Distribution of RHice inside thin and thick cirrus clouds over the Northern Mid-latitudes and in a Subtropical Region

In-situ based information on the distribution of RH_ice inside cirrus clouds is still retrieved mainly from focused research aircraft campaigns for dedicated regions and seasons, but a long-term and seasonal perspective is missing. We report on the distribution of RH_ice in clear sky as well as inside thin and thick cirrus clouds for the North Atlantic region and over subtropical Southeast Asia, with the focus on the occurrence of ice-supersaturated air masses.

The underlying data base builds on more than 7 years of continuous in-situ observations by the European research infrastructure IAGOS (www.iagos.org) which measures, among others, temperature, RH_ice and ice cloud particles, on instrumented passenger aircraft. Information on cloud coverage and cloud thickness were taken from ERA5 global reanalysis by means of the cloud ice water content (CIWC). The separation of clear-sky and in-cloud flight sequences was achieved by applying a novel ERA5 CIWC based cloud index validated by IAGOS in-situ RH_ice observations.

The analysed data set covers the period from June 2014 to December 2021. Four regions were identified for in-depth statistical analyses, with three of them located in the Northern midlatitudes (30–60°N), namely Eastern North America (105–65°W), the North Atlantic flight corridor (65–5°W), and Western Europe (5°W–30°E), and one in the Southeast Asian subtropics (0–30°N, 45–120°E).

We will present the novel cloud index and discuss the features of the resulting RH_ice probability distribution functions of the analysed regions, including seasonal variations, and potential implications for the underlying cirrus cloud formation processes.