Work

Impact of surface and near-surface processes on ice crystal concentrations measured at mountain-top research stations

Year: 2018

Type: article

Abstract: . In-situ cloud observations at mountain-top research stations regularly measure ice crystal number concentrations (ICNCs) orders of magnitudes higher than expected from measurements of ice nucleating particle (INP) concentrations. Thus, several studies suggest that mountain-top in-situ measurements are influenced by surface processes, e.g. blowing snow, hoar frost or riming on snow covered trees, rocks and the snow surface. A strong impact on the observed ICNCs on mountain-top stations by surface processes may limit the relevance of such measurements and possibly affects the development of orographic clouds. This study assesses the impact of surface processes on in-situ cloud observations at the Sonnblick Observatory in the Hohen Tauern Region, Austria. Vertical profiles of ICNCs above a snow covered surface were observed up to a height of 10 m. The ICNC decreases at least by a factor of two at 10 m, if the ICNC at the surface is larger than 100 L−1. This decrease can be up to one order of magnitude during in-cloud conditions and reached its maximum of more than two orders of magnitudes when the station was not in cloud. For one case study, the ICNC for regular and irregular ice crystals showed a similar relative decrease with height, which cannot be explained by the above mentioned surface processes. Therefore, two near-surface processes are proposed to enrich ICNCs and explain these finding. Either sedimenting ice crystals are captured in a turbulent layer above the surface or the ICNC is enhanced in a convergence zone, because the cloud is forced over a mountain. These two processes would also have an impact on ICNCs measured at mountain-top stations if the surrounding surface is not snow covered. Conclusively, this study strongly suggests that ICNCs measured at mountain-top stations are not representative for the properties of a cloud further away from the surface.

Source: Atmospheric chemistry and physics

DOI: 10.5194/acp-18-8909-2018

Authors: Alexander Beck, Jan Henneberger, Jacob P. Fugal, Robert O. David, Larissa Lacher, Ulrike Lohmann

Institutions: ETH Zurich, Institute for Atmospheric and Climate Science, Johannes Gutenberg University Mainz

Language: English

Topic: Atmospheric aerosols and clouds

Subfield: Global and Planetary Change

Field: Environmental Science

Domain: Physical Sciences

SDG: Climate action

Open Access status: gold

Estimated APC paid: $1,654