclinicity over the MRB. The GR composite 500 hPa geopo tential height anomaly pattern is characterized by a series of anomali es extending from the Bering Sea to the Gulf of Mexico. The western (e astern) anomalies tend to decay (amplify) with time. The composite exh ibits a positive anomaly over the Bering Sea, a negative anomaly over the Gulf of Alaska that moves eastward into the MRB during the precipi tation event, and a positive anomaly that moves eastward over western and central North America. The presence of large, slow-moving flow ano malies and an extended period of enhanced southwesterly geostrophic po w over the MRB in the composite suggests that a persistent influx of P acific moisture is required to moisten the atmosphere over the MRB suf ficiently for widespread precipitation An independent composite of dry MRB cyclone events exhibits substantially weaker southwesterly geostr ophic flow into the MRB relative to the wet GR composite.