Cumulus Thermals Throughout Different Convective Regimes: Sticky or Slippery?

Abstract:

Thermals—small, transient air bubbles—have proved to be useful when taken as the basic convective element in order to investigate cumulus convection dynamics. The thermals' transient nature is a key element of convection that can hardly be depicted by the classical steady plume, making the thermal framework a good candidate for the development of new convection parameterizations. In order to fully characterize the dynamics of thermals under different convective regimes, we track thousands of thermals in three 3D Large Eddy Simulations (LES) using WRF at 65m resolution: two sea-breeze type experiments with different island sizes and one case of daytime convective development over land based on observations from the Large-Scale Biosphere-Atmosphere (LBA) field study. Overall, thermals are not fundamentally different throughout the different simulations. Their internal vortical circulation is such that they rise much slower than the updraft core speed would suggest, while they mix vigorously with the environment without losing momentum. We find that drag on these thermals is not related to mixing, but rather can be parameterized as standard dissipative drag, proportional to W²R^-1. On average thermals suffer a drag comparable to that of a solid sphere moving through a quiescent fluid (with drag coefficient 0.4<c_d<0.8). However, this relationship is sensitive to convective intensity: as convection intensifies, the drag coefficient decreases, i.e., thermals become less `sticky' and more `slippery'.