Time for closure: Comparing CCN and cloud droplet number concentrations in Arctic mixed-phase clouds
Abstract
The radiative properties of mixed-phase clouds (MPCs) are controlled by the number and size of cloud droplets and ice crystals within them. In pristine environments, such as the Arctic, the limited number of aerosol particles, which can act as cloud condensation nuclei (CCN), directly control the number of cloud droplets in MPCs. The ability of an aerosol particle to act as a CCN depends on its size, hygroscopicity and the ambient water supersaturation. However, disentangling the impact of size and hygroscopicity on the ability of aerosols to act as CCN is difficult without an accurate measurement of supersaturation. Furthermore, in MPCs the presence of ice further complicates the estimate of CCN and subsequent cloud droplets due to the competition between ice and cloud droplets. Ice crystals can grow at the expense of cloud droplets or lower the humidity below water saturation near cloud base. Thus, accurately parametrizing the concentration of CCN and their activation into cloud droplets in Arctic MPCs for earth system models is challenging.
Due to orographic forcing, mountaintop observatories are frequently situated at or just above cloud base, making them excellent locations for observing the activation of CCN into cloud droplets. The Zeppelin Observatory located above the town of Ny-Ålesund, Norway, frequently experiences MPCs. During the fall of 2019, a holographic cloud imager (HOLIMO), which measures the phase-resolved size distribution of cloud particles between 6 μm and 2 mm, was mounted at Zeppelin Observatory. Simultaneous measurements of the aerosol size distribution and CCN concentration were also conducted. Additionally, the observatory is equipped with a counter-flow virtual impactor, which separates interstitial ambient aerosol from cloud particles, ultimately allowing for the properties of cloud residuals to be investigated. Indeed, the size distribution of the cloud residuals and their ability to act as CCN were also measured. Here we will present a closure study between the observed cloud droplet number concentration from HOLIMO and the CCN concentration in order to get an estimate of the ambient supersaturation of orographically enhanced Arctic MPCs. The size distributions of the cloud residuals and the retrieved supersaturation will then be used to estimate the hygroscopicity of activated CCN. Thus, this study provides essential information about the size and hygroscopicity of CCN involved in Arctic MPCs.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA063.0020D
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0345 Pollution: urban and regional;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES