Variations in In-Flight Icing Likelihood over the U.S. During Different Weather Regimes

Weather regimes can be used to identify archetypical mid- and upper-level flow patterns in the atmosphere (Lee et al. 2023). These regimes are defined using a form of machine learning based on historical reanalysis datasets (e.g. European Center for Medium Range Weather Forecasting (ECMWF) Reanalysis version 5 (ERA5), and their classification can be applied to Numerical Weather Prediction (NWP) forecasting systems (e.g. Global Forecast System (GFS)). While the regimes are largely associated with 500 hPa geopotential height anomalies, these anomalies have strong influence on mid- and upper-level flow patterns which guide the favored pathways for extratropical cyclones and resultant precipitation and clouds. Therefore, it is likely that weather regimes can be linked with in-flight icing conditions. Using a voice Pilot Report (PIREP) dataset spanning 25 years from 1994 to 2019, and daily weather regimes computed from ERA5 reanalyses over the same time period, the probability of an in-flight icing PIREP occurring is computed for the 25 year period (climatological value), as well as for all days during that period belonging to each of the weather regimes. The probability during each regime relative to the climatological probability can be expressed as a percent increase or decrease to help identify how much more or less likely days are with at least one in-flight icing PIREP of severity of trace or greater.

The spatial distribution of days with higher PIREP likelihood relates well to the placement of negative and positive mean 500 hPa geopotential height anomalies, respectively. This is especially distinguishable during the Pacific Ridge and Alaskan Ridge regimes, when the meridional extent of the 500 hPa geopotential height anomalies is more amplified. In the Pacific Ridge regime, most areas east of the Rocky Mountains show that days with at least 1 PIREP are 1.1 to 1.2 times less likely than average, while the west coast and intermountain west exhibit similar values but of the opposite sign (1.1 to 1.2 times more likely). During Alaskan Ridge regime days, the signal is nearly identically opposite, with the Pacific Northwest in particular indicating likelihoods of 1.4 times less likely. However, even in more zonally defined regimes when the 500 hPa height anomalies are weakly negative (weak troughing), days with at least 1 PIREP are more likely than average compared to when the height anomalies are weakly positive (weak ridging). In general there are fewer PIREPs during more zonally defined regimes, potentially due to a lower occurrence of strong extratropical cyclones and thus the signal is not as widespread but still discernible and statistically significant at the 95 % confidence level.

These results indicate that when provided with accurate weather regime forecasts, it is possible to infer whether an increase or decrease in icing conditions can be expected in some regions of the Contiguous United States. More information about the weather regimes, PIREP dataset, and potential applications will be discussed.

This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement No. 1852977. A portion of 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.