Measurements and simulations of orographic mixed-phase clouds in Switzerland

Aerosol-cloud interactions constitute the highest uncertainties in radiation forcing estimates. Uncertainties due to the phase and longevity of mixed-phase clouds (MPCs) influence the radiative balance and the hydrological cycle. Due to Wegener-Bergeron-Findeisen-process (WBF) which describes the glaciation of MPC due to the lower saturation vapor pressure over ice than over water, the MPCs are mostly expected to be short-lived. In contrast, in-situ measurements have shown that MPCs can persist over longer time.
We present measurements obtained at the high-altitude research station Jungfraujoch (JFJ, 3580 m asl) in the Swiss Alps partly taken during the CLoud-Aerosol Interaction Experiments (CLACE). During the winter season, the JFJ has a high frequency of super-cooled clouds and is considered representative for being in the free troposphere. In-situ measurements of the microstructure of MPCs have been obtained with the digital imager HOLIMO II, that delivers phase-resolved size distributions, concentrations, and water contents. The data set of MPCs at JFJ shows that for northerly wind cases partially-glaciated MPCs are more frequently observed than for southerly wind cases. The higher frequency of these intermediate states of MPCs at the JFJ suggests either higher updraft velocities, and therefore higher supersaturations with respect to water, or the absence of sufficiently high IN concentrations to quickly glaciate the MPC.

Because of the limitation of the in-situ information, i.e. point measurements and missing measurements of vertical velocities at JFJ, the mechanism of the long persistence of MPCs at JFJ cannot be fully understood. Therefore, in addition to measurements we investigate the JFJ region with a regional model study with a new version of the non-hydrostatic model COSMO that includes the online coupled Aerosol reactive trace gases model (ART) and the aerosol module M7. The combination of kilometer-scale simulations with measurements allows to systematically study the effect of vertical velocity and temperatures on MPCs at JFJ, the synoptic conditions, origins of air masses and aerosol concentration.

 

Speaker: Professor Ulrike Lohmann, ETH Zurich
Time:  27 May 2015, 11.00
Venue:  Yet to be assigned
   


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