The ice supersaturation biases limiting contrail modelling are structured around extratropical depressions

Abstract. Contrails are ice clouds occasionally formed along aircraft flight tracks, responsible for much of aviation's climate warming impact. Contrails persist in ice-supersaturated regions (ISSRs), but meteorological models often mispredict their occurrence, limiting contrail modelling. This deficiency is often treated by applying local humidity corrections. However, model performance is also affected by synoptic conditions (such as extratropical depressions).

Here, composites of ERA5 reanalysis model around North Atlantic extratropical depressions enable a link between their structure and ISSR modelling. ISSRs are highly structured by these systems: at flight levels, the ERA5 ISSR rate is 91 % less in the dry intrusion – in descending upper-tropospheric air – than above warm conveyors – where air is lifted. The contrast also occurs in composites of in situ aircraft observations, showing the model reproduces the fundamental relationship. However, performance in modelling individual ISSRs also differs across the structures. Compared to the warm conveyor belt, the infrequent ISSRs in the dry intrusion are less well captured by ERA5, with a 20–25 % drop in diagnosing confidence (precision) and 13–19 % drop in comprehensiveness (recall). Scaling humidity beyond ISSR occurrence rate corrections is able to dramatically increase the recall with a small precision cost and high specificity, demonstrating the potential value of scaling approaches designed with different intentions. However, the failure to improve precision, or the performance in the dry intrusion, implies that there is a need to account for the synoptic weather situation and structure in order to improve ISSR forecasts in support of mitigating aviation’s climate impact.