How do aerosol-cloud interactions influence the surface mass balance in East Antartica (CLIMB). (CLIMB)

The water cycle, cloud microphysics and cloud-aerosol-interactions are recognized as key elements of the Antarctic climate system by several international consortia, such as the Joint Programming Initiative Connecting Climate Change Knowledge for Europe (JPI Climate) or the International Panel on Climate Change (IPCC). Clouds and aerosols play a significant role in the radiative energy budget and aerosols impact cloud microphysics because they are cloud condensation and ice nuclei. In addition, clouds are an important part of the hydrological cycle serving as the agent linking water vapour transport into Antarctica with precipitation. Because precipitation is the only source term in the surface mass balance of the Antarctic ice sheet, it is one of the key factors affecting sea level. However, current knowledge on the interaction between clouds, precipitation and aerosol in the Antarctic is still limited, both from direct observations and from regional climate models. This is unfortunate, as the Antarctic ice sheet is expected to become a dominant contributor to sea level rise in the 21st century.

To investigate these complex processes, it is necessary to combine different instrumentation and modelling tools. At the Belgian Antarctic research station Princess Elisabeth (PES), such an observatory for aerosol, cloud and precipitation properties exists. The synergy of the data sets has been exploited in first case studies on the effect of aerosols on cloud and precipitation processes with an improved aerosol-cloud-precipitation parameterisation of the regional climate model COSMO-CLM2. First results show a strong sensitivity of cloud microphysics to the number of ice nuclei (IN), and less to the number of cloud condensation nuclei (CCN). This is in accordance with recent measurements in the Arctic. Measurements of IN in Antarctica are, however, rare and challenging because of their very low concentrations. CLIMB proposes therefore to do systematic measurements of ice nucleating particles (INP) at PES. These measurements will be combined with meteorological, aerosol and cloud microphysics observations – both made at PES (1390 m asl) and in-cloud, at the typical precipitating cloud level (around 1 – 2 km higher).

CLIMB’s main objective is to improve the understanding of the climatological effect of CCN and in particular of IN on clouds, precipitation, radiation and the surface mass balance in East Antarctica. CLIMB will generate an improved aerosol-cloud-precipitation parameterisation in a regional climate model for East Antarctica. CLIMB will deliver a unique data set of in-cloud meteorological, aerosol and cloud characteristics, combined with likewise, simultaneous boundary layer measurements. The data set will enable a detailed mapping of air mass origins and thus the transport mechanisms into East Antarctica, relevant for the surface mass balance.

The outcomes of CLIMB will be essential for reliable projections of the influence of Antarctica on the future global sea level. The manifold scientific collaborations based on the measurements of the existing atmospheric observatory at PES will benefit from the new data set and the prolonged time series for aerosol and cloud characteristics. The project results will be valuable for the scientific community and relevant for society and climate policy.

The starting point of CLIMB will be the continuation of the existing atmospheric observatory at PES, including aerosol in-situ properties, cloud occurrence, type and height, precipitation amount and vertically resolved aerosol extinction coefficient. During two austral summers, around 24 filter samples will be collected at PES for INP analysis. CLIMB proposes further to do measurements directly at cloud level – in the Vikinghogda mountains, around 12 km from PES. They are often in cloud, the summit area at 2600 m asl is accessible and sensors can be installed there. The following small-sized and battery-powered sensors will be installed: (i) a sensor for particle number size distribution; (ii) a Lufft smart disdrometer for precipitation type, intensity and droplet/crystal size; (iii) an automated sampling system for semi-volatile and volatile organic compounds; (iv) miniaturised, self-logging sensors for temperature, relative humidity and pressure. If the mountains are in-cloud can be checked continuously with a webcam at PES. A second Lufft smart disdrometer will be installed at PES. The same kind of miniaturised sensors for temperature, relative humidity and pressure will be installed at PES (1390 m asl) and on a mountain outcrop between PES and the Vikinghogda mountains (1600 m asl). Radio soundings will complete the vertical meteorological observations. The regional climate model COSMO-CLM2 will be run with the new data set in order to improve its parameterisation and to investigate the climatological effect of INP on clouds, precipitation, radiation and the surface mass balance in East Antarctica. The back trajectory and dispersion models FLEXTRA and FLEXPART will be applied in order to analyse air mass origin and transport mechanisms into Antarctica.