Freezing Drizzle Identification from the Automated Surface Observing System (ASOS): Field Evaluation of a Proposed Multi-Sensor Algorithm

The FAA and the aviation industry have identified freezing precipitation as the most important precipitation type for aviation safety. Surface observations of freezing drizzle can also be vital inputs to in-flight icing forecasts. Over 600 locations in the United States are now capable of providing automated observations of freezing rain, using the Rosemount 872C3 icing detector on the Automated Surface Observing System (ASOS). While the ASOS is highly effective in reporting freezing rain, it does not currently report freezing drizzle. Since the mid 1990's, the National Weather Service (NWS) has sponsored an ASOS Product Improvement Initiative intended to enhance the system's capabilities to identify and report freezing drizzle. This paper will present results of a field evaluation of a proposed multi-sensor ASOS freezing-drizzle algorithm, conducted over two winter seasons (1998-1999 and 1999-2000). The field evaluation collected data from more than 250 icing events, representing over 1200 hours of freezing precipitation. The paper will briefly describe the algorithm, present comparisons of the potential automated reports with manual observations, and quantify the hours of freezing drizzle that would have been reported had the algorithm been implemented in the ASOS. The presentation will conclude with the current status of recommendations to implement the proposed algorithm throughout the ASOS.