A New Technique for Determining Freezing Drizzle, Ice Pellets, and Frost from Archived ASOS Data

In support of the Federal Aviation Administration (FAA) Terminal Area Icing Weather Information for NextGen (TAIWIN) project, improvements to the detection and discrimination of icing conditions at the surface are necessary. Of specific importance to the FAA is the identification of Supercooled Large Drop (SLD) icing conditions which requires the detection and discrimination of freezing rain, freezing drizzle, and ice pellets. Ice pellets are included because they indicate SLD aloft. In order to meet this goal, it was necessary to identify the areas most impacted by SLD icing conditions before identifying or developing methods to help mitigate the risks associated with them. This required analyzing the climatological conditions associated with SLD icing (i.e. freezing drizzle, freezing rain, and ice pellets).

Prior to the introduction of the Automated Surface Observing System (ASOS), weather observations of present weather type including freezing rain, freezing drizzle, and ice pellets, were made by human weather observers. ASOS took the place of many of these observers when it was implemented in the 1990’s. Of the more than 900 ASOS stations across the United States, less than 200 stations still have human observers available to augment the weather observations. Human observers remained at high-impact airports to augment the ASOS and fill in observational gaps introduced by the ASOS, particularly in reporting precipitation type and other obstructions.

In its current form, the ASOS can only report freezing rain conditions. Freezing drizzle and ice pellets can only be reported in METAR observations if a human is present to augment the ASOS observations. This means, less than 200 stations have the ability to report freezing drizzle and ice pellets. Because of ASOS’ inability to detect and report freezing drizzle and ice pellets, developing a climatological record of occurrence of these precipitation types would be restricted only to the stations where humans are available to make these observations, creating large gaps in coverage.

To determine how often freezing drizzle and ice pellets may be affecting airports without human observers, it was necessary to develop an algorithm that could infer the presence of these precipitation types using archived One Minute Observation (OMO) and METAR data from the ASOS sites around the country. The details of this algorithm will be discussed and comparisons of the climatological occurrences of freezing drizzle and ice pellets across the country, based on METARs and observations derived from the freezing precipitation algorithm, will also be shown. In addition, a technique for detecting active frost formation conditions, which poses a ground deicing hazard to aircraft if they are not properly treated before takeoff, will also be discussed.

This research is in response to requirements and funding by the Federal Aviation Administration (FAA). The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.