Occurrence of seeding multi-layer clouds in the Arctic from ground-based observations

Studies of Arctic clouds often focus on low-level single-layer clouds (SLCs). Here, we use combined observations of soundings and cloud radar during the MOSAiC, ACSE, and AO2018 research cruises as well as from long-term observations at Ny-Ålesund, Svalbard and Utqiagvik, Alaska to investigate the occurrence of SLCs and multi-layer clouds (MLCs) in the Arctic and to assess the rate of ice-crystal seeding in cold MLCs. MOSAiC observations show cloudy conditions in between 70 % and 90 % of sounding-radar cases. SLCs dominate during October (90 % cases) with otherwise stable rates of around 40 %. MLCs are most abundant from November to March (40 % to 55 % of cases). Seeding occurs in about half to two thirds of the identified MLCs during MOSAiC for which the sub-saturated layer extends between 100 and 1000 m. The seeding rate increases by 5 to 10 percentage points as the assumed size of the falling ice crystals is increased from 100 to 400 µm. The observations reveal a somewhat higher rate of cloud-free conditions at latitudes south of 84° N. Cloud occurrence during MOSAiC and at Ny-Ålesund in July, when the geographical distance between observations was minimal, shows reasonable agreement. Comparisons of MOSAiC and other research cruises to the central Arctic also indicate consistent occurrence rates of different cloud types despite the likely effect of year-to-year variability. The comparison of data from ship campaigns and land sites suggests that the latter are not a good indicator of cloud occurrence in the high Arctic.