Sticky thermals: Evidence for a Dominant Balance Between Buoyancy and Drag in Cloud Updrafts

Summary

Quickly rising clouds are associated with many important phenomena, including hail, turbulence, and lightning. Despite the important impacts of fast updrafts, also called thermals, a surprisingly large uncertainty remains about the forces that generate these updraft speeds. What sets the speeds of these rising clouds? Do these cloud updrafts experience drag? If so, what are the magnitudes of the drag and buoyancy forces? Scientists funded by the Department of Energy’s Atmospheric System Research program provide answers to these questions in two recent studies.

Schematically, the acceleration of a cloud thermal can be written as acceleration equals buoyancy minus drag. The slippery-thermal hypothesis, advocated for in a previous study, states that drag is negligible and that the dominant balance in this equation is acceleration equals buoyancy. An alternative hypothesis, which is dubbed the sticky-thermal hypothesis in these studies, is that drag balances buoyancy.

To test these hypotheses, the scientists tracked cloud thermals. In one study, they tracked thousands of cloud thermals in a large-eddy simulation of deep convection, averaged their properties around a vertical axis through their top, and identified the thermal’s volume using its stream function. Averaging both the buoyancy and the drag over the cloud thermal, they found buoyancy and drag to be in very close balance. In another study, they tracked cloud thermals using stereo photogrammetry in which two synchronized cameras measured three-dimensional positions. Because they were able to measure speeds within the flow (Lagrangian speeds) as opposed to speed relative to a certain point (Eulerian speeds, such as those measured by Doppler radar), they could analyze the data using a simple momentum equation: acceleration equals buoyancy minus drag. They found that a substantial amount of drag (a drag coefficient on the order of one) was needed to match both the stereo-photogrammetric data and the known buoyancy of clouds from previous in situ measurements and the large-eddy simulations. Theoretical calculations reveal that wave drag could easily be the source of this drag. In other words, cloud thermals are sticky.

Funding

Funded by the Department of Energy’s Atmospheric System Research program.

References

Romps, D. M., and R. Öktem. 2015. “Stereo Photogrammetry Reveals Substantial Drag on Cloud Thermals,” Geophysical Research Letters 42(12), 5051–57. DOI: 10.1002/2015GL064009.

Romps, D. M., and A. B. Charn. 2015. “Sticky Thermals: Evidence for a Dominant Balance Between Buoyancy and Drag in Cloud Updrafts,” Journal of the Atmospheric Sciences, DOI: 10.1175/JAS-D-15-0042.1.