Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-1015
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
18 Nov 2016
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Aerosol-landscape-cloud interaction: Signatures of topography effect on cloud droplet formation
Sami Romakkaniemi1, Zubair Maalick2, Antti Hellsten3, Antti Ruuskanen1, Olli Väisänen2, Irshad Ahmad2, Juha Tonttila1,4, Santtu Mikkonen2, Mika Komppula1, and Thomas Kühn1,2 1Finnish Meteorological Institute, Kuopio, Finland
2Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
3Finnish Meteorological Institute, Helsinki, Finland
4Karlsruhe Institute of Technology, Karlsruhe, Germany
Abstract. Long-term in situ measurements of aerosol-cloud interactions are usually performed in measurement stations residing on hills, mountains, or high towers. In such conditions, the surface topography of the surrounding area can affect the measured cloud droplet distributions by increasing turbulence or causing orographic flows and thus the observations might not be representative for a larger scale. The objective of this work is to analyse, how the local topography affects the observations at Puijo measurement station, which is located in the 75 m high Puijo tower, which itself stands on a 150 m high hill. The analysis of the measurement data shows that the observed cloud droplet number concentration mainly depends on the CCN concentration. However, when the wind direction aligns with the direction of the steepest slope of the hill, a clear topography effect is observed. This finding was further analysed by simulating 3D flow fields around the station and by performing trajectory ensemble modelling of aerosol- and wind-dependent cloud droplet formation. The results showed that in typical conditions, with geostrophic winds of about 10 m s−1, the hill can cause updrafts of up to 1 m s−1 in the air parcels arriving at the station. This is enough to produce in-cloud supersaturations higher than typically found at the cloud base (SS of ~ 0.2 %), and thus additional cloud droplets may form inside the cloud. In the observations, this is seen in the form of a bi-modal cloud droplet size distribution. The effect is strongest with high winds across the steepest slope of the hill and with low liquid water contents, and its relative importance quickly decreases as these conditions are relaxed. We therefore conclude that, after careful screening for wind speed and liquid water content, the observations at Puijo measurement station can be considered representative for clouds in a boreal environment.

Citation: Romakkaniemi, S., Maalick, Z., Hellsten, A., Ruuskanen, A., Väisänen, O., Ahmad, I., Tonttila, J., Mikkonen, S., Komppula, M., and Kühn, T.: Aerosol-landscape-cloud interaction: Signatures of topography effect on cloud droplet formation, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1015, in review, 2016.
Sami Romakkaniemi et al.
Sami Romakkaniemi et al.
Sami Romakkaniemi et al.

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Short summary
Surface topography affects aerosol-cloud interactions in boundary layer clouds, and these measurement site dependent effects should be screened out from in situ observations before results can be generalized into a larger scale. Here we present modelling and observational results from a measurement station residing in a 75 m tower on top of 150 m hill, and analyze how landscape affects the air masses arriving to station, and what factors should be taken into account in data analysis.
Surface topography affects aerosol-cloud interactions in boundary layer clouds, and these...
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