A COMPARISON OF AN AUTOMATED FREEZING DRIZZLE ALGORITHM TO HUMAN OBSERVATIONS

In response to the aircraft engine damage caused by freezing drizzle at Denver International Airport (DIA, Rasmussen et al. 2006), an Automated Surface Observing System (ASOS) ice detector sensor was acquired and the Ramsey freezing drizzle algorithm (Ramsey, 1999) was incorporated into the DIA Weather Support to Deicing Decision Making (WSDDM). This system has run for several years, providing sensorbased observations of freezing drizzle to airline and airport operations personnel. Since its installation, no occurrences of freezing drizzle damage have been reported. The original engine damage noted in the Rasmussen paper occurred because the airport observers were often not reporting freezing drizzle in the METAR. Since the ASOS does not report freezing drizzle, human observers augment METARs for reports of freezing drizzle. Because of this, a study was undertaken to determine what types of precipitation are reported in the METARs (both the automated reports of precipitation type from the ASOS and the augmented reports from observers) during times when the Ramsey algorithm indicates the presence of freezing drizzle.