Similarity of Deep Continental Cumulus Convection as Revealed by a Three-Dimensional Cloud-Resolving Model

Abstract

A three-dimensional cloud-resolving simulation of midlatitude continental convection during the Atmospheric Radiation Measurement (ARM) program summer 1997 intensive observation period (IOP) is used to study the similarity of several second and third statistical moments, and second-moment budgets among five episodes of deep convection. Several parameter scales relevant to deep convection similarity are introduced. The dimensionless vertical profiles of the vertical velocity variance and its third moment, cumulus kinetic energy, the prognostic variables' variances and fluxes, their budgets, as well as several triple correlations cluster together, confirming the dynamical similarity of the simulated convective events. The dimensionless budgets of several second-order moments, such as convective kinetic energy (CKE), its vertical and horizontal components, variance, and vertical fluxes of the prognostic thermodynamic variables, as well as the momentum flux, are also presented. The most interesting aspect of the simulated CKE budget is that, in contrast to the boundary layer and shallow trade wind cumulus convection, the dissipation term is relatively small compared to the dominant buoyancy production, transport, and pressure correlation terms. The prognostic equation for the bulk CKE, defined as the vertically integrated mean CKE per unit area, is also discussed. It is found that the so-called bulk CKE dissipation timescale ranges in the simulation from 4 to 8 h. Therefore, the bulk CKE, mostly contained in the horizontal branches of mesoscale circulations associated with the deep convective systems, can persist much longer than the lifetime of an individual convective cloud. It is also found that the fraction of the bulk CKE associated with the vertical motions is about the same for all of the events considered, suggesting a strong correlation between the bulk CKE and the strength of the convective updrafts. It is shown that the bulk CKE dissipation timescale is inversely proportional to the square root of the bulk CKE itself. It is also found that the convective velocity scale is closely related to the convective available potential energy (CAPE) of the thermodynamic sounding taken immediately before a particular convective event.