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Comparison of in‐flight aircraft icing algorithms based on ECMWF forecasts

This study presents an objective verification of different icing algorithms for two periods from 5 December 2012 to 17 April 2013 and from 9 October to 11 December 2013. The predicted icing potential and severity are evaluated against pilot reports of aircraft icing below 23 000 feet over Eastern United States. The icing algorithms are applied to the deterministic forecasts of European Centre for Medium‐Range Weather Forecasts (ECMWF). Two of these algorithms are based only on forecasts of temperature (T) and relative humidity (RH), but the first uses fixed thresholds and the second uses a fuzzy logic approach. Although the second algorithm detects less icing events, it is more efficient with respect to the warning area, i.e. it forecasts icing over a smaller percentage of the airspace than the first algorithm. The impact of adding information from cloud liquid water (CLW) content and/or vertical velocity in the fuzzy logic scheme is assessed. Algorithms using T, CLW and vertical velocity as predictors are also tested. Most of the algorithms reveal a good performance in discriminating between icing and no‐icing conditions, but are less skilful in forecasting the icing severity. The results also show that the use of CLW is essential to avoid issuing too many warnings. Moreover, the algorithm combining T, RH, CLW and vertical velocity attains the best performance. This algorithm, SFIPO, has been operational at the Portuguese Meteorological Watch Office since June 2013, and an example of a severe icing episode in February 2014 over mainland Portugal is presented.