Entrainment of dry versus moist convective thermals
Submitter
Morrison, Hugh Clifton — University Corporation for Atmospheric Research
Area of research
Cloud Processes
Journal Reference
Science
Thermals, or rising bubbles of buoyant air, are a key “building block” of cumulus clouds and are nearly ubiquitous in cumulus convection. Nonetheless, the behavior of individual cloud thermals has been somewhat of a mystery. A particular question is how cumulus thermals take in the surrounding air and grow in size through a process called entrainment. Recent research has suggested that the growth rate of rising moist cloud thermals – which is related to their net entrainment of the surrounding air – is nearly zero. In contrast, dry thermals are known to have a radius growth rate that is substantial, increasing radius roughly 20-25% of the rate at which they rise under typical atmospheric conditions. The goals of this study were to confirm that this difference in moist and dry thermal growth rates is robust, and if so, to understand reasons for the difference.
Impact
Using idealized numerical modeling and theoretical analysis, it was confirmed that indeed moist thermals grow considerably slower as they rise compared to dry thermals, under otherwise the same atmospheric conditions. This difference is explained by the latent heating from cloud condensation within moist thermals that fundamentally alters their dynamical structure and limits entrainment. More broadly, these results help explain why individual cumulus clouds tend not to grow substantially wider as they rise, until they become negative buoyant, as seen in detailed large-eddy simulation models of cumulus clouds. There are also implications for convection parameterizations in weather and climate models since they often treat fractional entrainment rates as being inversely dependent on cloud width.
Summary
This study used idealized numerical modeling of dry and moist thermals as well as insight provided by theoretical analysis. Through the careful design of model experiments, dry and moist thermals growth rates could be compared under otherwise identical conditions, and it was confirmed that dry thermals have a radius growth rate that is about 1.7 times larger than that of moist cloud thermals. Analysis of large-eddy simulations of cumulus thermals under less idealized conditions confirmed the relatively slow growth rates of moist thermals. Idealized simulations with heating artificially added to the dry thermals showed that the slow growth rate of moist thermals is mostly caused by latent heating from condensation that is concentrated near the thermal core where the upward velocity of air is strongest. Evaporation near the margins of moist thermals had only a small contribution to limiting their growth rates. An important factor increasing growth rates of both moist and dry thermals was density stratification – the fact that air density decreases with height. Overall, these results suggest that slow spreading rates of cumulus thermals are a fundamental feature caused by condensation and latent heating that alters their distributions of buoyancy and thus flow dynamics compared to dry thermals (Figure 1).
Figure 1. Conceptual diagram contrasting buoyancy structure (blue shading) and subsequent lateral growth rates and net entrainment of dry and moist thermals. Red Xs mark the centers of rotation. For dry thermals, baroclinic generation of vorticity near the cloud edge and destruction in the interior lead to outward movement of the rotation center and rapid thermal spreading. Vorticity generation in the interior limits this outward movement and growth of moist thermals. From journal.