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R. Paul Ford
and
G. W. Kent Moore

Abstract

A case study of a small-scale polar front cyclone observed during the Canadian Atlantic Storms Program (CASP) is presented. The cyclone forms along an essentially two-dimensional front, which is in approximate thermal wind balance. This cyclogenetic event occurs where the Richardson number near the surface is small. The storm appears to grow in response to favorable low-level thermal advection rather than to any significant upper-level forcing.

As the wave amplifies, the initially two-dimensional frontal zone develops a three-dimensional structure. The cyclone has a horizontal wavelength of 1200 km and a vertical scale of 3–4 km. The wind field associated with the evolved system is strongly unbalanced in comparison with that in the initial state. A strong southerly low-level jet exists in the warm sector and a moderate northerly jet is observed in the air behind the cold front. The low-level warm sector is a region of reduced static stability.

Results will be compared with a frontal zone stability theory that describes how a two-dimensional primary frontal zone will evolve into a three-dimensional structure with secondary fronts. As we shall see, the three- dimensional structure of the observed system compares favorably with the theory.

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Yoshio Asuma
,
Soshi Iwata
,
Katsuhiro Kikuchi
,
G. W. Kent Moore
,
Ryuji Kimura
, and
Kazuhisa Tsuboki

Abstract

In the fall of 1994, the Beaufort and Arctic Storms Experiment (BASE) was held to collect information on the structure and evolution of mesoscale weather systems over the southern Beaufort Sea and the Mackenzie River delta of the western Canadian Arctic. As part of the experiment, X-band Doppler radar observations were carried out at Tuktoyaktuk, a village on the shore of the Beaufort Sea. In this paper, the precipitation features, structure, and moisture transport associated with two distinctly different weather systems that were observed during BASE are described with a variety of datasets. Climatologies of storm activity in the area suggest these two types of different weather systems, the so-called Pacific origin and storm track disturbances, are the most frequently observed in this region during the fall months.

The characteristic feature of a Pacific origin weather system is a pronounced layering of the air masses. In the upper layer, the air mass is of Pacific origin and is associated with a deep low in the Gulf of Alaska. As a result it is moist and is capable of producing precipitation. In contrast, the lower layer is initially of continental origin and is associated with a secondary lee cyclogenesis event in the Mackenzie River basin. As the secondary disturbance moves to the east, there is a shift in the wind direction that advects air from the Beaufort Sea into the lower layer. This results in a moistening of the lower layer that allows precipitation from the upper layer that had previously evaporated in the lower layer to be enhanced and reach the surface. The detailed structure of this type of storm is strongly affected by the topography of the region and the presence of open water in the southern Beaufort Sea.

The storm track weather system is markedly different and is associated with the passage of a mesoscale low over the southern Beaufort Sea. In this sort of system, there is a well-defined frontal structure of a type previously identified in the midlatitudes. Two different precipitation regimes are identified that are associated with the passage of the warm and cold front. In this sort of system, the sources of moisture are the Bering Sea and the open water in the southern Beaufort Sea.

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