Characterization of cirrus clouds in the arctic depending on ambient conditions

The increase of the average global temperature of the Earth’s atmosphere has been measured with various methods dating back to the 19^th century. In the past few decades scientists have shown that the arctic regions are warming even faster than the global average. This phenomenon has been labeled Arctic Amplification. Cirrus clouds are a potential contributor to this phenomenon. They reflect only a small part of the incoming solar radiation and can absorb and reemit earth’s long-wave radiation, thus potentially having a warming effect. Warm Air Intrusion (WAI) events transport warm, water-vapor- and aerosol-rich airmasses from the mid-latitudes into the arctic and can also contribute to arctic amplification. On the one hand the transported airmasses are already warm and contain significant amounts of water vapor which is a strong greenhouse gas. On the other hand, the cirrus clouds that form during such an event might have different and potentially stronger effects on the radiation budget of the atmosphere. Since it has also been shown that WAI events in the arctic are becoming more frequent or long-lasting, it is important to study the effects these events have on the macrophysical and optical properties of cirrus clouds in the arctic.

The HALO-(AC)³ field campaign took place in March and April of 2022. One of the central goals of the campaign was to study WAI events in the arctic regions of the Northern Hemisphere. Among others, the German research aircraft HALO was used to perform remote sensing measurements. In this study we use data collected during this campaign by the combined water vapor differential absorption and high spectral resolution lidar system WALES and the HAMP cloud radar. We selected two research flights: RF03, performed during an active warm air intrusion event (WAI case) and RF17, performed during undisturbed arctic conditions (AC case). For these flights we calculated the relative humidity over ice (RHi) and the backwards trajectories using the Lagrangian analysis tool LAGRANTO and the CLaMS-Ice model, which combines the Chemical Lagrangian Model of the Stratosphere (CLaMS) with two-moment ice microphysics. Our aim is to provide an in-depth analysis of the two types of cirrus clouds and find potential differences between them.

The clouds of the WAI case had a greater mean geometrical and optical depth as well as a slightly higher linear depolarization ratio, as measured by WALES. The distributions of RHi for the WAI case had its maximum slightly over saturation and a small negative skewness, while the AC case had its maximum at saturation with a bigger negative skewness. The supersaturations within and at close proximity to the WAI clouds reached high values over 127% more frequently than for the AC case. Surprisingly, the backwards trajectories revealed that the AC case had a significant part being of liquid origin and formed via heterogeneous nucleation, whilst the WAI case was predominantly of in-situ origin with homogeneous nucleation being the dominant process.