N2 – Low-level clouds
Stratocumulus clouds cover large areas of the oceans and strongly affect Earth’s energy balance and tropical circulations. Yet their future response to climate change is highly uncertain. Some studies predict a strong positive feedback and even the disappearance of stratocumuli, while others suggest they may be more resilient. Reducing this uncertainty is crucial for reliable climate projections. This is challenging because clouds are a multiscale phenomenon with key processes ranging from the micrometer scale of cloud droplets up to feedbacks on the global scale.
Our project therefore asks: How do interactions between small- and large-scale processes shape stratocumuli in future climates, and under which conditions might drastic changes occur?
We approach the question by combining detailed field measurements with modeling across a wide range of spatial scales. In this way, we hope to deliver the first robust constraints on the role of stratocumuli in future climate.
The research is structured into two interconnected work packages:
Work package 1: Local processes
Lead: Juan Pedro Mellado / Felix Ament
We investigate the processes that govern the break-up of stratocumuli into cumulus clouds with far less cloud cover. These processes span scales from meter-scale turbulence and raindrop dynamics to mesoscale circulations and the large-scale environment.
A central element is the STACCATO field campaign (South East Atlantic, 2028), which will provide unprecedented observations of mesoscale organization, diurnal cycles, and cloud-top processes. Airborne radars and satellite data will measure vertical motion, droplet properties, and mesoscale circulations. These observations will be combined with direct numerical simulations at meter resolution and ICON simulations at kilometer resolution. Together, they will test hypotheses about the stabilizing role of the inversion, the effects of precipitation and cold pools, and the mechanisms behind the closed-to-open cell transition.
Work package 2: Global response
Lead: Stefan Bühler / Sarah Kang
We explore how stratocumuli respond to rising CO2 and sea surface temperatures in global climate simulations, and how these changes feed back on radiation, tropical circulation, and rainfall. Using ensembles of global kilometer-scale ICON simulations, we will vary physical assumptions (e.g. turbulence, radiation coupling, microphysics) to map the plausible range of outcomes.
A key novelty is the integration of observational constraints from work package 1 and STACCATO into the global simulations, narrowing uncertainties and improving confidence in projections. This will allow us to determine whether extreme reductions in cloud cover — and the associated strong warming — are plausible, or whether stratocumuli are more resilient than some models currently suggest.