ASOS currently uses a heated tipping bucket rain gauge to measure and report liquid precipitation accumulation. The gauge performs very well during liquid precipitation events when intensities of precipitation are above 0.01 inches per hour up to 10 inches per hour; however, there have been numerous complaints from users that the gauge also reports precipitation amounts of 0.01 inches when no precipitation is occurring. These amounts have been termed false tips, which implies that there is something wrong with the gauge; however, in most cases the gauge is functioning properly, but is responding to condensation during heavy fog, dew, and frost. Even though the gauge may be functioning properly, a fog, dew, or frost induced tip is still meteorologically considered a false tip.
The ASOS Program Office has developed an algorithm that maximizes the number of false tips that are eliminated, while minimizing the number of valid precipitation tips that are discarded. This paper summarizes the results from 125 cases studies of false tips from selected ASOS sites during 1997 and 1998. The paper describes the test algorithm developed for the case studies and the proposed ASOS algorithm. The final ASOS algorithm will be a refined version of the test algorithm developed for the case studies. The differences between the algorithms are statistically insignificant.
The elimination of any false tip will be recorded in the ASOS EDIT Log, and the archived data containing the tip will be bracketed [0.01] as questionable. This method will assure that all reports that contain precipitation totals (6 and 7 groups, P group, Daily Summary, Monthly Summary and SHEF) will be recalculated to reflect the elimination of false tips.