Skip to main content

How nature's smallest clouds slow down large-scale circulations critical for climate

Periodic Reporting for period 2 - CloudBrake (How nature's smallest clouds slow down large-scale circulations critical for climate)

Reporting period: 2018-07-01 to 2019-12-31

The CloudBrake project strives to expose the coupling between winds and shallow convective clouds. Shallow convective clouds are frequently found in the midlatitudes in cold air outbreaks or on sunny days over land. They are especially plentiful over subtropical oceans, which are known for strong near-surface easterly winds - the trade-winds - which drive the surface evaporation that feeds the atmosphere with moisture and ultimately with clouds. The trade-winds are important because they define how airmasses converge into the ascending branch of the Hadley circulation, which produces the majority of tropical rainfall. The trade-winds also modulate ocean currents and upwelling, sea surface temperatures and turbulent fluxes at the ocean surface. Because the trade-winds are not only strong, but also relatively steady, they have a large unexploited wind energy potential.

The shallow clouds that accompany the trade-winds are not simply drifting along with the mean wind: they can modulate winds themselves. One way to do so is by transporting air with a certain momentum (wind speed) away from the surface to higher levels in the atmosphere, and transporting air with a different momentum from higher levels to the surface. This vertical mixing of clouds can influence the vertical profile of wind, and CloudBrake postulates that this process is critical for the winds that shape large-scale circulations in our atmosphere.

To study the causality behind relationships between clouds and wind in subtropical and midlatitude weather systems, CloudBrake combines high-resolution Large Eddy Simulations with the analysis of remote sensing data collected at ground-based observatories. New lidar techniques aboard aircraft are exploited to measure wind and turbulence closer to clouds and to validate low-level winds measured by the wind lidar aboard ESA's Aeolus satellite in the CloudBrake flight campaign (see Image). The simulations and measurements are used to study the influence of wind shear on cloudiness and the importance of momentum transport on (near-surface) winds, as well to constrain parameterizations of momentum transport by shallow convection and to test the impact of momentum transport on the Hadley circulation in large-scale models.
The first years of the project have focused on the understanding of small-scale processes that are important for a coupling of clouds and winds. To do this we have been performing idealized high resolution simulations (LES) and have carried out a number of observational studies.

Using LES we have studied how sensitive shallow clouds are to vertical shear in the large-scale wind profile. These simulations have shown interesting results, namely, that wind shear limits the vertical growth of cumulus clouds, influences cloud fractions, fastens the transition from stratocumulus to cumulus clouds and can lead to very different near-surface winds. These are important results, because it shows that wind shear controls properties of clouds that can regulate the effect of clouds on climate, whereas in most current studies, wind shear is not considered as a cloud-controlling factor. The influence on surface winds is important, because these regulate the heat exchange between the ocean and the atmosphere, and therefore subsequent cloud development and sea surface temperatures.

How the presence of clouds changes winds near the surface is also explored using measurements from the long-standing Cabauw and Barbados Observatories. How winds are changed at cloud level will be explored using data collected during the CloudBrake Flight campaign, which took place last June, whereby two aircraft from the German Aerospace Center (DLR) made simultaneous measurements of the wind profile and turbulent momentum fluxes on days with convective clouds over Germany.
We aim to collect even more wind measurements in a much more remote region, namely over the subtropical ocean east of Barbados, by deploying commercial wind lidar instrumentation on board a research vessel during the upcoming international EUREC4A field campaign in Jan-Feb 2020. These data will offer a very new view on how winds over oceans are modified under the presence of clouds, and also allow us to evaluate whether our current satellite observing systems can measure the small-scale interactions that are important.

In the second phase of our project, we move from smaller-scale towards larger scales. Using our insights into momentum transport from the high resolution simulations in idealized settings (LES) and high resolution simulations in realistic settings (hind-casts over the tropical Atlantic) we strive to quantify the boundaries of the influence of convective momentum transport on winds and we are examining the treatments of momentum transport by cumulus parameterizations in global models, such as the IFS forecast model and the ICON climate model.
Turbulence measurements with DLR's Cessna aircraft within shallow cumulus cloud fields