Abstract
Abstract. An advanced multiwavelength polarization Raman lidar was operated aboard the icebreaker Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition to continuously monitor aerosol and cloud layers in the central Arctic up to 30 km height. The expedition lasted from September 2019 to October 2020 and measurements were mostly taken between 85 and 88.5∘ N. The lidar was integrated into a complex remote-sensing infrastructure aboard the Polarstern. In this article, novel lidar techniques, innovative concepts to study aerosol–cloud interaction in the Arctic, and unique MOSAiC findings will be presented. The highlight of the lidar measurements was the detection of a 10 km deep wildfire smoke layer over the North Pole region between 7–8 km and 17–18 km height with an aerosol optical thickness (AOT) at 532 nm of around 0.1 (in October–November 2019) and 0.05 from December to March. The dual-wavelength Raman lidar technique allowed us to unambiguously identify smoke as the dominating aerosol type in the aerosol layer in the upper troposphere and lower stratosphere (UTLS). An additional contribution to the 532 nm AOT by volcanic sulfate aerosol (Raikoke eruption) was estimated to always be lower than 15 %. The optical and microphysical properties of the UTLS smoke layer are presented in an accompanying paper (Ohneiser et al., 2021). This smoke event offered the unique opportunity to study the influence of organic aerosol particles (serving as ice-nucleating particles, INPs) on cirrus formation in the upper troposphere. An example of a closure study is presented to explain our concept of investigating aerosol–cloud interaction in this field. The smoke particles were obviously able to control the evolution of the cirrus system and caused low ice crystal number concentration. After the discussion of two typical Arctic haze events, we present a case study of the evolution of a long-lasting mixed-phase cloud layer embedded in Arctic haze in the free troposphere. The recently introduced dual-field-of-view polarization lidar technique was applied, for the first time, to mixed-phase cloud observations in order to determine the microphysical properties of the water droplets. The mixed-phase cloud closure experiment (based on combined lidar and radar observations) indicated that the observed aerosol levels controlled the number concentrations of nucleated droplets and ice crystals.
Highlights
Rapid sea ice loss, unusual Arctic warming, and our incomplete knowledge about the complex processes controlling the Arctic climate motivated the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) (MOSAiC, 2020) expedition, the largest Arctic research initiative in history
Engelmann et al.: MOSAiC lidar observations northern Norway heading towards the central part of the Arctic and started drifting through the Arctic Ocean trapped in the ice at the beginning of October 2019
We present a new lidar technique, the so-called dual-field-of-view polarization lidar technique (Jimenez et al, 2020a, b) that allows us to retrieve the microphysical properties of the liquid-water droplets in the cloud top layer
Summary
Unusual Arctic warming, and our incomplete knowledge about the complex processes controlling the Arctic climate motivated the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) (MOSAiC, 2020) expedition, the largest Arctic research initiative in history. We continuously operated a state-of-the-art multiwavelength Raman lidar (Engelmann et al, 2016) aboard the research vessel Polarstern side by side with the ARM (Atmospheric Radiation Measurement) mobile facility 1 (AMF-1) (ARM, 2020) and collected tropospheric and stratospheric aerosol and cloud profile data throughout the expedition period from September 2019 to October 2020. Our most impressive and outstanding observation during the entire MOSAiC expedition was the detection of a persistent, 10 km deep aerosol layer of aged wildfire smoke (Ohneiser et al, 2021) We monitored this smoke layer in the upper troposphere and lower stratosphere (UTLS) from about 7–8 km up to 17–18 km height for more than 7 months from the beginning of the lidar observations in late September 2019 until May 2020. The TROPOS equipment was already aboard Polarstern 2 years ago and involved in the Arctic field campaign PASCAL (Physical feedbacks of Arctic boundary layer, Sea ice, Cloud and AerosoL) (Wendisch et al, 2019; Griesche et al, 2020, 2021)
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