The effects of warm air intrusions in the high arctic on cirrus clouds

Abstract. Warm Air Intrusions (WAI) are responsible for the transportation of warm and moist airmasses from the mid-latitudes into the high arctic (>70° N). In this work, we study cirrus clouds that form during WAI events (WAI cirrus) and during undisturbed arctic conditions (AC cirrus) and investigate possible differences between the two cloud types based on their macrophysical and optical properties with a focus on Relative Humidity over ice (RHi). We use airborne measurements from the combined high spectral resolution and differential absorption lidar, WALES, performed during the HALO-(AC)3 campaign. We classify each research flight and the measured clouds as either AC or WAI, based on the ambient conditions and study the macrophysical, geometrical and optical characteristics for each cirrus group. As our main parameter we choose the Relative Humidity over ice (RHi) which we calculate RHi by combining the lidar water vapor measurements with model temperatures. RHi is affected by and can be used as an indication of the nucleation process and the structure of cirrus clouds. We find that during WAI events the arctic is warmer and moister and WAI cirrus are both geometrically and optically thicker compared to AC cirrus. WAI cirrus and the layer directly surrounding them, are more frequently supersaturated, also at high supersaturations over the threshold for homogeneous ice nucleation (HOM). AC cirrus have a supersaturation dominated cloud top and a subsaturated cloud base. WAI cirrus also have high supersaturations at cloud top, but also at cloud base.