A Synoptic Climatology and Composite Analysis of the Alberta Clipper

Abstract

Surface and upper-air analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset are used to construct a climatology of 177 Alberta clippers over 15 boreal cold seasons (October-March) from 1986/87 to 2000/01. The Alberta clipper (hereafter simply clipper) occurs most frequently during December and January and substantially less frequently during October and March. These cyclones generally move southeastward from the lee of the Canadian Rockies toward or just north of Lake Superior before progressing eastward into southeastern Canada or the northeastern United States, with less than 10% of the cases in the climatology tracking south of the Great Lakes. Characteristics of the structure and evolution of clippers during a 36-h period leading up to departure of the cyclone from the lee of the Canadian Rockies and a 60-h period after departure as the cyclone traverses central and eastern North America are examined through composite analyses. Over the course of the predeparture period, a cyclone over the Gulf of Alaska approaches the west coast of North America, and through its interaction with the mountainous terrain of western North America spawns a surface lee trough, characterized by a thermal ridge at 850 hPa, to the east of the Canadian Rockies. This thermal ridge dampens considerably as the composite clipper moves into central North America away from the immediate lee of the Canadian Rockies. The composite clipper system evolves from a lee cyclone with its nonclassical thermal structure to a more classically structured midlatitude cyclone as it moves through central and eastern North America largely as a result of rotation of the low-level thermal gradient and the increasing westward tilt with height of the composite clipper over the last 36 h of the postdeparture period. The thermal gradient rotation is dynamically linked to convergence of the along-isentrope component of the Q vector and thus to the ascent that sustains the clipper and creates some of its characteristic sensible weather elements. Such dynamical forcing is a direct consequence of the persistent westward displacement of the 500-hPa vorticity maximum with respect to the composite clipper sea level pressure minimum that characterizes the postdeparture period.

Challenges of tracking extratropical cyclones in regional climate models

“…The CRCM V4.2.4 used in this study does not include these lakes and suffers from a strong near surface cold bias during fall and winter over this area (Markovic et al 2008;de Elía et al 2008). These factors may contribute to reduce lee cyclone formation in November and insufficient downslope adiabatic warming in December and January, which corresponds to the peak of the Alberta clippers season (Thomas and Martin 2007). The CRCM also underestimates cyclogenesis on the lee side of the American Rockies, although less markedly.…”

Regionalization of snowfall frequency and trends over the contiguous United States

“…The seven snowfall regions are: the southeast (Region 1, shown in red plus signs), the south central plains and southwest (Region 2, shown in orange x's), the Ohio River Valley and mid-Atlantic states (Region 3, shown in yellow boxes), the pacific Northwest (Region 4, shown in green triangles), and the Upper Midwest which is separated into three clusters. The Upper Mid-west region is sensitive to the position of the 'Alberta Clipper' type storm tracks from the northwest (Thomas and Martin, 2007) and 'Colorado Lows' coming from the southwest (Changnon et al, 2008 (Whittaker and Horn, 1981).…”

A Case Study of Anomalous Snowfall with an Alberta Clipper

“…Yet, the previous analyses, as well as the trajectories themselves, suggest modest synoptic-scale ascent values. This idea, coupled with typical atmospheric moisture values, would tend to suggest a less significant storm, with accumulations of the expected 8 to 15 cm range (e.g., Thomas and Martin [2]). Thus we turn to analyses of lake influence and static stability to determine a possible means of enhanced mesoscale upward motion in this system.…”

“…In mid-January 2004 up to 27 cm (11 in) of snow fell across western New York state, accompanied by thunder and lightning, as an Alberta clipper moved over the region. Such systems are not generally known for producing such intense precipitation, as noted by Bluestein [1] and Thomas and Martin [2], and the vigorous nature of this particular system surprised even seasoned local forecasters who are well acquainted with prolific snowfalls.…”

“…This track represents one of the major trajectories for cold-season North American cyclones (Petterssen [5]; Reitan [6]; Zishka and Smith [7]; Murray and Simmonds [8]; Hoskins and Hodges [9]; Simmonds and Keay [10]; Ulbrich et al [11]; Ward [12]). As a result of their fast motions and general lack of available moisture, clipper-type storms typically produce modest precipitation amounts (7.5-15 cm in a 3-6 h period) along a narrow path generally north of the track (Harms [13]; Beckman [14]; Hutchinson [4]; Thomas and Martin [2]). Enhanced snowfall amounts are possible via an influx of Great Lakes moisture (Harms [13]; Cinzani and Changnon Jr. [15]; Silberberg [16]; Angel and Isard [17]) or through the effect of intense upper-level forcing in the presence of reduced static stability (Silberberg [16]; Gallus Jr. and Bresch [18]).…”