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Patrick S. Market and James T. Moore

Abstract

A diagnostic study of a continental occluding extratropical cyclone (ETC) during 1–2 November 1992 is presented using initializations from the Mesoscale Atmospheric Prediction System (MAPS), a hybrid sigma–isentropic coordinate model. Whereas recent studies have concentrated on maritime ETCs and have used numerical model simulations, this study employs diagnostic, observational data and model initializations to develop an occlusion model. In addition, isentropic parcel trajectories from a diabatic trajectory model are examined to trace the origin and termination of air parcels associated with the development of the occluded front. The chosen storm was a moderately deepening (i.e., typical) ETC over a data-rich continental region. This storm developed over the central United States, where commercial aircraft and a network of wind profilers provided copious asynoptic data aloft, which was ingested by the MAPS. Analyses of this well-defined occluded cyclone tend to verify that the advancing cold front overtakes the retreating warm front, though it does not “ride up” the warm front, and that warm-sector parcels are lifted upward in the vicinity of the occluded front, thereby confirming that some of the parcels aloft over the surface occluded front do originate near the surface prior to occlusion. Discussion is also provided on the nature of the occluded front as a true frontal boundary.

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Douglas E. Tilly, Anthony R. Lupo, Christopher J. Melick, and Patrick S. Market

Abstract

The Zwack–Okossi vorticity tendency equation was used to calculate 500-hPa height tendencies in two intensifying Southern Hemisphere blocking events. The National Centers for Environmental Prediction–National Center for Atmospheric Research gridded reanalyses were used to make each of these calculations. The block intensification period for each event was associated with a deepening surface cyclone during a 48-h period beginning at 1200 UTC 28 July and 1200 UTC 8 August 1986, respectively. These results demonstrate that the diabatic heating forces height rises through the sensible and latent heating terms in these two Southern Hemisphere blocking events. The sensible heating was the larger contributor, second only to (about the same as) the vorticity advection term in the first (second) event. The vorticity advection term has been shown by several studies to be associated with block intensification.

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