Articles | Volume 17, issue 12
Atmos. Chem. Phys., 17, 7955–7964, 2017
Atmos. Chem. Phys., 17, 7955–7964, 2017

Research article 30 Jun 2017

Research article | 30 Jun 2017

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 Sami Romakkaniemi et al.
  • 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 cloud condensation nuclei (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 3-D 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 (SSs) higher than typically found at the cloud base of  ∼  0.2 %), and thus additional cloud droplets may form inside the cloud. In the observations, this is seen in the form of a bimodal 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.

Short summary
Surface topography affects aerosol–cloud interactions in boundary layer clouds. Local topography effects should be screened out from in situ observations before results can be generalised into a larger scale. Here we present modelling and observational results from a measurement station residing in a 75 m tower on top of a 150 m hill, and analyse how landscape affects the cloud formation, and which factors should be taken into account when aerosol effect on cloud droplet formation is studied.
Final-revised paper