LARGE-SCALE PRECIPITATION VOLUMES, GRADIENTS, AND DISTRIBUTION

Abstract

During the past two years, the Quantitative Precipitation Forecast Unit of the National Meteorological Center has collected, as a by-product of its verification program, a large quantity of data relating to observed precipitation amounts. A technique is developed to process these data into precipitation volumes for varying time periods. The technique is simple and provides a fast method of obtaining large-scale precipitation volumes on a day-to-day basis. Similarly, monthly volumes can be easily estimated. The volumes for a number of larger precipitation storms for periods of one to five days are presented, as well as monthly and annual volumes. An attempt was made to determine a normal isohyetal gradient for individual storms but was successful only with amounts exceeding 3 inches. The volumetric contribution by isohyetal intervals is examined and only in cases of the heaviest storms were the larger amounts found to be important to total volume. The distribution of heavy precipitation, as portrayed by isopleths of the number of times areas were enclosed by the 1 inch isohyet, is given for each month and for the year. Although the distribution is not too different from that which might be expected, the authors know of no other source for this information. An addendum permits a comparison of the 1961 and 1962 monthly and annual precipitation volumes. The significant decrease in the 1962 volume emphasizes the deficient rainfall reported over much of the Nation.