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JOSEPH T. SCHAEFER

Abstract

The generalized Ekman equation is often used for micrometeorological applications. Use of the noniterative numerical technique of superposition reveals that the vertical variation of both the eddy diffusivity and the thermal wind is important to the determination of the wind profile when this equation is applicable.

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Caren Marzban and Joseph T. Schaefer

Abstract

The correlation between tornadic activity in several regions of the United States and the monthly mean sea surface temperature over four zones in the tropical Pacific Ocean is examined. Tornadic activity is gauged with two mostly independent measures: the number of tornadoes per month, and the number of tornadic days per month. Within the assumptions set forth for the analysis, it is found that there appears to exist a statistically significant but very weak correlation between sea surface temperature in the Pacific Ocean and tornadic activity in the United States, with the strength and significance of the correlation depending on the coordinates at which the sea surface temperatures are assessed and the geographic region of the United States. The strongest evidence found is for the correlation between the number of days with strong and violent (F2 and greater) tornadoes in an area that runs from Illinois to the Atlantic Coast, and Kentucky to Canada and a cool sea surface temperature in the central tropical Pacific. However, there is only about a 53% chance of this relationship occurring in a specific month.

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Charles A. Doswell III and Joseph T. Schaefer

Abstract

No abstract available.

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Joseph T. Schaefer and Charles A. Doswell III

Abstract

By its very nature, interpolation in a vector field is ambiguous, owing to the somewhat arbitrary nature of the vector norm. Since a two-dimensional vector field cm be specified by two scalar quantities. which can be separately interpolated, the ambiguity can be resolved by forcing the interpolated wind field to preserve the vorticity and divergence fields associated with the raw data. A method to calculate divergence and vorticity directly from randomly spaced wind observations is developed and, using analytically generated data, shown to produce more accurate results than conventional computations. Two methods of retrieving the wind field from the analysed scalar fields are presented and also tested on the analytic field. Finally, total analysis, from wind observations to gridded wind fields, is demonstrated on real meteorological data.

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Donald L. Kelly, Joseph T. Schaefer, and Charles A. Doswell III

Abstract

While the climatology of excessive rain and tornadoes is well-documented, little is known of storms that produce high winds or large hail. The characteristics of the approximately 75 000 severe thunderstorms which occurred in the United States from 1955 through 1983 are analyzed in an attempt to rectify this situation.

The distribution of over 29 000 storms causing hail larger than 19 mm shows marked diurnal, seasonal, and geographic preferences. These storms occur most frequently during the midafternoon hours of May and June in a zone running from central Texas to Nebraska. Spring storms tend to occur south of the Kansas-Nebraska border and summer storms north of it.

Thunderstorm winds which produce either “structural” damage or are reported as faster than 25.8 m s−1 generated about 46 000 reports. These storms typically occur during midafternoon in June and July. While the geographic distribution of violent windstorms is similar to that hailstorms, a zone of weaker severe thunderstorm gusts lies from northern Iowa to central Ohio. During May, windstorms are predominant across the plains area, but by August thew storms are indigenous only to the northern Midwest.

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Mathias Drton, Caren Marzban, Peter Guttorp, and Joseph T. Schaefer

Abstract

Tornadic activity in four U.S. regions is stochastically modeled based on data on tornado counts over the years 1953–98. It is shown that tornadic activity on a given day is mostly affected by the activity on the previous day. Hence, the process can be modeled as a Markov chain. A parametric nonhomogenous Markov chain model is developed based on the well-known increase of tornadic activity in the spring and summer months. This model, with only eight parameters, describes tornadic activity quite well. The interpretability of the estimated parameters allows a diagnosis of the regional differences in tornadic activity. For instance, a comparison of the values of the parameters for the four regions suggests that in the South tornado persistence is specific mostly to the early part of the year. Finally, within the framework of probabilistic forecast verification, it is shown that the Markov chain model outperforms the climatological model, even though the former is far simpler in terms of the number of parameters (8 and 366, respectively). The superior performance of the model is confirmed in terms of several measures of performance in all four regions. The exception is the southern Tornado Alley, where the reliability of the model forecasts is nonsignificantly inferior to that of the climatological ones.

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Edward W. Ferguson, Joseph T. Schaefer, Steven J. Weiss, Larry F. Wilson, and Frederick P. Ostby

Abstract

The tornado events of 1982 are reviewed. Significant and interesting aspects of the 1047 reported storms are noted. The synoptic patterns associated with four major tornado days are examined.

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