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Nathaniel B. Guttman, J. R. M. Hosking, and James R. Wallis

Abstract

Precipitation quantile values have been computed for 9 probabilities, 8 durations, 12 starting months, and 1 1 1 regions across the United States. L-moment methodology has been used for the calculations. Discussed are the rationale for selecting the Pearson type III (gamma) and Wakeby distributions, and the confidence that can be placed in the quantile values. Results show that distribution functions become more asymmetrical as the duration decreases, indicating that the median may be a better measure of central tendency than the mean. Portraying the quantile values as a percentage of the median value leads to smooth spatial fields.

Computation of quantile values was the first known large-scale application of L-moment methodology. In spite of the complexity of the techniques and the extensive use of personnel and computer resources, the results justify the procedures in terms of preparing easy to use probability statements that reflect underlying physical processes.

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Nathaniel B. Guttman, J. R. M. Hosking, and James R. Wallis

Extreme rainfall amounts that resulted in severe flooding during the spring and summer of 1993 along the Missouri and Mississippi Rivers are examined from a historical and probabilistic viewpoint. Long-term average precipitation amounts and the departures of the 1993 summer rainfall from these averages are presented. Also, climatic regionalization and precipitation probabilities developed for the National Drought Atlas using L-moment techniques have been applied to the drainage area that contributed to the flooding. The exceedance probabilities of monthly and multiple-month observed precipitation amounts have been calculated. The results show that the three-month period May–July experienced unusually heavy rainfall when compared to prior years, and that July was particularly wet. Recurrence intervals for the rainfall events vary widely depending on the specific time period and locality, but the observed precipitation was an extreme event.

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M. N. Raphael, G. J. Marshall, J. Turner, R. L. Fogt, D. Schneider, D. A. Dixon, J. S. Hosking, J. M. Jones, and W. R. Hobbs

Abstract

The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases.

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