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D. R. DAVIS and C. E. DEAN

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D. R. DAVIS and W. C. BRIDGES

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A weak tropical depression moved out of the Gulf of Mexico on Sept. 19–20, 1969. With the blocking action of a surface High and in the absence of steering currents aloft, the Low became stationary on the Florida coast for approximately 48 hr. Torrential rains occurred in a small area 60–65 mi to the east and 50 mi inland from the point where the Low made landfall. Record-breaking floods resulted. The 23-in. maximum point rainfall was about 9 in. greater than the previous maximum rainfall of record produced by a 1924 tropical storm in the same area. The location of the area of maximum rainfall with respect to the point of landfall of the Low's center closely follows the pattern previously reported for the more intense hurricanes and tropical storms.

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S. B. Trier, C. A. Davis, and W. C. Skamarock

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Idealized numerical simulations are used to quantify the effect of quasi-balanced lifting arising from the interaction of the ambient vertical shear with midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems on thermodynamic destabilization over a range of ambient vertical shear strengths and vortex characteristics observed in Part I. Maximum upward displacements occur beneath the midtropospheric potential vorticity anomaly, near the radius of maximum tangential vortex winds. The location of the region of upward displacements relative to the ambient vertical shear vector depends on the relative strength of the vortex tangential flow and the ambient vertical shear, and ranges from downshear for vortices of moderate strength in strong ambient vertical shear to 90° to the left of downshear for strong vortices in weak ambient vertical shear. Although significant upward displacements occur most rapidly with small vortices in strong ambient vertical shear, maximum upward displacements are associated with large vortices and occur in approximately average vertical shear for MCV environments.

The simulations suggest that in larger and stronger than average MCVs, the lifting that results from the MCV being embedded in a weakly baroclinic environment is, alone, sufficient to saturate initially moist and conditionally unstable layers immediately above the boundary layer. The horizontal location of the resulting thermodynamic instability is approximately coincident with the maximum lower-tropospheric upward displacements. Since in the absence of sustained deep convection the vortices develop substantial vertical tilt, the destabilized region in the lower troposphere lies nearly underneath the vortex center at its level of maximum strength, consistent with observations that redevelopment of organized, long-lived (e.g., t ≥ 6 h) deep convection is most often found near the midtropospheric MCV center. This location for convectively induced stretching of preexisting vertical vorticity is optimal for maintaining the vortex against the deleterious effect of differential advection by the ambient shear.

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David C. Smith IV and George P. Davis Jr.

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Gulf Stream rings exhibit a number of propagation tendencies after their release from the current. Among those tendencies identified by observational, analytic, and numerical techniques are β-induced westward self-propagation, advection by the mean flow, and eddy–jet interaction. In this study, we numerically examine eddy propagation tendencies associated with the latter in a two-layer fluid. Results indicate that cyclones (anticyclones) can interact with the anticyclonic (cyclonic) shear of the jet. Two eddy response are seen: meridional eddy motion away from the jet when the initial eddy–jet separation is small and westward motion parallel to the jet for larger eddy–jet separation distance. The results also indicate circumstances that may be necessary for eddy–jet coalescence.

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C. G. Davis, S. S. Bunker, and J. P. Mutschlecner

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Two codes, ATMOS1 and ATMOS2, have been developed to model respectively, wind fields and pollutant transport in three dimensions with particular application to complex terrain. ATMOS1 models wind fields with the use of a mass conservation error minimization principle employing available observations. ATMOS2 employs these wind fields to model transport by advection and diffusion using a Crowley second-order flux estimation. Both codes employ terrain-following vertical coordinates which provide critical resolution of the boundary layer and simplify boundary conditions at the surface. A companion paper provides three examples of models produced by the codes with comparison to observation.

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Daran L. Rife, Christopher A. Davis, and Jason C. Knievel

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The study describes a method of evaluating numerical weather prediction models by comparing the characteristics of temporal changes in simulated and observed 10-m (AGL) winds. The method is demonstrated on a 1-yr collection of 1-day simulations by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) over southern New Mexico. Temporal objects, or wind events, are defined at the observation locations and at each grid point in the model domain as vector wind changes over 2 h. Changes above the uppermost quartile of the distributions in the observations and simulations are empirically classified as significant; their attributes are analyzed and interpreted.

It is demonstrated that the model can discriminate between large and modest wind changes on a pointwise basis, suggesting that many forecast events have an observational counterpart. Spatial clusters of significant wind events are highly continuous in space and time. Such continuity suggests that displaying maps of surface wind changes with high temporal resolution can alert forecasters to the occurrence of important phenomena. Documented systematic errors in the amplitude, direction, and timing of wind events will allow forecasters to mentally adjust for biases in features forecast by the model.

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William C. Ackermann, Stanley A. Changnon Jr., and Ray Jay Davis

The Illinois State Water Survey, a state water resources research agency, initiated efforts in 1971 to develop and secure a law for Illinois that would permit and regulate weather modification activities. Such legislation was deemed a prime requirement, not only for the proper execution of scientific experiments on weather modification in Illinois but for the general benefit of citizens of Illinois through encouragement to properly conducted activities and protection from improperly conducted weather modification operations. (It was our intention to develop a “model law” that reflected the best aspects of weather modification legislation and experience in other states, and which would serve as a model for future legislation in other states.) The efforts began in October 1971 and were completed in September 1973 with the signing of the Illinois Weather Modification Control Bill and its accompanying appropriation bill. This paper describes the type of law desired, the activities performed to secure the law, and the primary aspects of the enacted Illinois law.

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Robert C. Mcarthur, James R. Davis, and David Reynolds

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The purpose of this paper is to illustrate how the construction of a knowledge-based system (KBS) to support nowcasting, can be used to guide and facilitate the development of objective pattern recognition algorithms for use with meteorological data. We believe that a KBS based on the semantic interpretation of weather data, using the concept of weather scenarios, can assist the development and use of objective algorithms for pattern recognition in two ways:

1) it focuses the development of pattern recognition algorithms on only those phenomena which are most useful to operational forecasters;

2) its top-down logic constrains when, where, and how objective algorithms should be applied.

We first describe our understanding of nowcasting expertise and the use of pattern recognition (“manual”) by human forecasters. We then briefly review the current use of automatic pattern recognition in nowcasting, present the elements within a scenario and discuss a KBS architecture for using scenarios. Finally, we close by discussing the practical benefits of merging a qualitative KBS with algorithmic pattern recognition techniques.

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Russ Davis, T. P. Barnett, and C. S. Cox

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Variability of near-surface Currents over a 20-Day period in a 15O km diameter region of the central North Pacific is described using vertical profiles from a current meter and the tracks of 25 drifting buoys. Energetic fluctuations of order 0.10 m s−1 having time scales of a few days and vertical scales in excess of 100 m were found, apparently coherent with the wind forcing. Buoy tracks disclose a small-scale (<15 km) short-period (less than a few days) variability with speeds of the order 0.05 m s−1 and an energetic mesoscale motion with speeds of the order 0.07 m s−1, space scales of the order 40 km and time scales exceeding 20 days. Additionally, the difference between the mean current observed over the experiment. having a speed of aboutO.03 m s−1, and the climatological norm inferred from ship-drift. with a speed of about 0.10 m s−1, suggests a larger scale variability not adequately resolved.

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Christopher Davis, Chris Snyder, and Anthony C. Didlake Jr.

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Tropical cyclone formation over the eastern Pacific during 2005 and 2006 was examined using primarily global operational analyses from the National Centers for Environmental Prediction. This paper represents a “vortex view” of genesis, adding to previous work on tropical cyclone formation associated with tropical waves. Between 1 July and 30 September during 2005 and 2006, vortices at 900 hPa were tracked and vortex-following diagnostic quantities were computed. Vortices were more abundant during periods of an enhanced “Hadley” circulation with monsoon westerlies around 10°N in the lower troposphere. This zonally confined Hadley circulation was significantly stronger during the genesis of developing vortices. Developing vortices were stronger at the outset, with a deeper potential vorticity maximum, compared to nondeveloping vortices. This implies that developing disturbances were selected early on by favorable synoptic-scale features.

The characteristic time-mean reversal of the meridional gradient of absolute vorticity in the lower troposphere was found to nearly vanish when the aggregate contribution of strong vortices was removed from the time-mean vorticity. This finding implies that it is difficult to unambiguously attribute development to a preexisting enhancement of vorticity on the synoptic scale. The time-mean enhancement of cyclonic vorticity primarily results from the accumulated effect of vortices. It is suggested that horizontal deformation in the background state helps distinguish developing vortices from nondevelopers, and also biases the latitude of development poleward of the climatological ITCZ axis.

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