CIRRUS-HL: Picturing high- and mid-latitude summer cirrus and contrail cirrus above Europe with airborne measurements aboard the research aircraft HALO

Abstract

Accurately determining and reducing the climate impact of aviation and its uncertainties is one of the pressing challenges of our times. Contrail cirrus are estimated to contribute more than half of the total effective radiative forcing from aviation, yet the uncertainties in their optical and radiative properties are large. In contrast to mid-latitude cirrus, high-latitude cirrus are less anthropogenically influenced, thus they are more pristine. However, little is known about Arctic cirrus properties and their role in the amplified warming of this region. The CIRRUS-HL (Cirrus in High Latitudes) mission using the High Altitude and LOng range research aircraft HALO provides measurements in mid and high latitudes during the summer (June/July) 2021, exploiting HALO’s capabilities and a comprehensive cloud-aerosol-trace gas and radiation instrumentation. The results of 24 HALO flights provide new insights into both natural cirrus and contrail cirrus properties in high (60°N-76°N) and mid latitudes (38°N-60°N). In particular, we find lower ice water content (-42%) and lower number concentrations (-88%) of cirrus particles with larger mean diameters (+22%) in high latitudes. Ice supersaturated regions were frequently observed in mid and high latitudes, with median in-cloud relative humidity over ice between 105% and 122%. Mean aerosol number concentrations in the mid latitudes were reduced by up to 80% compared to pre-COVID times. Less air traffic during the COVID-19 lock-downs, reduced contrail cirrus coverage and lower ice nucleating particle concentrations in high latitudes help to explain the observed differences in cirrus properties. The extensive data set will be used to improve weather and climate models.