THE EFFECT OF LARGE-SCALE FLOW ON LOW-LEVEL FRONTAL STRUCTURE AND EVOLUTION IN MIDLATITUDE CYCLONES

Observational and modeling studies documented in the literature indica te that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-l evel cyclone/frontal structure and evolution through a combined observ ational and idealized modeling approach.Analyses of two observed cyclo ne cases embedded in large-scale diffluence and confluence, respective ly, are presented to illustrate two possible cyclone/frontal structure s and evolutions. Specifically, the cyclone moving into a diffluent, h igh-amplitude ridge becomes meridionally elongated and possesses a str ong meridionally oriented cold front and a weak warm front. The cold f ront rotates into the warm front, forming an occluded front in the man ner of the Norwegian cyclone model, as indicated by the narrowing of t he thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving into confluent, low-amplitude zonal flow becomes zonally elongated and possesses strong zonally oriented warm a nd bent-back fronts and a weak cold front. The frontal structure in th is case is reminiscent of the Shapiro-Keyser cyclone model, exhibiting a fracture between perpendicularly oriented cold and warm fronts (i.e ., the so-called frontal T-bone structure). The idealized simulations employ a nondivergent barotropic model in which potential temperature is treated as a passive tracer When a circular vortex acts on an initi ally zonally oriented baroclinic zone, cold and warm fronts, a frontal fracture. a bent-back front, and eventually a Norwegian-like occlusio n develop. When a circular vortex is placed in a diffluent background flow, the vortex and frontal zones become meridionally elongated, and the evolution resembles the Norwegian occlusion with a narrowing therm al ridge. When a circular vortex is placed in a confluent background f low, the vortex and frontal zones become zonally elongated, and the ev olution resembles the Shapiro-Keyser model with a frontal fracture, fr ontal T-bone, and bent-back front. Although the idealized model qualit atively reproduces many of the frontal features found in the observed cyclones analyzed in the present study, one significant difference is that the maximum potential temperature gradient and fronto-genesis alo ng the cold and warm fronts may differ by a factor of 2 or more in the observed cases, but remain equal along the cold and warm fronts throu ghout the idealized model simulations. Possible reasons for this asymm etry in the strength of the observed cold and warm fronts are discusse d.