Characterisation of Arctic Cirrus by Airborne Water Vapor and High Spectral Resolution Lidar

Ice clouds in the Arctic are expected to have different radiative properties compared to mid latitude cirrus, because of the different humidity and temperature profile and also the prevalent aerosol loading in northern latitudes which govern their formation. During the late winter and early spring 2022 the HALO-(AC)3 campaign was conducted out of Kiruna (Sweden) to probe artic clouds with an airborne remote sensing payload. For this purpose, the German research aircraft HALO was equipped with a water vapor Differential Absorption Lidar (DIAL), a cloud radar, micro-wave radiometers, radiation measurements in the visible, near infrared and thermal region and a drop-sonde dispenser. A total of 25 flights where performed mainly over the sea between Svalbard and Greenland and up to nearly 90°N.

The primary observable to study ice cloud formation is the relative humidity, which is not directly measurable by lidar, but can only be computed with the aid of additional temperature information. By comparison with a large number of dropsondes launched during flight, we will show that the temperature field from ECMWF IFS analyses and short-term forecasts provides sufficient accuracy to retrieve the relative humidity for ice cloud studies. Using this method we will analyse different scenarios of arctic cirrus formation: under stable artic conditions, during a warm air intrusion and while a cold air outbreak. An interesting special case is the modification of cirrus properties by the presence of an aerosol layer which is most probably composed of long range transported Sharan dust.