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Last modified
7/28/2009 2:29:19 PM
Creation date
7/18/2007 2:18:33 PM
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Weather Modification
Title
Applied Weather Associates Responses to Corps of Engineers Questions on the AWS Study of Cherry Creek PMP, Sept 2006
Prepared For
USACE
Prepared By
Applied Weather Associates
Date
1/29/2007
County
Douglas
Weather Modification - Doc Type
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<br />-r:r .'ib,. <br /> <br />13, Page 79, Why was the CCC isohyetal analysis not used for the 1997 For Collins storm? <br /> <br />A W A response to Question13. <br /> <br />The CCC did an excellent job of collecting bucket survey rainfall observations for this storm. <br />The CCC subjectively included NEXRAD spatial data in producing their hand drawn isohyetal <br />pattern, A W A recognizes the value of using NEXRAD data to determine the spatial distribution <br />of rainfall and has incorporated its use in the Storm Precipitation Analysis System (SPAS). In <br />general, our experience has shown that using NEXRAD, the area sizes associated with storm <br />isohyetals tends to be smaller since without NEXRAD, either smoothed base maps or some <br />mathematical algorithm (e.g. minimum curvature) are used which tend to increase areas of <br />extreme rainfall. A W A made the decision to use the GIS derived isohyetal pattern primarily <br />because it showed larger areas of extreme rainfall than the CCC isohyetal analysis and hence <br />provided larger rainfall volumes. This provided an increased degree of conservatism in the <br />rainfall analysis. The rainfall associated with the 1997 Ft Collins storm using this conservative <br />approach turned out to be significantly less than other storms used in this study (see Figure 10.2, <br />page 124), The CCC isohyetal pattern could have been digitized and used in the study but would <br />not have influenced the results of the study, <br /> <br />14. Page 82. Why was the CCC isohyetal analysis not used for the 1997 Pawnee Creek storm? <br /> <br />A W A response to Question 14. <br /> <br />This response is effectively the same as for question 13. <br /> <br />The CCC did an excellent job of collecting bucket survey rainfall observations for this storm, <br />The CCC subjectively included NEXRAD spatial data in producing their hand drawn isohyetal <br />pattern. A W A recognizes the value of using NEXRAD data to determine the spatial distribution <br />of rainfall and has incorporated its use in the Storm Precipitation Analysis System (SPAS). In <br />general, our experience has shown that using NEXRAD, the area sizes associated with storm <br />isohyetals tends to be smaller since without NEXRAD, either smoothed base maps or some <br />mathematical algorithm (e.g. minimum curvature) are used which tend to increase areas of <br />extreme rainfall, A W A made the decision to use the GIS derived isohyetal pattern primarily <br />because it showed larger areas of extreme rainfall than the CCC isohyetal analysis and hence <br />provided larger rainfall volumes. This provided an increased degree of conservatism in the <br />rainfall analysis. The rainfall associated with the 1997 Pawnee Creek storm using this <br />conservative approach turned out to be significantly less than other storms used in this study (see <br />Figure 10.2, page 124). The CCC isohyetal pattern could have been digitized and used in the <br />study but would not have influenced the results of the study. <br /> <br />15, Page 100. How was the conclusion of "not physically possible" reached based on the sub- <br />cloud layer winds? Note: 1 st sentence indicates thunderstorm produced flash flood east of the <br />continental divide in Colorado have sub-cloud layer winds southerly to easterly. Page 99 <br />indicates that sub-cloud layer winds feeding the NWS SSS would be southeasterly to easterly, <br />
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