Surface Ice Accretion Rates from the Automated Surface Observing System (ASOS): An Issue for Deicing Holdover Times

The Rosemount 872C3 icing detector on the Automated Surface Observing System (ASOS) has previously been shown to provide reliable estimates of surface ice accretion amounts, in terms of both ice mass and thickness. An ongoing National Weather Service Product Improvement Initiative for the ASOS has resulted in the derivation of ice-accretion rates from over 250 icing events covering the winters of 1998-1999 and 1999-2000. A detailed analysis of ASOS sensor data can provide estimates of surface ice-accretion rates over periods as short as ten minutes. The paper will include histograms of short-period ice-accretion rates in freezing rain and freezing drizzle, representing a "climatology" of ice accretion rates in the central and eastern United States. The ASOS data have identified a potentially serious issue related to METAR reports of freezing-drizzle intensity: FAA holdover tables for aircraft deicing operations are based on ice-accretion rates, which are typically inferred from METAR reports of precipitation intensity. National guidelines for reporting the intensity of freezing drizzle are based on visibility. ASOS data indicate that actual surface ice-accretion rates can differ significantly from METAR reports of freezing drizzle intensity based on visibility. The paper will present comparisons of icing rates derived from the ASOS icing sensor with rates derived from visibility, and will show that the use of METAR reports of freezing drizzle intensity can lead to the selection of deicing holdover times that are far too long. The presentation will conclude with the current status of recommendations to the NWS to change current guidelines regarding determination of the intensity of freezing drizzle.