Fairweather cumulus are here to stay after all!

Some climate models predict that the trade wind cumuli will decrease significantly: as temperatures increase, the clouds transport more moisture to higher layers of air. Conversely, drier air descends from above. The absolute humidity thus increases overall, but the relative humidity in the lowest layers of air close to the cloud base would decrease. This makes a less favorable environment for cloud formation. And if cloudiness decreases in general, less sunlight is reflected overall, which leads to more warming, more mixing-induced drying, fewer clouds, and so on. Climate models that predicted higher than average warming with rising CO₂ tended to express this mixing desiccation hypothesis more markedly than models that predicted lesser warming.

The German-French-Barbadian EUREC⁴A field campaign was conducted from the Caribbean island of Barbados in February 2020. It was designed and led by Prof. Bjorn Stevens, Managing Director of the MPI-M, and Dr. Sandrine Bony, Research Director at the LMD. The aim of the field campaign was to test the ideas stemming from analyses published by their respective groups in 2013 and 2014. Building on a now 12-year collaboration between the MPI-M and the Caribbean Institute for Meteorology and Hydrology (CIMH), led by Dr. David A. Farrell, as well as a series of earlier field campaigns, the researchers and their teams developed the methods EUREC⁴A would need to successfully test the mixing-desiccation hypothesis.

Two early career researchers were on site in Barbados: Dr. Geet George, who was working on his thesis with Bjorn Stevens at the MPI-M at the time, and Dr. Raphaela Vogel, who after finishing her PhD at the MPI-M had moved to Paris to work as a postdoctoral research with the team of Dr. Bony at LMD. She is now a research scientist at the University of Hamburg. “We performed the first real measurements to test the hypothesis,” said Dr. Vogel. The results proved to be a treasure trove of data overturning the prevailing assumption about the behavior of subtropical cumuli. In order to be able to make a statement about the large-scale cloud circulation on the mesoscale, “you have to measure the boundary conditions from an altitude and the clouds at their base at the same time. For that, it helps to have two aircraft,” explained Dr. George.

Accordingly, the German research aircraft HALO flew at an altitude of 10,000 meters. The data were gathered with the help of dropsondes launched through a shaft in the aircraft – Geet George’s role in the campaign. “During the flights, I was busy in the back of the aircraft dropping out a sensor package suspended by a parachute, or dropsondes, if you want to put it technically,” he said, describing the flight days on site. As the dropsondes descend through the cloud layer to the ground they radio the ambient temperature, humidity and position back to the aircraft where it is recorded.

The French research aircraft, fondly known as the ATR-42, employed a newly developed technique to “scan” the cloud base horizontally. This was done by emitting pulsed radio and light beams that would be scattered back by cloud droplets. The backscattered radiation allows conclusions to be drawn about the presence of clouds and rain. All in all, five research aircraft, four research vessels and advanced, ground-based remote sensing technology were deployed in the field study.

The mixing of air in the clouds was of particular interest. At approximately 200 km in diameter, the flight area was too large to measure this directly, so an alternative, indirect approach was required. The approach was proposed by Drs Bony and Stevens and initially tested using data from a preparatory field campaign in 2016. Dr. Vogel refined the approach with the help of detailed fluid dynamical simulations that she prepared and analyzed at the start of her PostDoc at LMD. “We determined the thickness of the lowest atmospheric layer, referred to as the sub-cloud layer, as well as all parameters that can affect this thickness. When doing so, we focused on the effect of the mesoscale circulation and the mixing in of warmer, drier air from above – in other words: what goes into the sub-cloud layer and what goes out of it. From the balance of this mass budget, the strength of the convection can ultimately be determined: if more goes in than out, more air rises in the clouds. If the reverse is true, it is less,” explained Dr. Vogel.