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WEATHER AND CIRCULATION OF MARCH 1980

Record Precipitation in the South and Central High Plains

Robert E. Livezey

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Robert E. Livezey

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Robert E. Livezey

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Evaluation of general circulation model (GCM) experiments presents one of the most challenging statistical inference problems in the study of climate. The problem is similar and comparable in difficulty to that encountered in empirical studies of global climate, because the data sets take the form of small samples of large numbers of cross-correlated climate statistics. Thus, in the absence of detailed a priori hypotheses the ability to detect all but the strongest of climate signals is severely limited.

Most studies directed at this problem have followed the lead of Chervin and Schneider and have emphasized parametric techniques to solve the univariate or “local” significance problem. Hasslemann was apparently the first to point out in the context of GCM problems that 1) a collection of “local” tests has dubious value in the absence of a “global” test, and 2) a sensitive global test is difficult to construct with multivariate methods without drastic a priori reduction in test dimensionality. Hasslemann's parametric strategy has subsequently guided a number of workers. Recently, in the vein of Mielke et al., Livezey and Chen, and Preisendorfer and Barnett, among others, have presented permutation or Monte Carlo approaches. These nonparametric methods obviate the need for limiting the choice of test statistics to those with known distributions.

Examples are presented of the kinds of questions that GCM climate experiments address and how they have been answered, together with considerations on significance testing in future experiments. The principal goals of the narrative are to provide a comprehensive, critical overview of the topic for the nonspecialist, and a compact, discriminating guide to the subject's extensive literature for the GCM climate experimenter.

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Robert E. Livezey

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WEATHER AND CIRCULATION OF MARCH 1981

Drought Worsens in the East and Northern Plains

Robert E. Livezey

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WEATHER AND CIRCULATION OF NOVEMBER 1981

Widespread Warmth with Storminess in the Far West

Robert E. Livezey

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WEATHER AND CIRCULATION OF NOVEMBER 1980

A Late Heat Wave and Hurricane and Early Snow

Robert E. Livezey

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Robert E. Livezey

The widely held belief that monthly and seasonal predictions, while containing some information, are not skillful enough to be of economic benefit, is reexamined through an extended example of winter seasonal temperature skill as it relates to the natural gas industry. A case is made that forecasts of mean conditions for periods as long as a season can be made with appreciable reliability for certain parameters, places, seasons, and situations, thereby making them potentially beneficial to certain users.

Several opportunities to improve the reliability of these forecasts over the next several years are described in the context of operational systems currently used to make predictions for the United States. Finally, the levels and types of research necessary to realize the potential benefits to users of skillful long-range forecasts are outlined. It is argued that it makes little sense, from a scientific or societal point of view, to neglect research on prediction of intraseasonal to interannual time scales (the long-range problem) in the face of growing concern and interest in climate fluctuations (such as global warming) on longer time scales.

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Robert E. Livezey and Richard Tinker

Various aspects of the severe heat wave that affected the midwestern and eastern United States in mid-July 1995 and led to hundreds of heat-related deaths are examined. First, the event is placed in historical context through examination of relatively long records at several affected sites. Next, the origins of both the strong high pressure cell and the unusually large moisture content of the air mass are traced. This is followed by a brief summary that concludes with the suggestion that longer-term processes played minor roles at best in the event. Finally, microclimatic factors in the Chicago metropolitan area are considered for their role in exacerbating conditions in the city most severely affected by the heat wave.

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Robert R. Dickson and Robert E. Livezey

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It is known that the Southern Oscillation Index (SOI) and the mean sea surface temperature off the Peru Coast are highly coherent and that variations of the latter are dominated by infrequent warming episodes. The present study examines the relative contribution of these warming episodes to the covariance of statistically significant correlations between the fall SOI and winter mean 700 mb heights in the Northern Hemisphere. The degree of dominance of the warming episode years in this context is evaluated by Monte Carlo methods.

It was found that, for the 30-year period studied, data pairs following tropical east Pacific warming events contributed disproportionately to major correlation maxima in much of the Northern Hemisphere. Such covariance concentrations, however, were found to be fairly likely outcomes (probability > 9%) if groups of years are chosen at random from the appropriate covariance arrays. Thus, we conclude that the influence of the fall SOI upon the subsequent winter mean 700 mb height distribution is a rather pervasive one, not limited to tropical east Pacific warming situations.

In contrast to other areas, correlation maxima in the North American sector received disproportionately small covariance contributions from the warming episode years. In northwest Canada, the contribution of those years was small and opposite in sign to the total covariance.

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