How do airborne particles, so-called aerosols, affect the formation and life cycle of clouds and precipitation? In order to come one step closer to solving this question, atmospheric scientists from the Leibniz Institute for Tropospheric Research (TROPOS) and the Leipzig Institute for Meteorology (LIM) at Leipzig University will observe the atmosphere at one of the cleanest places in the world for at least a year. For the DACAPO-PESO project, they transported two containers with extensive measurement technology from Leipzig to Punta Arenas on the Strait of Magellan in southern Chile. There, the two institutes cooperate with the Laboratory of Atmospheric Physics at Magellan University (UMAG), which, in addition to providing the infrastructure for the Leipzig instruments, also contributes its own measurements to the project. The choice fell on Punta Arenas because the city is located on a comparable geographical latitude as Germany and will thus enable comparisons between the northern and southern hemispheres. The measurement campaign is part of the International Year of Polar Prediction (YOPP), which aims to improve weather and climate forecasts for the polar regions through intensive measurements.
"The experiment will allow us to explain more precisely the observed regional differences in the efficiency of ice formation in clouds and to describe the role of aerosols in more detail," says Dr. Patric Seifert, scientist at TROPOS and initiator of DACAPO-PESO. "The synergies of the versatile remote sensing instruments and the development of new evaluation algorithms based on them will provide more precise insights into the clouds. One objective will be, to clarify whether the ice crystals in the virtually aerosol-free atmosphere of Punta Arenas grow more frequently through riming in liquid water layers than through aggregation as compared to the northern hemisphere," explains Junior Prof. Heike Kalesse from Leipzig University.
The measuring station in Punta Arenas was set up in November and December 2018 by three scientists from TROPOS and UMAG. Patric Seifert returned to Leipzig shortly before Christmas and is satisfied with the progress made so far: "All instruments are working reliably and the first interesting observations have already been made. Pure liquid water clouds were observed at temperatures of around -20°C, which is extremely rare during comparable measurements in Germany. This could be an indication for few ice germs in the southern hemisphere."
The formation of clouds and precipitation essentially depends on three parameters: Humidity, temperature, as well as the availability and type of aerosol particles that serve as nuclei for cloud drops and ice crystals. Even though it is widely accepted that water vapour and temperature dominate cloud processes, scientists still debate about the actual influence of aerosol particles on the weather. It is known that every cloud droplet and every ice crystal that forms at temperatures higher than -40°C requires the availability of a drop or ice nuclei.
In Germany and other regions in the mid-latitudes of the northern hemisphere, most precipitation (in the free troposphere) occurs between two and twelve kilometres above the ground. These air layers are characterised by aerosol particles from man-made air pollution, desert dust and forest fires. In the mid-latitudes of the southern hemisphere, these particles are largely absent because there are more oceans and much less industry, deserts and forests. If the amount of aerosol particles available in the atmosphere changes, the amount of cloud droplets and ice crystals formed also changes. It can thus be explained that liquid water clouds over regions of Europe or Southeast Asia that are heavily contaminated with aerosols consist of considerably more droplets. In contrast, liquid water clouds in less polluted regions such as over the oceans or polar regions consist of fewer and often larger droplets.
A similar influence of aerosol particles on the formation of ice crystals has already been observed by TROPOS scientists in clouds above Leipzig: Clouds formed under the influence of Sahara dust form ice more frequently and at higher temperatures than clouds formed in clean air. The life cycle of a cloud thus takes a different course because the formation of the tiny ice crystals and cloud droplets are the beginning of numerous processes that accompany the development of the cloud and ultimately lead to the formation of precipitation.
The knowledge about the connections between aerosol particles and clouds comes mainly from measurements in the more polluted northern hemisphere. However, relatively little is known in detail about how these processes take place under the much cleaner conditions of the southern hemisphere. Therefore, the idea for the field experiment DACAPO-PESO (Dynamics, Aerosol, Cloud and Precipitation Observations in the Pristine Environment of the Southern Ocean) was born at TROPOS, which should provide a reference data set in the Southern midlatitudes. In order to be able to compare the situation in the northern hemisphere with that in the southern hemisphere, the choice was therefore made for a location that lies approximately at the same latitude as Germany and therefore has similar temperature and climate conditions. With the exception of South America, the southern hemisphere is covered by oceans in these latitudes and long-term measurements at sea are not practicable to this extent. Therefore, the choice fell on Chile, where the researchers from Leipzig (52°N) found a cooperation partner with Magellan University in Punta Arenas (53°S).
South America is currently in the focus of international atmospheric research: Almost 3000 km to the north, a large-scale measurement campaign by various universities and institutions from the USA is currently undergoing in Argentina and southern Brazil: RELAMPAGO-CATI investigates the formation of thunderstorms to improve the prediction of hail and tornadoes.
In recent years, the available techniques for the continuous observation of aerosols and clouds have progressed extremely. Lidar and radar equipment such as the Leipzig Aerosol and Cloud Remote Observations System (LACROS) can be used to record the structure of cloud and aerosol layers as well as precipitation from the ground with high resolutions in the range of seconds and meters. In the DACAPO-PESO measurement campaign, a total of three lidar systems are used to record aerosol properties. In addition, three differently configured Doppler radar systems observe the structure of clouds and precipitation. The radars and a so-called Doppler lidar also provide information on the vertical movement of the air, which is so important for cloud formation. These vertical movements lead to cooling. This increases the relative humidity in the air and cloud formation can occur. Rain sensors record the size and type of precipitation arriving on the ground, a microwave radiometer determines the amount of water vapour and liquid water present in the atmosphere, and radiation measuring instruments document the influence of aerosols and clouds on the energy arriving on the Earth's surface. Most of the instruments are part of the LACROS station of the TROPOS, which is during DACAPO-PESO extended by a new Doppler radar of LIM and a lidar of the UMAG.
All measurements together provide a detailed picture of the weather in the clean atmosphere over Punta Arenas. The long time series of measurements of at least one year will provide a comprehensive data set. This will serve the scientists as a reference for the comparison with the weather conditions in the highly aerosol-burden temperate latitudes of the northern hemisphere. The evaluation is partly carried out within the framework of the European infrastructure projects ACTRIS, Cloudnet and EARLINET as well as in cooperation with the Consortium of the Year of Polar Prediction (YOPP), the Priority Programme SPP2115 "PROM" of the German Research Foundation (DFG) and a project of the European Social Fund (ESF) led by LIM.
Since TROPOS was founded in the 1990s, researchers at the institute have been working on the question of how aerosols and clouds interact. They discovered, for example, that ice forms in clouds in the northern hemisphere at much higher temperatures than in clouds in the southern hemisphere. This was the result of lidar-assisted investigations at TROPOS, UMAG in Chile, Stellenbosch University in South Africa and during transatlantic cross-sections of the research vessel Polarstern, which were already collected in 2008-2010. In Central Europe, around 70 percent of clouds already form ice at temperatures around -18 degrees Celsius. In southern Chile and South Africa, on the other hand, only 20 and 35 percent respectively form ice at the same temperature. The reason for such a contrast is most probably the larger number and greater diversity of aerosol particles floating in the air in the northern hemisphere.
The two project coordinators Seifert and Kalesse are confident that DACAPO-PESO will significantly increase the level of knowledge on the interaction between aerosols, clouds, and precipitation, especially due to the good personnel support. Thanks to approved projects of the DFG, the ESF and own funds of TROPOS and UMAG, the DACAPO-PESO team already comprises 4 experienced researchers and 4 PhD students. The data set will provide an unprecedented wealth of information on aerosols and clouds in the mid-latitudes of South America.
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