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<br />5 <br /> <br />California, Hydrometeorological Report No. 36 (U.S. Weather Bureau 1961). <br />Hereafter they will be referred to as HMR No. 43 and HMR No. 36, respectively. <br /> <br />2. CONVERGENCE COMPONENT OF PMP <br /> <br />2.1 Introduction <br /> <br />2.1.1 Method of Determining General-Storm PMP <br /> <br />We noted in chapter 1 that the method for determining general-storm PMP in <br />this study was to make separate estimates of orographic and nonorographic PMP; <br />to judge the regional, seasonal, depth-area, and depth-duration variations <br />of each component; and then to add the components for an estimate of total <br />PMP. This method is .comparable to that used for general-storm PMP estimates <br />to the west and north (HMR No. 36 and No. 43). Development of nonorographic <br />PMP, or convergence PMP, is the subject of this chapter. <br /> <br />2.1.2 Definition of Convergence PMP <br /> <br />Nonorographic precipitation can be defined as precipitation resulting from <br />atmospheric processes not affected by terrain. Lifting and therefore cooling <br />of moist air are necessary for major precipitation. Lifting or vertical <br />motion can be produced by horizontal convergence of air at lower levels; <br />hence, the term "convergence" for nonorographic precipitation. Under this <br />definition all precipitation in regions with no abrupt changes in elevation <br />is classified as convergence. Convergence and orographic precipitation can <br />occur simultaneously. <br /> <br />2.1.3 General Storm Relation to Local Storm <br /> <br />In the United States east of approximately the 105th meridian, many extreme <br />small area rainfalls have occurred within longer storm periods in which gen- <br />eral rains cover larger areas. In contrast, experience has shown that the <br />greatest short-duration rainfalls over small areas in the intermountain <br />region come from intense local storms (thunderstorms) as opposed to general- <br />storm situations. For the Southwestern States, therefore, separate estimates <br />of local-storm PMP are given in chapter 4. While most extreme point rain- <br />falls of record in the Southwest States have been isolated with regard to <br />space and time, this does not negate the occurrence of lesser thunderstorm <br />rains imbedded in the general PMP storm prototype. The point to be empha- <br />sized is that the local thunderstorm, the greatest potential rainfall threat <br />for small areas and short durations, is an isolated event in time and space <br />in the Southwestern States, while less intense thunderstorm occurring within <br />general-storm rains are the key for general-storm convergence PMP. <br />