Abstract View
Analysis of Cloud Condensation Nuclei Number, Hygroscopicity and Cloud Formation Over the Swiss Alps
PARASKEVI GEORGAKAKI, Aikaterini Bougiatioti, Jörg Wieder, Zamin A. Kanji, Fabiola Ramelli, Jan Henneberger, Claudia Mignani, Maxime Hervo, Alexis Berne, Ulrike Lohmann, Athanasios Nenes, LAPI, EPFL (Switzerland) / ICE-HT, FORTH (Greece)
Abstract Number: 363
Working Group: Aerosols, Clouds and Climate
Abstract
Orographic mixed-phase clouds (MPCs) provide a natural laboratory to investigate the complex web of interactions and feedbacks between atmospheric dynamics, cloud microphysics and terrain. Due to their frequent occurrence over mountain barriers, orographic MPCs play a major role in alpine weather, hydrology and climate. Here we analyze observations collected in the Alpine region of Davos, Switzerland during the Role of Aerosols and Clouds Enhanced by Topography on Snow (RACLETS) field campaign during February and March 2019. Observations of aerosol size distributions, cloud condensation nuclei, hygroscopicity and lidar-derived vertical velocities coupled with a state-of-the art droplet activation parameterization are used to quantify how changes in aerosol and dynamical parameters modulate the predicted droplet number and maximum supersaturation developed in the MPCs. The proposed method performed remarkably well yielding a droplet closure within ~25%. Strongly influenced by the velocity distribution (expressed by its standard deviation, σw) that ranges from 0.1 to 0.6 ms-1, droplet formation is driven by aerosol variability (aerosol-limited regime) when aerosol levels are between 300 and 1500 cm-3. We also find that droplet number never exceeds a characteristic limit of ~150-450 cm-3, which depends solely on the value of σw. The mean CCN-derived hygroscopicity parameter value is ~0.25, which is representative of continental aerosol. Fresh emissions from anthropogenic activities in the valley floor in Davos, apart from increasing cloud droplet number, reduce cloud supersaturation and make the clouds insensitive to aerosol and sensitive to vertical velocity variations. The hourly mean values of σw at the high mountaintop site are greater by 0.1 ms-1 than the corresponding values recorded at the valley site, indicating that clouds forming are less susceptible to aerosol fluctuations. Identifying regimes where dynamics can potentially overshadow variations in aerosol concentration is key for interpreting and constraining the aerosol indirect effects.