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Huw C. Davies

-scale development and subsynoptic across-frontal circulation patterns (e.g., Keyser et al. 1989 ; Martin 1999 ); explore the relative amplitude and nature of the forcing at different elevations (e.g., Trenberth 1978 ; Durran and Snellman 1987 ); infer qualitatively the major regions of ascent (e.g., Hoskins and Pedder 1980 ; Sanders and Hoskins 1990 ); examine the contribution of diabatic heating to cyclogenesis (e.g., Chang et al. 1984 ; Tsou et al. 1987 ; Strahl and Smith 2001 ); study jet flow and

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Clifford Mass and Brigid Dotson

vegetation, climate, and terrain. The region’s tall trees, many reaching 30 to 60 m in height, act as “force multipliers,” with much of the damage to buildings and power lines associated not with direct wind damage but from the impact of falling trees. Strong winds, predominantly during major cyclones, account for 80% of regional tree mortality, rather than old age or disease ( Kirk and Franklin 1992 ). Heavy precipitation in the autumn, which saturates Northwest soils by mid-November, enhances the

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David M. Schultz

itself, such as those with the environment of the front. On the other hand, internal processes are those associated with the cold front itself. Because such processes may not be mutually exclusive, it may be possible that some mechanisms overlap, as discussed further in section 4a . Those mechanisms external to the front include synoptic-scale forcing interaction with lee troughs and drylines interaction with fronts in the mid- and upper troposphere frontogenesis associated with inhomogeneities in

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David M. Schultz and Philip N. Schumacher

implicitly, our interpretation of some of the other previous literature is that CSI is treated sometimes as an instability and sometimes as a forcing mechanism for ascent. An example is illustrated by those who wish to separate the effect of CSI from that due to frontogenesis, when in fact these two processes often cannot be considered independently (see section 4 ). The ingredients-based methodology firmly labels CSI as the instability, clearly separate from the lifting mechanism. Applying the

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Daniel Keyser and M. A. Shapiro

force. The significant role of rotation infronts distinguishes them from a number of relatedphenomena in which divergence is dominant, such asinternal gravity waves, gravity or density currents andsquall lines. Furthermore, lineal phenomena generatedprimarily through localized surface-based differentialheating rather than synoptic-scale deformation (e.g.,sea-breeze fronts) usually are excluded from dynamicalconsiderations of fronts. The time scale in which frontsform from "smooth" synoptic

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Paul M. Markowski

understood. b. Formation Fujita (1958a) originally attributed hook echo formation to the advection of precipitation from the rear of the main echo around the region of rotation associated with the tornado cyclone and updraft. Browning (1964, 1965b) also documented hook echoes and attributed their evolution ( Fig. 6 ) to essentially the same process described by Fujita (1958a) . Fujita (1965) later attributed hook echo formation to the Magnus force. He explained that this force pulled the spiraling

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John Molinari and Michael Dudek

models,the lack of observations makes it impossible to verifyany current closure. As a result, this fundamental aspectof mesoscale cumulus parameterizations remains onan ad hoc basis. The ambiguities arising from questionable scaleseparation assumptions and closure conditions maycause problems in practice as well. Frank (1983) notedthat when rotational constraints are weak, local divergent circulations may become dominant in mesoscalemodels. Grid-scale forcing may initiate parameterizedconvection

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Markus Gross, Hui Wan, Philip J. Rasch, Peter M. Caldwell, David L. Williamson, Daniel Klocke, Christiane Jablonowski, Diana R. Thatcher, Nigel Wood, Mike Cullen, Bob Beare, Martin Willett, Florian Lemarié, Eric Blayo, Sylvie Malardel, Piet Termonia, Almut Gassmann, Peter H. Lauritzen, Hans Johansen, Colin M. Zarzycki, Koichi Sakaguchi, and Ruby Leung

in which uncertain model parameters are adjusted to match the model output with a chosen set of observations ( Hourdin et al. 2017 ). One can argue that it might be possible to “tune away” the time step sensitivity by using different parameter values for different step sizes; however, there exists the danger that such tuning might result in error compensation that cannot be guaranteed for simulations under different forcing scenarios. Revision of the model and subsequent reduction of the time

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Dayton G. Vincent

significant portion of the forcing of low-level1956 MONTHLY WEATHER REVIEW ,VOLUME 1220 150E200 mb JANUARY ;~~_~ ~~ .... ~.7~m. . - " ~ I 180 I,~)OW 120W 90WL>ON~ON 0 lOS20S170E 180 170W 160 150 140W Tlme

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Tammy M. Weckwerth and David B. Parsons

variations of several grams per kilogram can be concentrated in a few kilometers. Prior to the project, drylines were believed to be the primary surface-based forcing mechanism in the region for new convection forming as synoptic features approached (e.g., Rhea 1966 ). The local topographic variations are small and generally not critical to the triggering of convection, although the Texas Caprock area does exhibit an increased frequency of convection. The region has a nocturnal precipitation maximum (e

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