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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

temperature fieldwas the main forcing mechanism. The process studied occurs frequently in the warmer months of the year, and a conceptual model is proposedfor the process of frontogenesis within the trough and frontolysis of the preexisting midlatitude front. Importantcomponents include differential thermal advection, the action of Coriolis turning in association with ageostrophicvertical circulations, and horizontal deformation of the thermal field.1. Introduction A climatology of surface troughs

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John A. Augustine and Fernando Caracena

-troposphericenvironments that indicate the expected location and degree of nocturnal mesoscale convective system (MCS)development over the central United States. The authors concentrate on two features: 1 ) the forcing for thelow-level jet (LLJ), and 2) the frontogenetic character of lower-tropospheric fronts, or other types of airmassboundaries, with which MCSs are associated. Results show that very large, long-lived, nocturnal MCSs are likelyto mature downwind of a late afternoon surface geostrophic wind maximum if that

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Graham A. Mills and Ian Russell

1992) Between 17 and 22 April 1990 widespread flooding occurred throughout Queensland, New South Wales, andVictoria, with several record rainfalls recorded. This paper investigates the forcing of the rainfall, assesses thequality of the operational numerical weather prediction model guidance during this period, and presents somenumerical sensitivity experiments both to diagnose forcing mechanisms and to identify reasons for modelweaknesses. All diagnostics used in this study could be made

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David M. L. Sills, James W. Wilson, Paul I. Joe, Donald W. Burgess, Robert M. Webb, and Neil I. Fox

at least one adjacent scan angle and on either a previous or subsequent scan (to ensure spatial and temporal continuity). To allow for convergent (divergent) vortices in the lower (upper) portions of the mesocyclone, the inbound and outbound velocity maxima used to calculate MDV did not have to be at the same range from the radar. 4. The prestorm environment a. Synoptic-scale forcing On 3 November, the synoptic-scale environment over the eastern third of Australia was characterized by a

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Stephen F. Corfidi

midpoint of the twentieth century. On the evening of 20 March 1948, a tornado struck Tinker Air Force Base, Oklahoma, without warning, causing more than $10 million in damage and several injuries. The funnel moved diagonally across the airfield, destroying aircraft and shattering the control tower’s windows. The following day, the commanding general of the Oklahoma City Air Materiel Area, Fred S. Borum, directed the base weather officers at work that evening, Major E. J. Fawbush and Captain R. C

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Hui Yu and H. Joe Kwon

upper-level trough could be either favorable or unfavorable to TC development. On the favorable side, it has been suggested that the asymmetric structures of the outflow layer associated with upper-level synoptic-scale systems could produce large-eddy imports of angular momentum. As a result of the small inertial stability in the upper troposphere, the response to external forcing can penetrate to the vortex center ( Holland and Merrill 1984 ). Up until now, many composite and case studies ( McBride

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Paloma Borque, Paola Salio, Matilde Nicolini, and Yanina García Skabar

1. Introduction Environments where mesoscale convective systems (MCSs) develop have some common characteristics, including an upper-level short-wave trough, a quasi-stationary frontal zone, and a strong low-level jet (LLJ) ( Maddox 1983 ; Laing and Fritsch 2000 ). Consistently, organized convection in subtropical South America develops essentially under environmental conditions characterized by synoptic-scale forcing associated with a cold front over northern Patagonia, downstream from a

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Ben C. Bernstein, Tiffany A. Omeron, Frank McDonough, and Marcia K. Politovich

, and location relative to surface weather features. Upper-air maps were tagged for the synoptic-scale forcing mechanisms operating in each area. Forbes et al. (1993) performed a similar study, comparing pilot reports to surface and upper-air charts, sounding data from the National Weather Service (NWS), and the STORM-FEST field program and output from an early nonoperational version of NCEP’s Eta Model ( Messinger et al. 1988 ). That study was limited to data from February to March 1992 and

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Gregory Thompson, Roelof T. Bruintjes, Barbara G. Brown, and Frank Hage

measures used in WRIPEP; however, more testing may be required since the 500-m requirement only encompasses one or two model levels and thus may be too restrictive. c. AFGWC RAOB icing scheme An algorithm developed by the Air Force Global Weather Central ( Knapp 1992 ) for guidance of their flight operations was also included in WRIPEP. This icing algorithm was originally written for use with rawinsonde data but was tested in WRIPEP on model data. The algorithm, shown in Table 3 , has three

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Jared A. Rackley and John A. Knox

barrier ( Smith 1982 ). This weakens the northward-directed component of the Coriolis force and the flow turns to the south and accelerates in response to the now-unbalanced pressure gradient force. As the ageostrophic northerly flow accelerates, the Coriolis force strengthens once again and acts to deflect the flow in a toward-barrier direction, causing both an accumulation of mass and orographic ascent along the eastern Appalachian slopes. The mass accumulation and adiabatic cooling due to the

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