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Joseph T. Schaefer

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Joseph T. Schaefer

Abstract

Nonlinear biconstituent diffusion is proposed as a possible trigger for cumulus development. Properties of this second-order effect are examined. It is shown that this process can exist in the atmosphere and that it can produce sustained convective cells of appreciable magnitude. One possible atmospheric example is illustrated.

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Joseph T. Schaefer

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Joseph T. Schaefer

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Techniques that have evolved during the hundred years that scientific severe thunderstorm forecasts have been prepared are reviewed. The early empirical rules developed by Finley, Showalter and Fulks, Fawbush, Miller and Starrett, and others have been corroborated by more recent theoretical work. While significant efforts have been devoted to defining the severe thunderstorm environment, it is now obvious that these storms can occur under a variety of synoptic conditions. Severe thunderstorm forecasting consists in not only identifying the time and place that an environment compatible with such storms will exist but also in identifying suitable triggering mechanisms in that environment.

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JOSEPH T. SCHAEFER

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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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Joseph T. Schaefer

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The dryline, a narrow non-frontal zone of sharp moisture discontinuity, has long been known as a preferential location of thunderstorm development. Through an examination of several years of data, a conceptual model of the dryline life cycle is developed.

The dryline originates along the trailing edge of a continental air mass and is coincident with an old frontal surface. As it moves, it is located on the surface projection of the western edge of the low-level inversion. The dryline is destroyed either by a new cold air outbreak or by becoming too diffuse to be easily recognizable.

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Joseph T. Schaefer

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A form of the critical success index (CSI) is used by the National Weather Service to indicate the value of warnings. This verification statistic assumes that the times when an event was neither expected nor observed are of no consequence. It can be shown that the CSI is not an unbiased indicator of forecast skill but is proportional to the frequency of the event being forecast. This innate bias is demonstrated theoretically and via example. An unbiased verification statistic appropriate for forecast of rare events is presented and applied to severe convective weather warnings. Comparisons of this score to the CSI show the extent of the penalty the CSI extracts from forecasters who work in areas that are not climatically prone to given events.

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Joseph T. Schaefer

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A dryline is a narrow zone, other than a classical polar front, across which a sharp horizontal surface moisture gradient occurs. It often exists and moves under synoptically quiescent conditions. One explanation of dryline motion is that it is caused by turbulent vertical mixing. A slab symmetric numerical model of the boundary layer is developed to test this hypothesis. This model, when applied to a typical dryline environment, produces dryline motions similar to those observed in the atmosphere. The model is applied to several real dryline cases, and a close correspondence between simulated and observed motions are found. It is concluded that vertical mixing is the primary cause of dryline motion.

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Caren Marzban and Joseph T. Schaefer

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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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Joseph T. Schaefer and Richard L. Livingston

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Since the probability of precipitation (PoP) appears in the forecasts of the National Weather Service (NWS), it is important that both the forecaster and user fully grasp the subtleties of the meaning of PoP. A brief review of the theory of PoP is presented. It is shown that although the PoP is defined as the average point probability, the guidelines outlined in the National Weather Service Operations Manual (NWSOM) also make the PoP the expected areal coverage of precipitation across the forecast area.

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