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HALO-(AC)³ crew

HALO-(AC)³ crew during a measurement flight in the cabin of DLR's HALO research aircraft.

Credit: © Marlen Brückner / University Leipzig

In mid-March 2022, the large-scale international HALO-(AC)3 research campaign will begin investigating transformations of air masses in the Arctic. Three German aircraft will be deployed, scientists from the UK and France will also be involved during joint flights with two further aircraft. The team of researchers will be focusing particularly on northwards-flowing warm air reaching into the central Arctic, which are often called warm air intrusions. The counterpart, cold air outbreaks with southwards-moving cold air from the Arctic, will also be investigated. The goal is to investigate the processes causing the above-average increase in temperatures in the Arctic during the last decades. At two to three degrees Celsius over the last 50 years, this increase is much larger than the warming that has taken place in other regions on Earth. This phenomenon is referred to as Arctic amplification. The effects of this increase in temperature are not limited to the climate system in the Arctic, but are suspected to modify the regional weather in the mid-latitudes, as well. Thus, the HALO-(AC)³ campaign aims to contribute to a better understanding of the processes behind the dramatic climate changes that are currently taking place in the Arctic.

During the first measuring flights since 12 March 2022 a massive warm air intrusion into the Arctic has been observed. At this event, several unusual phenomena have been detected, such as heavy rain over sea ice and massive clouds, reaching almost as high as in the tropics. With the arrival today of further research aircraft, planned coordinated measurement flights will begin on 19 March 2022, to better understand the complexity of these events.

Manfred Wendisch from the Institute for Meteorology at Leipzig University is the scientific coordinator of the five-week HALO-(AC)³ measuring campaign, which will study changes in air masses on their way into and out of the Arctic. What exactly happens to these air masses, particularly with regard to cloud formation, will be observed in detail using state-of-the-art instruments. These changes to air masses cannot be characterised by local, ground-based instruments, as there are only a limited number of meteorological measurement stations in the central Arctic region. For this reason, three measurement aircraft will be applied as part of HALO-(AC)³ to observe the changes of the air masses on their way into and out of the Arctic. The researchers aim to determine changes to the air masses using a quasi-Lagrangian observation method. In this technique, each flight pattern is adapted to the direction of movement of the relevant air mass, allowing changes to clouds, humidity, temperature and many other parameters to be measured directly. The data obtained in this way will be used to assess the accuracy of numerical weather forecasting models, which are necessary for predicting the future changes of the Arctic climate. Thus, the campaign will help to close an important gap in climate research knowledge, which was identified by the Intergovernmental Panel on Climate Change (IPCC) in the second part of its latest assessment report.

Manfred Wendisch summarises the goal of HALO-(AC)³ as follows: "Predicting the future of the Arctic climate remains a challenge. We want to carry out an extensive flight campaign – HALO-(AC)³ – that uses innovative observation methods to help reduce major uncertainties in the projection of future climate development in the Arctic."

Coordinated measuring flights with three research aircraft

Three German research aircraft will be used to perform measurements for HALO-(AC)³. The first is HALO (High-Altitude and Long-Range Research Aircraft), a modern research aircraft operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). "HALO will operate at higher altitudes as a remote sensing platform, as it is capable of covering distances of up to 10,000 kilometres at altitudes of up to 15 kilometres," explains Andreas Minikin of the DLR Flight Experiments facility. Meanwhile, in tandem with HALO, the Polar 5 and Polar 6 aircraft will conduct detailed measurements of the air masses at lower altitudes. The two polar aircraft have been used in the Arctic for more than 10 years under the direction of the Alfred Wegener Institute (AWI) Helmholtz Centre for Polar and Marine Research. Covering distances of between 1500 and 2000 kilometres, they will complement HALO by conducting measurements at altitude ranges below six kilometres. Andreas Herber, an AWI researcher and the coordinator for Polar 5 and Polar 6, adds: "Although the polar aircraft have a shorter range, they have the major advantage of being able to fly 'low and slow', so they can capture snapshots of very specific processes taking place in, below and above clouds, and in the planetary boundary layer."

The aircraft are equipped with state-of-the-art instrumentation that can survey the entire atmosphere from the ground up to an altitude of 10 kilometres. The most important measurement parameters include cloud properties, temperature and humidity profiles, energy fluxes and the properties of aerosol particles and trace gases. Susanne Crewell, an atmospheric researcher at the University of Cologne, explains: "By coordinating the flight patterns of the three aircraft, we can track the air masses as they evolve in space and time. These measurements make it possible to take a closer look at the smallest cloud structures, all the way down to individual cloud particles, and to investigate the effect of Arctic sea ice on cloud properties. By combining various measurements, we will be able to obtain an almost complete picture of the air masses under investigation." Important tools for this are what are referred to as 'dropsondes', which are released from the aircraft and descend to the ground on small parachutes. As they descend through the atmosphere, they acquire measurements of temperature, air pressure and humidity.

In parallel with the aircraft measurements, the Leibniz Institute for Tropospheric Research (TROPOS) and the Leipzig University will conduct profile measurements using a tethered balloon at the AWI-research station AWIPEV near Ny-Ålesund on Svalbard. In addition to meteorological measurements, small-scale exchange processes, radiation and aerosol parameters will also be examined from the ground up to a height of one kilometre. A first set of these measurements were collected in autumn 2021 during the transition to the polar night and will now be repeated at the transition to the polar day, to link ongoing ground measurements with aircraft measurements. The extensive data set acquired by the HALO-(AC)³ campaign will be enhanced by measurements from ground-based remote sensing instruments at the AWIPEV research station, together with the latest satellite remote sensing methods and state-of-the-art numerical climate models.

Studying clouds with lidar and radar

A team from the DLR Institute of Atmospheric Physics in Oberpfaffenhofen is participating with a comprehensive set of instruments for determining cloud properties on Polar 6 in Svalbard. In addition, DLR operates a lidar instrument and, together with other partners, a cloud radar on HALO. "The focus of the measurements is primarily on the increased moisture input into the Arctic due to the increased exchange of air masses with the mid-latitudes and the resulting changes in the atmosphere – particularly altered cloud properties," explains DLR atmospheric researcher Silke Groß. Polar 6 is also used specifically to investigate the differing cloud properties over sea ice and the open ocean.

AWIPEV research base

View of the observatory of the German-French research base AWIPEV in Ny Ålesund, Svalbard, with the lidar system KARL. A laser beam leaves the observatory through the lidar hatch.

Credit: © Alfred-Wegener-Institut / Rene Bürgi