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Gary P. Ellrod and David I. Knapp

Abstract

An objective technique for forecasting clear-air turbulence (CAT) is described. An index is calculated based on the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft. The forecast technique has been evaluated and is now in operational use at two forecast centers with international aviation responsibilities: the National Meteorological Center (NMC) in Washington, D.C., and the Air Force Global Weather Central (AFGWC) in Omaha, Nebraska. The index is also an operational forecast tool at the Canadian Atmospheric Environment Service (AES), and the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri, both responsible for domestic aviation forecasts. The AFGWC index also includes horizontal convergence in its calculation. Thresholds were selected empirically by comparing index values with the location and intensity of observed CAT. Verification indicates that the index is quite reliable. The probability of detection (POD) varied from 70%–84%. False-alarm ratios (FAR) ranged from a low of 22% for the NMC aviation model to more than 40% for the AFGWC global model. An average threat score of 0.17 was calculated for the aviation model 24-h forecast. The operational capabilities of the NMC and AFGWC indices are compared in two CAT episodes that differ in synoptic-scale conditions and times of the year.

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Steven Businger, David I. Knapp, and Gerald F. Watson

Abstract

A storm-following climatology was compiled for the precipitation distributions associated with winter cyclones that originate over the Gulf of Mexico and adjacent coastal region. The goal of this research is to investigate the roles of the Gulf of Mexico and Atlantic Ocean as sources of moisture for these storms, and to investigate geographic/orographic influences on the precipitation distributions. A second objective of this research is to provide forecasters with a potential guide with which to evaluate numerical model forecasts of quantitative precipitation for these storms. A 24-y climatology (1960–1983) was compiled of storms that originated over the Gulf of Mexico and adjacent coastal region, and produced wide-spread areas of precipitation (total ≥ 25 mm). Sixty-six storms satisfied these criteria, and three dominant storm tracks were identified. Six-h totals of hourly precipitation data were objectively analysed for individual storm belonging to each of the three tracks, and grid-point values were composited in a storm-following coordinate system. Charts of mean precipitation distributions and frequency of occurrence were constructed to display the evolving precipitation fields surrounding storms belonging to each track. The resulting climatology is presented.

To provide an example of the application of the precipitation climatology, results from a GALE Case study am presented.

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