Aerosol–landscape–cloud interaction: signatures of topography effect on cloud droplet formation
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...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-06-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/7955/2017/acp-17-7955-2017.pdf |
| Summary: | 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<sup>−1</sup>, the hill can cause updrafts of up to 1 m s<sup>−1</sup> 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. |
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| ISSN: | 1680-7316 1680-7324 |