Sensitivity estimations for cloud droplet formation in the vicinity of the high-alpine research station Jungfraujoch (3580 m a.s.l.) release_vfiseiya3zgbtb2aqpldlntlwi

by Emanuel Hammer, N. Bukowiecki, B. P. Luo, Ulrike Lohmann, Claudia Marcolli, E. Weingartner, U. Baltensperger, Christopher Hoyle

Published in Atmospheric Chemistry and Physics by Copernicus GmbH.

2015   Volume 15, Issue 18, p10309-10323

Abstract

<strong>Abstract.</strong> Aerosol radiative forcing estimates suffer from large uncertainties as a result of insufficient understanding of aerosol–cloud interactions. The main source of these uncertainties is dynamical processes such as turbulence and entrainment but also key aerosol parameters such as aerosol number concentration and size distribution, and to a much lesser extent, the composition. From June to August 2011 a Cloud and Aerosol Characterization Experiment (CLACE2011) was performed at the high-alpine research station Jungfraujoch (Switzerland, 3580 m a.s.l.) focusing on the activation of aerosol to form liquid-phase clouds (in the cloud base temperature range of −8 to 5 °C). With a box model the sensitivity of the effective peak supersaturation (SS<sub>peak</sub>), an important parameter for cloud activation, to key aerosol and dynamical parameters was investigated. The updraft velocity, which defines the cooling rate of an air parcel, was found to have the greatest influence on SS<sub>peak</sub>. Small-scale variations in the cooling rate with large amplitudes can significantly alter CCN activation. Thus, an accurate knowledge of the air parcel history is required to estimate SS<sub>peak</sub>. The results show that the cloud base updraft velocities estimated from the horizontal wind measurements made at the Jungfraujoch can be divided by a factor of approximately 4 to get the updraft velocity required for the model to reproduce the observed SS<sub>peak</sub>. The aerosol number concentration and hygroscopic properties were found to be less important than the aerosol size in determining SS<sub>peak</sub>. Furthermore turbulence is found to have a maximum influence when SS<sub>peak</sub> is between approximately 0.2 and 0.4 %. Simulating the small-scale fluctuations with several amplitudes, frequencies and phases, revealed that independently of the amplitude, the effect of the frequency on SS<sub>peak</sub> shows a maximum at 0.46 Hz (median over all phases) and at higher frequencies, the maximum SS<sub>peak</sub> decreases again.
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