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Abdul Jabbar Abdullah

86JOURNAL OF METEOROLOGYmVOLUME 6CYCLOGENESIS BY A PURELY MECHANICAL PROCESSBy Abdul Jabbar AbdullahMassachusetts Institute of Technology(Manuscript received 14 July 1948)ABSTRACTIn this paper an attempt is made to study the mechanical influence of a surge of cold air on cyclogenesis.It is assumed that a quasistationary front exists and that cold air starts pushing at the rear of the underlyingcool air. It is shown that the cool air acquires an energy which is of the same order of magnitude as

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David J. Bodine and Kristen L. Rasmussen

rainwater mixing ratios are also observed ahead of the leading line beneath the developing convective cores. As noted previously, θ and both decreased at low levels during surge A. The light blue contours in Fig. 12 show perturbations of −2 and −4 K, and are used as an approximation of the cold pool. At 0430 UTC, perturbations show an expanded cold pool depth and intensity with the outermost −2-K contour reaching 3 km AGL and the −4-K contour enclosing a larger volume compared to 0410 UTC. The

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J. R. Garratt, P. A. C. Howells, and E. Kowalczyk

)ABSTRACT Observations of intense, d~ summertime cold fronts in southeast Australia provide evidence for frontaldeformation (cold air surges) in the coastal region well to the west of the main mountain range. This compareswith the severe deformation that occurs when such cold fronts, known as Southerly Busters, interact with themountains to the east. Three-dimensional numerical'simulations of unsteady gravity currents, in the absenceof orography, provide some insight into the factors affecting the

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Chenjie Huang, Y-L. Lin, M. L. Kaplan, and J. J. Charney

-tropospheric northeasterly winds at the northeast periphery of a high pressure ridge that was just off the western U.S. coast advected the dry air southwestward toward the California coast; 3) the increasing curvature of the upper-level jet-front system and the strong cold-air advection over the southwestern United States strengthened the meso- α -scale subsidence; 4) the first dry-air surge within the PBL in southern California before 0000 UTC 26 October was caused by moisture divergence associated with the surface

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Jose Marengo, Angel Cornejo, Prakky Satyamurty, Carlos Nobre, and William Sea

1. Introduction Freezing weather in southern Brazil is caused by outbreaks of polar air during wintertime (May–August). These cold surges occur several times a year (from zero to eight), producing low temperatures in midlatitudes, and are sometimes so strong that extensive freezes affect southern Brazil with considerable cooling in central and northern Amazonia ( Serra and Ratisbona, 1942 ; Parmenter 1976 ; Satyamurty et al. 1990 ; Seluchi and Nery 1992 ; Marengo et al. 1997 ). The freezing

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Pao-Shin Chu and Soon-Ung Park

planetary-scale circulation dueto cold surge activity have been studied in detail, basedon 200 mb wind data, by Chang and Lau (1980, 1982).They demonstrated that the increase in cold air ad ~ This work was performed while both authors were affiliated withthe University of Wisconsin-Milwaukee, Milwaukee, WI 53201.vection over northern China is responsible for the enhancement of the East Asian local Hadley circulationthrough increased sinking motion in northern midlatitudes. In response to the surge

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RenéD. Garreaud

1. Introduction Synoptic-scale incursions of cold air that penetrate deep into the Tropics are frequently observed to the east of major north–south-oriented mountain ranges, producing dramatic weather changes over a wide range of latitudes. Particular emphasis has been placed on cold surges bounded by the Himalayan Plateau over southeast Asia and their influence on the Tropics during the winter monsoon (e.g., Ramage 1971 ; Lau and Chang 1987 ; Boyle and Chen 1987 ; Wu and

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Ooi See Hai, Azizan Abu Samah, Sheeba Nettukandy Chenoli, Kumarenthiran Subramaniam, and Muhammad Yunus Ahmad Mazuki

climatological base period for the reanalysis data is chosen as 1981–2010 while that for TRMM is from 1998 to 2013. To detect all of the surge-induced events due to the cold-air outburst from the Siberian high moving toward the equatorial South China Sea, we adopt the cold surge index from Chang et al. (2005) , which is chosen as the averaged 925-hPa meridional wind between 110° and 117.5°E along 15°N ( Fig. 5a , described in more detail below). By adapting the index definition from Chang et al. (2005

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Len G. Keshishian, Lance F. Bosart, and W. Edward Bracken

polar air returning poleward behind a retreating surfaceanticyclone, 3) subsided Pacific air crossing the southern Rockies, and 4) a new polar or arctic air mass movingsouthward east of the Rockies. The !nverted trough separates 2 from 4, and a weak warm front delineates 1 from2. The primary cold front marks the boundary between 1 and 3, while a secondary cold front, originating as anortherly wind surge along the eastern slopes of the Rockies and appearing as a bent-back cold front, separates3 from

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José A. Marengo, Carlos A. Nobre, and Alistair D. Culf

Introduction Surges of cold polar air that move across southeastern Brazil and the Amazon region from the south, greatly modifying the atmospheric structure and climatic conditions, are known locally as friagens (friagem in singular). These events can produce severe frosts in the coffee growing areas of southern Brazil and substantial cooling in the Amazon basin. The cold front heading these thrusts of polar air may reach as far north as the equator. The events are relatively common in the

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