HALO | Scientific News from AWARES

As we reported earlier in this blog, we participated in the airborne field campaign HALO-(AC)³ In March and April 2022. The goal of the campaign was to improve the understanding of the airmasses transformation when transported into or out of the Arctic. Two types of airmass transports were of particular interest: First, moist and warm air intrusions that transport high amounts of heat and moisture from the mid-latitudes into the Arctic. Second, marine cold air outbreaks that lead to the formation of cloud streets and convective cells when cold and dry air from the central Arctic is transported southwards over the relatively warm North Atlantic. In our study, we analyse the weather (and sea ice) conditions during the HALO-(AC)³ campaign.

Map of the study area of the HALO-(AC)³ campaign including the flight tracks of the research aircraft HALO, Polar 5 (P5) and Polar 6 (P6). The study area has been separated into three subregions.

We separated the campaign into a warm and a cold period with the help of northwards humidity transport (IVT) and the so-called cold air outbreak index (MCAO index). The cold air outbreak indicates the strength of the temperature difference between the surface and the lower atmosphere. High differences suggest cold air outbreak conditions with strong interactions between the cold ocean and the atmosphere. The warm period was dominated by northward winds and warm air intrusions while the cold period featured several cold air outbreaks.

(a) Northward water vapour transport (IVTnorth) and (b) marine cold air outbreak (MCAO) index for the campaign duration in 2022 (black line). Grey shading indicates the climatology over the years 1979-2022. The red box shows the warm period, while the blue box illustrates the cold period.

During an extremely strong warm air intrusion, record breaking near-surface temperatures occurred in the central Arctic compared to the March 1979-2022 climatology. Also at Ny-Ålesund, the weather station recorded the highest near-surface temperatures for March since the beginning of the measurements in 1975. This warm air intrusion was detected as so-called Atmospheric River, a thin but long band of extremely strong moisture transport. Over the sea ice northwest of Svalbard, record breaking rainfall rates occurred.

Average 2 m temperature in March 2022 north of 80°N (red line). Thin black lines show the temperature for each year between 1979 and 2022 and the thick black line illustrates the average over those years.

At the beginning of the cold period, a strong cold air outbreak led to an extremely dry atmosphere over Ny-Ålesund with integrated water vapour content of just 1.1 kg m-2 (24 March 2022). Less than 3 % of all radiosondes launched since 1993 recorded drier conditions.

Humidity measurements from radiosondes (weather balloons) launched at Ny-Ålesund (Svalbard) during HALO-(AC)³. The colours indicate the specific humidity (fraction of water vapour mass to total air mass) while the black line shows the total humidity content of the troposphere (lowest layer of the atmosphere).

During the cold period, we also observed the Arctic version of a hurricane, a Polar Low. Polar Lows are characterised by convective (cumulus) clouds, relatively strong winds (at least gale force) and precipitation, while extending only over a few 100 kilometers. They also have a relatively cloud free centre like the eye of a hurricane. We analysed the environmental conditions for the formation of the Polar Low.

Photo taken from the research aircraft HALO during the flight to observe the Polar Low.

Luckily, the weather conditions were quite favourable to achieve the goals of the campaign because we could capture both types of airmass exchange between mid-latitudes and the Arctic. The publication has been submitted to the European Geosciences Union journal Atmospheric Chemistry and Physics.

What a successful day! P5, P6 and HALO had their first joint research flight on Sunday!

If you ask yourself “What is HALO? And what is special about a joint flight?” then read on…

First, let’s start with something you already know from the previous posts: P5 and P6 are polar aircraft, which participate in the HALO-(AC)³ campaign and which are stationed in Longyearbyen, Spitsbergen. The instruments belonging to the University of Cologne, namely the radar MiRAC and the microwave radiometer HATPRO, are installed on P5, which is a remote sensing aircraft. Contrary, P6 is equipped with in-situ instruments recording atmospheric parameters and cloud particles near the aircraft. Thus, P5 is mainly flying above the clouds at an altitude of around 3 km, whereas P6 is flying at lower altitudes directly through the clouds. Joint flight activities of the two polar aircraft provide in-situ and remote sensing observations of a lot of different parameters from the same cloud at the same time!

HALO at the airport in Kiruna, Sweden. (Photo: Henning Dorff)

Next, HALO comes into play. HALO stands for “High Altitude and Long Range Research Aircraft” and is operated by the German Aerospace Center (DLR). HALO carries remote sensing instruments similar to P5, but is able to fly at higher altitudes of about 10 km and travel much longer distances. During HALO-(AC)³ it is stationed in Kiruna, the northernmost city of Sweden, together with other scientists from our group. The aim is to observe how air masses change in the Arctic by measuring the temperature, humidity, cloudiness or aerosol concentration along the wind direction.

As often as possible, joint flights with all three aircraft will be realized, which is really difficult to coordinate! Another difficulty are the bad weather conditions in Longyearbyen making flights with the polar aircraft impossible. And sometimes the different platforms are interested in different clouds or air masses.

During the first joint flight, the flight conditions were very calm! Our instruments worked well and observed precipitating cloud structures over sea ice and open water. In one of the following posts we will have a first look at the observations!

If you are interested in a video diary with short daily updates on the work routine in Longyearbyen, just follow AWIexpedition on Instagram!

Scientific News from AWARES

The Atmospheric Water Cycle and Remote Sensing group from the University of Cologne

Tag Archives: HALO

Weather conditions during the HALO-(AC)³ campaign

Long-awaited first research flight: Joint adventure with three aircraft!