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Page 12 WATER TALK Summer/Fall 1995 <br />r- aced reciitati <br />ti ate <br />Greg Spoden, State CI(matologist <br />The accurate measurement of rainfall and <br />snowfall has long been. a challenge for <br />hydrology. agriculture. and many other <br />disciplines that require a full. accounting of <br />precipitation patterns over space and time. <br />Minnesota is fortunate to have one. of the <br />highest density precipitation monitoring <br />networks iri the country. However, difficulty <br />often arises when attempting to extrapolate <br />these point estimates to broader areas. <br />Fortunately, an important tool for augmenting <br />rain gauge data is now available. <br />The National Weather Service is nearing the <br />final stages of a radar modernization effort. The <br />new generation of radar has greater resolution <br />and is more sensitive than the older radar <br />technology. Also, improved computing <br />capabilities and signal. processing techniques <br />dramatically improves. the quantity and quality <br />of the information derived from the radars. One <br />of the more exciting components of this new <br />information flow is radar-based precipitation <br />estimation. <br />With weather radar, objects such. as rain, <br />snow, drizzle, and hail reflect a transmitted <br />beam back to a receiver. The strength of the <br />reflected beam and the time lapsed to "echo" <br />back to the radar site identifies the intensity <br />and position of a precipitation event. (The <br />modern Doppler radars can also derive the <br />radial velocity of objects moving towards or <br />away from the radar site.) The intensity level of <br />the "echo" is proportional to the rainfall./ <br />snowfall rate. Therefore, for a given location,. a <br />known precipitation rate spanning a known <br />amount of time can lead to an estimated <br />precipitation total. <br />thunderstorm), the curvature of the earth, and <br />hail "contamination" are problem sources as <br />well. However, computing power can overcome <br />many of these difllculties. Computers are able <br />to "mosaic"radar data from multiple sites. <br />Radar sites covering Minnesota will eventually <br />include: Chanhassen, LaCrosse, Duluth, Sioux <br />Falls, and Mayville (between Fargo and. Grand <br />Forks). Amulti-radar composite aids the task of <br />filtering the data of false echoes and overcomes <br />probrems due to attenuation and the earth's <br />curvature. Meteorologists quality control the <br />data manually as well, looking for hail cells and <br />other "anomalous propagation." The final <br />results are radar-based precipitation estimates <br />for nearly any time interval of choice, at a <br />resolution of 2 x 2 kilometers (about the size of <br />a section of land). <br />'The new technology does not mean that rain <br />gauges will no longer be needed. Even with <br />powerful new radars and signal processing <br />capabilities,, there remains a significant error <br />component. In fact, the effort to identify and <br />quantify these errors wffl require a solid <br />ground-based data collection network.. <br />Nonetheless, the radar technology as it stands <br />today is akeady providing the Division of Waters <br />with spatial detail of precipitation patterns <br />previously unattainable in near real-time. A <br />private-sector vendor of value-added National <br />Weather Service radar data provides imagery via <br />the INTERNET at no charge. Using recently <br />acquired connections to the INTERNET, DOW <br />employees are able to access the radar-based <br />precipitation estimates with the click of a <br />button. <br />(INTERNET users interested in browsing for the <br />radar-based precipitation estimates should see <br />the World Wide Web home page: http://www. <br />intellicast.com/weather/msp/wxTnsp.html) ffi <br />Unfortunately, there are many factors that <br />can lead to errors in radar-based precipitation <br />estimates. Precipitation rates vary with the <br />atmospheric conditions of the moment. The . <br />reflectivity properties of ice crystals are different <br />from those of water droplets. Also, problems <br />arise from ground clutter and signal <br />degradation with distance from the radar. <br />Attenuation (a weakened signal because there is <br />a thunderstorm between the radar and the next <br />