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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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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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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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Roland A. Madden and Paul R. Julian

convection moves from the Indian to the Pacific Ocean. This agrees with our conclusions. However, their work indicated that the singularrole of surface frictional stresses that we proposed isnot correct during northern winter. At that time, theyshowed that surface wind stresses over the central Pacific are nearly out-of-phase with AAM. If they werethe only driving force, the wind stresses would lead theAAM by 0.25 cycles. We computed the coherence andphase between AAM and the torque due to surfacewind

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Andrew Staniforth and Jean Côté

time levels (section 2c).b. Forced advection in multidimensions Consider the forced-advection problem dF -~- + G(x, t) = R(x, t), (6)where dF OF - + V(x, t). VF, (7) dt at dx dt V(x, t), (8)Here, x is the position vector (in 1-, 2- or 3D), V isthe gradient operator, and G and R are forcing terms

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