AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Measured and Modelled Cloud Droplet Activation of Aerosol Particles at the High-Alpine Research Station Jungfraujoch
MARTIN GYSEL, Christopher R. Hoyle, Clare S. Webster, Harald E. Rieder, Athanasios Nenes, Emanuel Hammer, Erik Herrmann, Nicolas Bukowiecki, Ernest Weingartner, Martin Steinbacher, Urs Baltensperger, Paul Scherrer Institute
Abstract Number: 384 Working Group: Aerosols, Clouds, and Climate
Abstract The interaction of aerosol particles with warm clouds has been studied at the high-alpine research station Jungfraujoch in a series of Cloud and Aerosol Characterisation Experiments (CLACE). Here we assess the sensitivity of cloud droplet number concentration (CDNC) to variability of updraft velocity and potential cloud condensation nuclei (CCN) number concentration. The cloud droplet number concentration was inferred from the total (including cloud droplet residuals) and interstitial (only non-activated) particle number size distributions measured in-situ within liquid clouds by means of two specific inlets. No direct measurement of updraft velocity at cloud base is available, however, a comparison of detailed microphysical box-model simulations with the in-situ measurements showed that the updraft can be estimated in reasonable approximation from the surface wind measured at the observation site.
In a first approach we used a regression analysis of the 2399 data points from 4 field campaigns to develop a simple statistical model to predict the CDNC. The number concentration of potential CCN alone is sufficient to explain 79% of the variance of observed CDNC. The model performance can be slightly improved by including additional predictors related to aerosol emissions. Including an explicit dependence on updraft velocity does not improve the correlation, indicating that cloud formation mostly occurs in the CCN-limited regime.
The droplet formation regime was further assessed by applying a state of the art cloud droplet formation parametrization to the aerosol size distribution and updraft data set. Excellent agreement between simulated and measured CDNC with high correlation and almost equal mean values was achieved. Sensitivity analyses using this parametrization confirmed that cloud formation at the Jungfraujoch with considerable orographic forcing mostly occurs in the CCN limited regime, i.e. CDNC is approximately linearly dependent on available CCN, while variations of updraft only become limiting for small updraft <~1 m/s.