Nathaniel B. Guttman

A Historical Perspective of U.S. Climate Divisions

The history of climatic divisions in the contiguous United States has been pieced together from fragmentary documentation. Each of the 48 contiguous states has been subdivided into climatic divisions. Divisional boundaries are now standardized, and a set of climatic variables for time-invariant divisional boundaries has been compiled for the period of record beginning in 1895. This paper documents the origins of climatic divisions, the computational methodology of an area-invariant divisional dataset maintained by the National Climatic Data Center, and the strengths and weaknesses of divisional data.

The Use of L-Moments in the Determination of Regional Precipitation Climates

Abstract As part of a national study of water management during periods of drought, the U.S. Army Corps of Engineers is underwriting the preparation of a national drought atlas. One of the variables being analyzed for the atlas is precipitation. A statistical technique known as L-moments is the basis for the analysis. Central to the L-moment technique is the aggregation of site-specific precipitation data into homogeneous regions. This paper concerns a methodology for defining regions of similar precipitation climates that are homogeneous with respect to the statistical distribution of annual precipitation. Included are a discussion of the data, of the necessity for regionalization, and of the iterative use of clustering and an L-moment-based homogeneity test to determine the regions. The methodology resulted in 104 precipitation regions within the continental United States. The number of stations in each region varied from 1 to 97. Problems were encountered mainly in mountainous and in and areas. They we...

Regional Precipitation Quantile Values for the Continental United States Computed from L-Moments

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.

On the Sensitivity of Sample L Moments to Sample Size

Abstract Parametric probability distributions can be fit to a dataset by equating sample L moments to those Of the fitted distribution. This study examines the mean and mean squared departures of sample L moments of monthly precipitation data from large sample values as sample size increases. Mean departures decrease as the sample size increases with values near zero generally occurring with about 30 to 40 or more observations for the central tendency measure, about 40 to 50 or more for the dispersion measure, and about 60 or 70 for the skewness and kurtosis measures. It was also found that the root-mean-square departures appear to decrease linearly with the square root of the sample size. The results are intended to provide guidance for determining sample sizes when applying at-site L moments to monthly precipitation data.

National drought atlas developed

A National Drought Atlas for the United States is nearing completion. The idea for the atlas was inspired by the severe droughts that plagued parts of the country from 1987 to 1989, covering much of the Great Plains states, the Ohio Valley, and the southeast. In California, the drought lasted 6 years. In general, water managers were caught off-guard by the droughts severity and duration. The atlas is the first nationwide study of drought frequency that planning groups can use to answer a large number of “what-if” questions. They can determine the most likely durations of critical drought, when such droughts are likely to occur, how much drier than “normal” a given drought may be, and what droughts have occurred in a given region.

The 1993 Midwest Extreme Precipitation in Historical and Probabilistic Perspective

Abstract 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.

Heavy winter precipitation in southwest Arizona

During December 1992, according to the Weekly Climate Bulletin of the Climate Analysis Center in Washington, D.C., heavy precipitation inundated parts of Arizona causing more than 400% of normal precipitation to fall in the southwestern part of the state. Heavy precipitation continued to fall during the next 2 months, causing extensive flooding along the Gila River. Phoenix Weather Service Forecast Office monthly storm data reports indicated flooding along the Santa Cruz and San Pedro Rivers on December 29. From January 7 to 20, roads, bridges, homes, businesses, and farmland suffered considerable flood damage from Graham County westward to Yuma County as rivers and streams swelled. Several thousand people were isolated in their homes as flood waters cut off roads. The January storm data report shows that the combination of a northward-displaced subtropical jet stream, with its abundant moisture supply and associated low pressure disturbances and a southward-displaced polar jet stream, with its storm track, led to the abnormally wet period from late December to mid-January. In February, severe flooding was reported in several areas as water rose in the Painted Rock Reservoir; water accumulating behind the dam produced the largest lake in the state. After exceeding the 2.5 million acre-feet capacity of the reservoir, water began spilling over the dam and damaging homes, crops, farmland, roads, and bridges. About 3,500 residents were evacuated, and the National Guard responded to the flooding with various relief efforts including helicopter support operations. The U.S. and Arizona Departments of Agriculture reported flood damage in excess of

COMPARING THE PALMER DROUGHT INDEX AND THE STANDARDIZED PRECIPITATION INDEX

50 million.