Distribution and morphology of non-persistent and persistent contrail formation areas in ERA5

Abstract. The contrail formation potential as well as its temporal and spatial distribution are estimated using meteorological conditions of temperature and relative humidity from the ERA5 re-analysis provided by the European Centre for Medium-Range Weather Forecasts. Contrail formation is estimated with the Schmidt–Appleman criterion (SAc), solely considering thermodynamic effects. The focus is on a region ranging from Eastern United States to central Europe. Around 18,000 flight trajectories from the In-service Aircraft for a Global Observing System (IAGOS) are used as a representative subset of transatlantic, commercial flights. The typical crossing distance through a contrail-prone area is determined based on IAGOS measurements of temperature T and relative humidity r, then based on co-located ERA5 simulations of the same quantities. For IAGOS, 50 % of the crossings of persistent contrail (PC) regions are shorter than 9 km, while in ERA5 the median is 155 km. Time-averaged IAGOS data lead to a median crossing length of 66 km. The difference between the two data sets are attributed to the higher variability of r in IAGOS compared to ERA5. Binary masks of PC formation are created by applying the SAc on the two-dimensional fields of T and r from ERA5. In a second step the morphology of PC regions is also assessed. Half of the PC regions are found to be smaller than ≈35000 km² (at 200 hPa) and the median of the maximum dimension is shorter than 760 km (at 200 hPa). Furthermore, PC regions tend to be of near-circular shape with a tendency to a slight oval shape and a preferred alignment along the dominant westerly flow. Seasonal, vertical distributions of PC formation potential P are characterized by a maximum between 250 and 200 hPa. P is subject to seasonal variations with a maximum in magnitude and extension during the winter months and a minimum during summer. The horizontal distribution of PC regions suggests that PC regions are likely to appear in the same location on adjacent pressure levels. Climatologies of T , r, wind speed U, and resulting PC formation potential are calculated to identify the constraining effects of T and r on P. PC formation is primarily limited by too warm conditions below and too dry conditions above the formation region. The distribution of PCs is slanted towards lower altitudes from 30° N to 70° N, following lines of constant T and r. For an observed co-location of high U and P it remains unclear whether PC formation and the jet stream are favored by the same meteorological conditions or if the jet stream itself favors PC occurrence. This analysis suggests that some PC regions will be difficult to avoid by rerouting aircraft because of their large vertical and horizontal extents.