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<br />Additionally, it will present a detailed reconstitution of the rainfall on July <br />1ih and contrast it to the rainfall associated with several additional storm events <br />which occurred over the Buffalo Creek basin during the post-fire period of 1996, <br />Finally, the report will present information on the meteorological causes of the <br />primary July 1 ih flash flood event and contrast it to the causes of the other <br />lessor events which occurred. <br /> <br />2.0 Use of radar to describo storm rainfall <br /> <br />The ability to use radar to obtain a map of current rainfall has been <br />pursued for over 30 years by engineers, meteorologists and hydrologists, in <br />general, most current radar-rainfall techniques rely an assumed relationship <br />between the strength of the radar reflectivity and the intensity of the rainfall rate, <br />This relationship is described by the equation below: <br /> <br />(1) <br /> <br />Z = A Rb <br /> <br />where, Z is the radar reflectivity, R is the rainfall rate. A is an empirically derived <br />co-efficient related to the cloud physics of the storm cloud water droplets and b <br />is another empirical co-efficient related to the type of storm cloud present This <br />relationship has proven to produce highly variable results. Since the values of <br />both A and b must be assumed to allOw the equation to be solved correctly, <br />many opportunities for errors in the calculation are possible. <br /> <br />The algorithms used to estimate the rainfall are standard for use around <br />the country and have not proven to be responsive to local cloud variations, The r- <br />squared or "goodness" correlation factor of the rain to radar reflectivity statistical <br />relationship has varied from 0.15, to 0.90 on a daily basis and for most storm <br />seasons has been about 0.60. The good "r's" (values >0.7!5) have been <br />achieved for the low volume and low intensity rain events, ~Ienerally those of less <br />than O,25"/hr accumulation rates. The high intensity, high volume, "front-end <br />dumper" thunderstorms have produced r-values of 0,15 to OA5. Thus the <br />standard products appear to be unreliable at this point. The storm rainfall has <br />been both overestimated and underestimated for periods of less than three <br />hours for storms within 25 miles of each other. <br /> <br />Finally, hail "pollution" of the equation has proven to be a troublesome <br />problem. The strength of the radar return signal is related objectively to the <br />diameter of the rain droplet size. The strong radar return si!~nal produced by <br />wet hail stones frequently causes an over-estimation of the rainfall rate, <br />Attempts have been made to reduce this over-estimation by adjusting the <br />coefficients A and b in Equation 'I. A satisfactory solution to this problem <br />continues to prove quite elusive. <br /> <br />4 <br />