A New Automated Technique for Detecting Freezing Drizzle

In its current state, the Automated Surface Observing System (ASOS) operated primarily by the National Weather Service (NWS) and the Federal Aviation Administration (FAA) can only detect three precipitation types: rain, snow, and freezing rain. An algorithm was written for ASOS to detect freezing drizzle, but it was never accepted for formal implementation. Due to a new aircraft certification rule issued by the FAA restricting some aircraft operations during freezing precipitation conditions, there is a need for automated detection of freezing drizzle within the terminal area. This is especially important because not all airports have human observers to augment the ASOS observations during periods of freezing drizzle. Thus, a new automated freezing drizzle algorithm has been developed that utilizes existing instrumentation on the ASOS to detect freezing drizzle conditions. This algorithm has been tested using the One-Minute Observation (OMO) archived data from select stations around the country. Periods of known freezing drizzle (based on METARs and other observations) were used to verify the algorithm. These statistics, along with a description of the algorithm and new freezing drizzle climatologies for select stations, will be presented.