Stability assessment of forecasts regarding ice supersaturation and persistent contrails

Kurzfassung

In order to achieve the ambitious 2 ◦C target of the Paris Agreement, it is necessary to reduce the climate effect of all sectors, including aviation. The reduction of non-CO2 effects, especially through contrail avoidance, seems a quick and relatively easy way to reduce the climate impact of aviation. Several studies have shown that this would be achievable with only a very small increase in fuel consumption on the order of 0.1 %, but these studies assumed that perfect weather forecasts were available for the prediction of contrails and for flight planning which aims at avoiding contrails. Obviously, this is an invalid assumption, as predictions are always subject to errors. It needs to be clarified what this assumption implies for contrail avoidance. Our results show that the quality of the forecast of persistent contrails decreases with increasing temporal gap between flight planning and the actual flight time. During this time, the regions where contrails can form may develop differently in nature than in the forecast model due to non-linear dynamics of the atmosphere. This results in correct forecasts, but also in false alarms and misses. In an example case with a large, yet realistic, time gap on 623·103 km planned flight distances persistent contrails were predicted, but occurred actually on 464·103 km. Additionally, 135·103 km of contrails occurred nexpectedly (misses). The false alarms and misses can spoil the climate benefit of contrail avoidance measures. An analogous mechanism would apply for the prediction of other non-CO2 effects of aviation. Several ways to improve the contrail forecast are conceivable. In particular, assimilation of relevant data, such as relative humidity in the upper troposphere, would help to keep the forecast model close to reality. A better representation of ice supersaturation (relative humidity w.r.t. ice over 100 %) in the models seems necessary as well. Meanwhile, considering the most recent available weather forecast could already help to minimise false alarms and misses.