Measurements of cloud-aerosol interactions in ice and mixed phase clouds during CLACE campaigns at the Jungfraujoch mountain top site, Switzerland

In this paper we present results obtained form 2 campaigns conducted as part of the Ice NUcleation Process Investigation And Quantification (INUPIAQ) project to investigate the role of a range of aerosol types in regulating the ice crystal number concentration alongside secondary ice processes in natural supercooled cloud enveloping the summit of Jungfraujoch (JFJ) in Switzerland. The campaigns were conducted at the JFJ summit site (3580m asl) only in January and February 2013, and at the summit site and an upwind site (at Schilthorn 2970 m asl), using the cloud as a natural reaction chamber, in January and February 2014.

INUPIAQ is a NERC funded UK project contributing to an ongoing International series of CLoud-Aerosol Interaction Experiments (CLACE) held at the Jungfraujoch site, involving participating groups from several European countries (the CLACE team). In this paper we will focus mainly on measurements of cloud microphysics and aerosol, together with WRF modelling undertaken as part of INUPIAQ. At the summit site, measurements were made of the liquid and ice phase microphysics including measurement of cloud: particle number; particle size distributions and ice crystal habit; liquid and ice water contents (the latter measured and/or derived). For the 2014 project, WRF modelling was also carried out of the airflow in the region and of the cloud microphysical development, all prior to the 2014 field experiment. On the basis of this modelling it was concluded that the Schilthorn site (to the WNW of JFJ) was frequently below cloud base when the JFJ summit was in cloud with good flow connection between the sites. Hence, Schilthorn was used as the upwind site (in a northerly flow situation) and equipment was deployed for measuring the properties of the aerosol prior to interaction with the cloud. Aerosol measurements included: aerosol size distribution; aerosol chemical composition (using an Aerodyne compact ToF Aerosol Mass Spectrometer, a DMT Single Particle Soot Photometer (SP2) for black carbon and aerosol coating characterisation, a WIBS device for measurement of biological aerosol - all at high time resolution, and with filters for offline analysis of non volatile components). A Halo Photonics Lidar (from NCAS) was also deployed just upwind of the JFJ summit at Kleine Scheidegg (2061 m asl), to provide data on cloud base height and cloud structure at JFJ and sites between JFJ and Schilthorn (key to interpreting changes in the summit microphysics) and to provide data to test WRF model simulations

It was found that the phase partitioning of the cloud at the summit varied substantially between individual cloud events and within cloud events as they evolved. The cloud varied between consisting mostly of supercooled water to being nearly totally glaciated. The cause of these long term changes will be discussed in relation to changes in the aerosol entering the cloud.

It was also observed that very rapid changes between predominantly glaciated and predominantly supercooled liquid cloud also occurred. These are partly likely due to the effects of secondary ice processes occurring and partly to the changes in the structure of the cloud arriving at the summit site, and will also be discussed.

Results from the combined cloud and dynamics model WRF, and other models, will be used as an aid to interpreting the data.