Investigating the Variability of Winter Precipitation Rates across an Airport Terminal Area

The winter precipitation variability across the terminal area is of significant interest to the Federal Aviation Administration (FAA) Terminal Area Icing Weather Information for NextGen (TAIWIN) project. This variability has never been studied in-depth and different precipitation rates across the terminal area can have significant impacts on aviation operations. This is particularly true in instances where snow bands may move across the airport or if precipitation is only occurring over portions of the terminal area. While all winter precipitation types are of interest, this research has focused on the variability of snow, freezing drizzle and freezing rain.

Current airport operations rely on point observations for determining precipitation rate over the terminal using a single precipitation gauge and/or visibility sensor, but these observations are rarely indicative of the precipitation rate over the entire terminal area. Assumptions of uniform precipitation rate can result in aircraft and runway de-icing and anti-icing fluids failing sooner than expected, allowing snowfall accumulation and/or ice accretion to build up on aircraft and runway surfaces. Further, new aircraft icing regulations (§25.1420, enacted January 2015) require the discrimination between the subsets of Supercooled Large Drop (SLD) environments, freezing drizzle and freezing rain, in the terminal area to support operations by aircraft affected by the new rule. To address these issues, mesoscale and microscale spatial variability of precipitation rates need to be understood. Two separate but similar studies were performed to examine frozen and freezing precipitation variability. For the frozen study, precipitation gauges and present weather sensors were deployed around Bloomfield, CO and Superior, CO across an area equivalent in size to an airport terminal area with distances between gauges varying from 0.1km to 9.8km. For the freezing study, archived one-minute observation data from six Automated Surface Observing Systems (ASOS) sites around the Chicago area (KORD, KMDW, KARR, KDPA, KPWK and KUGN) were utilized to derive ice accretion rates. Statistical analyses were conducted on the precipitation rate and ice accretion rate measurements, in each respective study, to quantify the changes in variability that can exist over an airport terminal area. Results from the analysis of the events, potential impacts to aviation operations based on these results, and possible correlations to other meteorological parameters will 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.