Colloquium: Katarina Grubbe Hildebrandt (FPT)

Minimisation of the climate impact of flights in the short term may be achieved by contrail avoidance, by avoiding ice supersaturated regions (ISSRs), which cause contrails to persist. ISSRs occur when the temperature is below 235.15 K and the relative humidity over ice is above 1, but numerical weather prediction (NWP) models struggle to predict such regions. This study validates the prediction of ISSRs in the ERA5 reanalysis using IAGOS in-situ measurements from a pressure level (350 to 175 hPa), seasonal, yearly (2011 to 2022) and regional perspective. Validation of ERA5's temperature showed a cold bias in all extratropic regions and at low altitudes in the tropics, which can result in ERA5 predicting ISSRs when none are observed. Increasing temperature deviations between IAGOS and ERA5 were observed between years 2020 and 2022. In terms of relative humidity over ice, ERA5 and IAGOS showed good agreement below ice supersaturation, but ERA5 struggled in simulating values close to or above a value of 1. For the latter conditions, ERA5 showed a dry bias in the extratropics, below the tropopause. A wet bias was found in some regions and seasons, including South Asia in June, July, and August. This may be due to inconsistencies in the modelling of the atmospheric ice content in ERA5. Years 2013 and 2014 also showed a possibility of a wet bias in ERA5 in some regions, but this was most likely the result of regional changes in sampling by IAGOS. The wet bias caused false detection of ISSRs in ERA5. Meanwhile, the dry bias led to an underestimation of the occurrence of ISSRs, with 25 to 50\% remaining undetected. This causes an overestimation of no persistent contrail regions and an underestimation of persistent contrail and reservoir regions. When ERA5 overestimates the ISSR occurrence, it leads to an overestimation of persistent contrail and reservoir regions, but an underestimation of no persistent contrail regions. Hence, improvements are necessary to successfully avoid or ensure unnecessary avoidance of ISSRs, where the latter could cause an increase in climate impact of flights. This should be done before implementation in contrail avoidance strategies, otherwise the benefit of such strategies may be defeated.

Supervisor: Dr. Feijia Yin