Impacts of the Implementation of the Automated Surface Observing System (ASOS) on the Reports of Precipitation Type in Airport Terminal Areas Around the United States

The introduction of the Automated Surface Observing System (ASOS) in the late 1990’s and early 2000’s had significant impacts on the weather observational network across the United States. Many locations where humans had provided the weather observations (including present weather) were modified to either include ASOS with the human observers (referred to as human augmented sites) or the human observers were replaced with ASOS. While some observation types such as temperature, humidity and pressure were minimally impacted by this automation, other observations such as precipitation type have likely seen significant impacts.

In its current state, ASOS can only detect three precipitation types: rain, snow, and freezing rain. Other precipitation types such as drizzle, freezing drizzle and ice pellets can only be reported at the sites that have augmented human observers. The Terminal Area Icing Weather Information for NextGen (TAIWIN) project, funded by the Federal Aviation Administration (FAA), has a need for the identification, discrimination, and reporting of these precipitation types as they can indicate icing conditions in the terminal area either at the surface, aloft, or both. It is therefore important to understand the current and past weather observing capabilities across the United States. To determine the impact of the implementation of ASOS on reported present weather types across the United States, a study was undertaken to examine the reported occurrences of freezing drizzle, freezing rain and ice pellets in METARs for a period of time before the implementation of ASOS (1979-1990) and after the implementation of ASOS (2005-2016). An analysis of these periods will be presented showing the impacts across the United States. Additionally, the analysis will be broken down by Service Level A and B airports (where human observers still augment the ASOS observations), Service Level C airports (where there is limited human augmentation available), and Service Level D airports (where no human augmentation is available). Because Service Level C and D airports were most impacted by the implementation of ASOS, the focus of the analysis will be on these airports.

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.