Drifting with the clouds

Pursuing a Lagrangian approach means the researchers track changes in a moving air mass instead of observing changes in the atmosphere over a fixed location. “The CMET balloons drift along with the air mass for as long as several days, providing us with data on the humidity, temperature and wind. We can adjust the balloons’ altitude by satellite link, which allows us to capture targeted profiles ranging from a few hundred metres to several kilometres above the ground,” says Felix Pithan. “The first balloon launched in the project was able to take measurements for 17 hours at a time, covering a distance of 642 kilometres. It crossed a large part of the Fram Strait west of Svalbard and drifted over the sea ice for several hours,” reports the AWI researcher enthusiastically. Over the next few months, the team will analyse how the air masses surrounding the balloons cool along the way from the open ocean to the Central Arctic. In this regard, they will also use high-resolution models, which, unlike global climate and weather models, can be used to simulate individual clouds. 

In the course of the five-year project, Felix Pithan and his team plan to use the results to arrive at a better understanding of the accelerated climate change in the Arctic – by the end of this century, climate models project warming of up to 20 degrees in winter and a doubling of precipitation compared to the end of the 20^th century. However, climate models have considerable difficulties when it comes to simulating key processes in the Arctic, such as the transition from liquid to frozen cloud particles. “The transition from liquid to ice clouds is a critical step in the cooling of initially warm air masses over the Arctic. Once we understand this process better, we’ll be able to more precisely predict how much the climate will warm in response to a given CO₂ concentration, because we will have reduced a substantial source of uncertainty concerning climate sensitivity,” says Felix Pithan. 

A further goal is to determine why more water vapour will be transported in the Arctic in the future – is this chiefly because the water content of the incoming air masses is changing, or is the atmospheric circulation changing, transporting more warm and moist air masses to the North Pole? In addition, the team will explore how changes in the Arctic are affecting atmospheric circulation.

For their research, the team will focus on comparatively warm, moist air masses transported to the Arctic. Once there, they cool and lose moisture through cloud formation and precipitation. Then cold, dry air masses are transported back southward, where they absorb heat and moisture over the open ocean. This perspective explains two different types of atmospheric conditions observed in winter during Arctic expeditions like MOSAiC: on the one hand, cloudy conditions, in which liquid water droplets in the clouds protect the sea-ice surface from further cooling; on the other, clear skies, where the sea-ice surface can emit heat into space with very little interference. 

When there is less ice cover in the Arctic in winter, more water evaporates over the open ocean. As such, it seemed likely that this water could be the source of and reason for the increased moisture in the Arctic in winter. “However, a Finnish research team found that the majority of additional evaporated water was rapidly exported back out of the Arctic, and that the additional moisture seemed to be mainly imported from lower latitudes if you tracked the air masses,” Felix Pithan explains. “Accordingly, for the new balloon launches we’re adopting an air-mass-tracking perspective, which will afford us new insights.” Ultimately, Felix Pithan will combine those insights with the standard atmospheric measurements gathered at the AWIPEV station, and with the results of an aerial survey campaign with the AWI research aircraft Polar 6, launching from Longyearbyen, Svalbard, which he’ll join in following his stay at the station.