This mechanism explained the behavior of climate models with particularly high climate sensitivity, i.e., a very large increase in the mean global surface temperature as a result of a doubling of the atmospheric CO2 concentration. In a study published in Nature this week, Raphaela Vogel and her colleagues Anna Lea Albright, Jessica Vial, Geet George, Bjorn Stevens and Sandrine Bony refute this ‘mixing-desiccation’ hypothesis using data collected in the recent EUREC4A field campaign.
EUREC4A stands for Elucidating the role of clouds-circulation coupling in climate and was specifically designed to test this hypothesized cloud feedback mechanism. During the one-month field campaign in early 2020, more than 800 dropsondes were dropped from the German HALO research aircraft and used by the authors to estimate the convective mass flux at cloud base. At the same time, the cloud-base cloud fraction was measured with a lidar and radar which was aligned laterally from the French research aircraft ATR. This allowed the atmosphere to be analyzed simultaneously from multiple angles.
With these novel observations, the authors demonstrate that the mixing-desiccation mechanism is not active in nature. Mesoscale motions (20-500 km) and the entrainment rate, which denotes the mixing in of drier ambient air from higher layers, contribute equally to variability in mixing. However, since they have opposite effects on humidity, deeper mixing associated with increased mass flux does not desiccate clouds at their base. As the observations are at the ~220 km scale of a typical climate model grid box, they provide a physical test of the capacity of climate models to represent the interplay of the processes regulating trade cumulus clouds. The comparison shows that within a model, the magnitude, variability, and coupling of mixing and cloudiness behave coherently across timescales: from day-to-day variability of weather conditions (here: synoptic variability of air masses) to climate change scales. These relationships, however, vary strongly among climate models, and regarding the synoptic response, they differ strongly from the EUREC4A observations. Models with large trade cumulus feedbacks tend to represent a mixing-desiccation mechanism that is not apparent in the observations, and also exaggerate variability in cloudiness. The data render those models implausible, as they physically refute a strong trade cumulus feedback.
By showing that mesoscale motions inhibit the mixing-desiccation mechanism, the study refutes an important physical hypothesis for a large trade cumulus feedback and thus a critical line of evidence for a large climate sensitivity. This is good news, as it shows that extreme increases of Earth’s surface temperatures with warming are less likely.
Bjorn Stevens, Director of the department “The Atmosphere in the Earth System” at the Max Planck Institute for Meteorology and co-author of the study, says: “It is quite rare that low cloud feedbacks, long an enigma for climate science, could be broken down to a concrete hypothesis that is directly testable with observations. Bringing the science to this level was an achievement we are very proud of.”
Original publication:
Vogel, R., Albright, A., Vial, J., George, G., Stevens, B., Bony, S. (2022), Strong cloud-circulation coupling explains weak trade cumulus feedback. Nature, doi.org/10.1038/s41586-022-05364-y.
Contact:
Dr. Raphaela Vogel
Universität Hamburg, Meteorologisches Institut
Email: raphaela.vogel@uni-hamburg.de
Prof. Dr. Bjorn Stevens
Max Planck Institute for Meteorology
Email: bjorn.stevens@mpimet.mpg.de