Diagnosing nonlocal effects and coherent structure scales in moist
convection using a large-eddy simulation
Abstract
The anelastic theory of effective buoyancy has been generalized to
include the effects of momentum flux convergence. Mediated by the
nonlocal perturbation pressure, the dynamics tends to average over
details of the forcing, yielding acceleration robust to small-scale
variations of the flow. Here we demonstrate in a large-eddy simulation
(LES) with a 100-m horizontal grid spacing that including the anelastic
nonlocal dynamics can help capture the mean evolution of convection
without fully resolving the fine-scale coherent turbulent structures
embedded in the flow. Instances of convection in the LES are identified.
For these, the buoyancy and dynamic contributions to the vertical
momentum tendency are separately diagnosed. The diagnoses show that
buoyancy is the leading effect in the vertical acceleration while
strongly interacting with the vertical momentum flux convergence. In
comparison, the influence of the horizontal momentum flux convergence on
the vertical motion are substantially weaker. The sensitivity resulting
from averaging over fine-scale features are quantified. For
deep-convective cases, these contributions at the cloud scale
($\sim8$ km) exhibit a robustness—as measured in a
root-mean-square sense—to horizontally smoothing out turbulent
features of scales $\lesssim3$ km.
As expected, such scales depend on the size of the convective element of
interest, while dynamic contributions tend to be more susceptible to
horizontal smoothing than does the buoyancy contribution. By verifying a
key attribute of the pressure-mediated dynamics in an LES, results here
lend support to simplifying the representation of moist convection under
the anelastic nonlocal framework for global climate models and
storm-resolving simulations.