Temporal and Spatial Patterns of Ice Supersaturation: A 3D climatology over the North Atlantic Region

Abstract

As the most important greenhouse gas in the Earth's atmosphere, the presence of water vapor in the upper troposphere and lower stratosphere (UTLS) is essential for influencing global radiation patterns and surface climate conditions. Even minor changes in water vapor levels within the mostly dry lower stratosphere (LS) can impact the vertical water vapor gradient, making it a crucial factor in the decadal variability of surface temperature.In condensed form, water holds significant importance for planetary radiation. Clouds play a dual role by reflecting incoming solar radiation into space and absorbing/emitting longwave radiation from the surface. Estimating the impact of cirrus clouds on the radiation budget is particularly challenging as it depends on a variety of factors, such as altitude, humidity and the microphysical properties of the cloud.During the lifetime of a cirrus cloud, the radiative impact can even change from a warming to a cooling effect and vice versa. For the formation of cirrus clouds, ice supersaturated regions (ISSRs) play an important role. However, the required amount of supersaturation is dependent on the nucleation mechanism, with at least ∼ 45% supersaturation for homogeneous freezing and as low as ∼ 20% for heterogeneous freezing.We present a statistical intercomparison of the In-service Aircraft for a Global Observing System (IAGOS) dataset with ERA5, the latest reanalysis product of the European Centre for Medium-Range Weather Forecasts (ECMWF). Furthermore, a machine learning based algorithm is developed to improve the accordance of relative humidity with respect to ice (RHi) of reanalysis data with in-situ measurements, enabling large scale analyses of water vapor in the UTLS region. With this tool, we build three-dimensional climatologies of RHi and ISSRs over the North Atlantic region and show their seasonal and regional variability. This will help foster a general understanding of the occurence of cirrus clouds and their impact on weather and climate.