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DECEMBER 19~ 1 SILVERMAN, ROGERS AND DAHL 1469 point measurements of precipitation but are theo- retically generated by a continuous spatial distri- bution function so that the true precipitation char- acteristics are known quantities. 2. Approach Storm characteristics such as type, structure, du- ration, and speed and direction of movement affect the size, shape, and precipitation gradient of the isohyetal pattern it produces. The distribution of sur- face rainfall will vary greatly between storms of sim- ilar area and duration, particularly for convective storms in which cells d,evelop and coexist at various times and locations during the storm's lifetime (Crane, 1979). The isohyetal pattern of moving con- vective storms, therefore, will frequently contain multiple peaks and will rarely be regular iri shape. An example of a convective storm isohyetal pattern is shown in Fig. 1. The complexity and variability of convective storm isohyetal patterns make it difficult to model the en- tire storm pattern in a general manner. However, we can consider that the isohyetal pattern of the storm is composed of smaller isolated precipitation areas which are characterized by closed isohyets and model these components individually. Therefore, we adapt the methodology of Schickedanz (1974) and, for the purpose of this analysis, use the surface raincell which he states is the smallest definable rain-pro- ducing entity of a storm and shows that it generally reflects conditions within the convective cells aloft. Schickedanz (1974) defined a raincell in the follow- ing manner: "A raincell in a multicellular system is a closed isohyetal entity within the overall enveloping isohyet of a rain-producing system; that is, it de- scribes an isolated area of significantly greater in- tensity than the system-enveloping isohyetal. When raincells develop apart from a multicellular storm system, the system-enveloping isohyet will not be present, and the single cell is uniquely defined by the separation between rain and no rain." He defined a storm as an entity of rain, one or more raincells and/ or areas of rain, that can be identified with a specific synoptic weather classification and is separated from other entities by 32 km and/or 1 h between rain end and start times. According to these definitions, it can be seen from Fig. 1 that the illustrated storm is com- posed of three raincells. The present analysis of the sampling variance of raingage networks will be based on the raincell. The gage density that is required to quantitatively resolve the smallest size raincell that contributes meaning- fully to the total storm rainfall also determines the upper bound of gage density (densest) that is re- quired to resolve the entire storm with at least the same accuracy. Local random variability may also contribute to I. FIG, 1. An example of an isohyetal pattern of total rainfall for a Montana convective storm illustrating its raincell structure, Isohyets are in mm, the sampling variance of raingage networks. Storm and raincell isohyets are generally drawn fairly smoothly by precipitation data analysts. However, if isohyets were drawn to rigorously fit the point rain- fall data, they would be quite irregular. How much of this irregularity is due to measurement-related errors (e.g., evaporation, gage inclination, splash, reading, exposure and wind effects) and how much is due to local, storm-related random variability is not known. Of all the measurement-related errors, exposure and wind have the greatest effect on gage catch, the others being estimated by Kurtyka (1953)2 to be an average of 1.5%. Larson and Peck (1974) have shown that the effect of wind speed on gage catch increases with increasing wind speed. The average gage catch deficiency for rain is 12% at 4 m S-I, increasing to 18% at 8 m S-I. These errors can be reduced, but not eliminated, by the use of gage shields and proper site selection. Woodley et al. (1975) examined the ac- 2 Kurtyka, J, C" 1953: Precipitation measurements study. Rep, of Investigation No, 20, Illinois State Water Survey Division, 178 pp.