Characterization of aircraft icing conditions in Western Europe and the North-East Atlantic. Case studies using aircraft reports, satellite, and synoptic data

A sample of 115 aircraft icing events in the Western Europe and Northeastern Atlantic sector, identified in aircraft pilot reports (PIREPs), is analyzed using satellite observations and products. Most of the icing events occurred between October and February, although a few cases were identified during late spring and even summer months. Icing conditions were generally reported at mid-troposphere, with 82.7% and 78.6% of moderate and severe icing, respectively, identified between FL100 and FL250 (≈ 3 - 7.6km). Satellite observations allow the identification of icing-prone conditions and also provide an independent means of validating some of the data in the aircraft reports. It is shown that aircraft icing occurs mainly within opaque clouds, with ice cloud-tops, or mixed-phase (ice and water). Accordingly, most events were associated with middle and high opaque clouds, with 10.8 μm brightness temperatures (BT10.8) between -40 and -8°C. Moreover, a detailed analysis of three events, using model data, satellite, and synoptic observations, illustrates the occurrence of aircraft icing in precipitating clouds. Within those cases, one occurred below the upper layer of a thick nimbostratus associated with an occluded low centered in the Bay of Biscay. The second event, reported as severe, as in the former case, took place within low clouds over southern England in association with northwesterly winds driven by a complex low. Finally, a moderate event happened over southern Portugal, in association with a cold front, near the cloud top of nimbostratus. In all cases, the icing index operational at the Portuguese Weather Service, based on the European Centre for Medium-range Weather Forecasts (ECMWF) model, was able to predict prone-icing conditions, with higher severity in the severe cases.