ASOS Heated Tipping Performance Assessment and Impact on Precipitation Climate Continuity
COLORADO STATE UNIV FORT COLLINS DEPT OF ATMOSPHERIC SCIENCE
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The National Weather Service NWS has been installing Automated Surface Observing Systems ASOS at all first order weather stations since 1991 as part of the NWS modernization program. This program was a joint effort between the NWS, Federal Aviation Administration FAA, and the Department of Defense DOD to automate the measurement of surface meteorological elements. The introduction of this system has brought with it inherent differences in the measurements of the meteorological parameters induced by instrument changes and spatial variation brought on by the decision to change the locations of official observing sites. The instrument of concern in this study was the gauge measuring accumulated liquid precipitation, the Heated Tipping Bucket Rain Gauge HTB. ASOS uses a non-linear correction algorithm to produce a corrected accumulated precipitation measurement which is intended to correct one-minute precipitation totals with a more significant correction being made during higher rain rate events. Early comparisons of HTB to the Universal Rain Gauge UNIV at sites located in the Midwestern U.S. uncovered a significant pattern of undermeasuring accumulated liquid-precipitation by the HTB. ASOS HTB Modification 35 MOD 35 began in early 1996 to correct problems with the HTB including its tendency to undermeasure liquid precipitation. Thus, the scope of this investigation was to assess the operational performance of the HTB MOD 35 gauge at 4 sites and to quantify the impact of the ASOS HTB on climate continuity at 13 sites located around the country.