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Lance F. Bosart and, Alicia C. Wasula, Walter H. Drag, and Keith W. Meier

Abstract

This paper begins with a review of basic surface frontogenesis concepts with an emphasis on fronts located over sloping terrain adjacent to mountain barriers and fronts located in large-scale baroclinic zones close to coastlines. The impact of cold-air damming and differential diabatic heating and cooling on frontogenesis is considered through two detailed case studies of intense surface fronts. The first case, from 17 to 18 April 2002, featured the westward passage of a cold (side-door) front across coastal eastern New England in which 15°–20°C temperature decreases were observed in less than one hour. The second case, from 28 February to 4 March 1972, featured a long-lived front that affected most of the United States from the Rockies to the Atlantic coast and was noteworthy for a 50°C temperature contrast between Kansas and southern Manitoba, Canada.

In the April 2002 case most of New England was initially covered by an unusually warm, dry air mass. Dynamical anticyclogenesis over eastern Canada set the stage for a favorable pressure gradient to allow chilly marine air to approach coastal New England from the east. Diabatic cooling over the chilly (5°–8°C) waters of the Gulf of Maine allowed surface pressures to remain relatively high offshore while diabatic heating over the land (31°–33°C temperatures) enabled surface pressures to fall relative to over the ocean. The resulting higher pressures offshore resulted in an onshore cold push. Frontal intensity was likely enhanced prior to leaf out and grass green-up as virtually all of the available insolation went into sensible heating.

The large-scale environment in the February–March 1972 case favored the accumulation of bitterly cold arctic air in Canada. Frontal formation occurred over northern Montana and North Dakota as the arctic air moved slowly southward in conjunction with surface pressure rises east of the Canadian Rockies. The arctic air accelerated southward subsequent to lee cyclogenesis–induced pressure falls ahead of an upstream trough that crossed the Rockies. The southward acceleration of the arctic air was also facilitated by dynamic anticyclogenesis in southern Canada beneath a poleward jet-entrance region. Frontal intensity varied diurnally in response to differential diabatic heating. Three types of cyclogenesis events were observed over the lifetime of the event: 1) low-amplitude frontal waves with no upper-level support, 2) low-amplitude frontal waves that formed in a jet-entrance region, and 3) cyclones that formed ahead of advancing upper-level troughs. All cyclones were either nondeveloping or weak developments despite extreme baroclinicity, likely the result of large atmospheric static stability in the arctic frontal zone and unfavorable alongfront stretching deformation. Significant frontal–mountain interactions were observed over the Rockies and the Appalachians.

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Thomas J. Galarneau Jr.,, Lance F. Bosart, and, and Anantha R. Aiyyer

Abstract

The pioneering large-scale studies of cyclone frequency, location, and intensity conducted by Fred Sanders prompt similar questions about lesser-studied anticyclone development. The results of a climatology of closed anticyclones (CAs) at 200, 500, and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented to assess the seasonally varying distribution and hemispheric differences of these features. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP–NCAR reanalysis 200-, 500-, and 850-hPa geopotential height fields for the period 1950–2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once.

The climatology results show that 200-hPa CAs occur preferentially during summer over subtropical continental regions, while 500-hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850-hPa CAs occur preferentially over oceanic regions beneath upper-level midocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter.

Three case studies of objectively identified CAs that produced heal waves over the United States, Europe, and Australia in 1995, 2003, and 2004, respectively, are presented to supplement the climatological results. The case studies, examining the subset of CAs than can produce heat waves, illustrate how climatologically hot continental tropical air masses produced over arid and semiarid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets.

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