ALBERT

Description: Ice nucleating particles (INPs) affect the radiative properties of cold clouds. Knowledge concerning their concentration above ground level and their potential sources is scarce. Here we present the first highly temperature resolved ice nucleation spectra of airborne samples from an aircraft campaign during late winter in 2018. Most INP spectra featured low concentration levels (〈3 · 10−4 L−1 at −15°C). −2 −1 However, we also found INP concentrations of up to 1.8·10 L at −15°C and freezing onsets as high as −7.5°C for samples mainly from the marine boundary layer. Shape and onset temperature of the ice nucleation spectra of those samples as well as heat sensitivity hint at biogenic INP. Colocated measurements additionally indicate a local marine influence rather than long‐range transport. Our results suggest that even in late winter above 80°N a local marine source for biogenic INP, which can efficiently nucleate ice at high temperatures, is present. Clouds are a key factor in the energy budget of the Arctic atmosphere. Ice nucleating particles (INPs) can modify the radiation properties and lifetime of clouds by affecting the relative abundance of liquid and frozen droplets in a cloud. Despite this important ability, knowledge about the INP concentration above ground level is limited as airborne INP measurements are very scarce in the Arctic. Here we present results from an aircraft campaign, which took place during the late winter of 2018 in latitudes above 80°N. We found INP concentrations at above −15°C, which are similar to those found in midlatitudes. These INPs also initiate freezing already at high temperatures. We found indications that the INPs are biogenic and originate from a local, marine source, rather than being transported from midlatitudes into the Arctic. Due to the presence of numerous cracks, open leads and polynyas in the sea ice in the investigation area, the ocean may provide a source for these biogenic INP in an environment, where sources on land are still shrouded in snow and ice. However, in a warming Arctic contributions from different sources might change, making the characterization of the current state important.

Description: To understand the feedbacks driving the amplified changes in the Arctic a quantification of the contribution of the involved processes is necessary. Here a detailed study of low-level Arctic clouds on the surface radiation budget is presented. These clouds frequently occur below the lowest detection range of most state-of-the-art remote-sensing instruments and were observed in summertime during 25% of the time over the marginal sea ice zone. The low altitude of these clouds poses challenges on their observation and characterization by remote-sensing techniques. Ground-based remote sensing and surface radiation flux measurements performed during the Arctic cruise PS106 in 2017 were combined with radiative transfer simulations to study low-level clouds. A multiwavelength lidar PollyXT with near-range observations capabilities down to 50m and a cloud radar with a lowest detection limit at 165m altitude were operated continuously. The liquid-water microphysical properties of clouds missed by the cloud radar were estimated using measurements of a microwave radiometer HATPRO and the lidar-detected cloud base. Thereby the surface radiative effect of these clouds was quantified. A closure between the observed and modelled radiative surface fluxes was achieved with a realistic representation of low-level liquid-containing clouds in the radiative transfer model. When omitting these low-level clouds, the cloud radiative effect at the surface was misestimated by 43Wm−2. The presented study highlights the importance of improving cloud retrievals for low-level liquid-containing clouds as they are frequently encountered in the high Arctic, together with observational capabilities, both in terms of cloud remote sensing and radiative flux observations.