On the Factors Controlling the Development of Shallow Convection in Eddy-Diffusivity/Mass-Flux Models

Abstract

This study addresses key aspects of shallow moist convection, as simulated by a multiplume eddy-diffusivity/mass-flux (EDMF) model. Two factors suggested in the literature to be essential for the development of convective plumes are investigated: surface conditions and lateral entrainment. The model consistently decomposes the subgrid vertical mixing into convective plumes and the nonconvective environment. The modeled convection shows low sensitivity to the surface plume area. The results indicate that plume development in the subcloud layer is controlled by both surface conditions and lateral entrainment. Their impact significantly changes in the cloud layer where the surface conditions are no longer important. The development of shallow convection is dominated by the interactions between the plumes and the large-scale field and is sensitive to the representation of the variability of thermodynamic properties between the plumes. A simple two-layer model of steady-state convection is proposed to help understand the role of these processes in shaping the properties of moist convection.

Towards Unifying the Planetary Boundary Layer and Shallow Convection in CAM5 with the Eddy-Diffusivity/Mass-Flux Approach

“…where ψ is the transported quantity, w the vertical velocity, K the eddy-diffusivity/viscosity coefficient for the local transport, i the index of the ith updraft (and thus, a i , w i , and ψ i refer to the values for the ith updraft), I the total number of updrafts (here, I = 20, which is sufficient to minimize the sensitivity of the results to I, as shown in [18]), a i the updraft area, and primes denote deviations from the grid-mean values denoted with overbars. Below, we describe how the two components are parameterized.…”

“…The mass-flux formulation follows [18], neglecting the non-local transport within the environment. The updrafts are rooted at the surface, regardless of the model's resolution, and extend up to the level of vanishing vertical velocity.…”

“…The updrafts are designed to transport moist-conserved thermodynamic variables (i.e., liquid water potential temperature θ l and total water mixing ratio q t ), horizontal momentum, and optionally, other fields. Note that, in general, the two thermodynamic variables do not account for the precipitating water, which has to be treated separately due to sedimentation, yielding additional sources for θ l and q t [18].…”

“…Variability among the plumes is ensured by using both different boundary conditions for integrating their properties in the vertical and stochastic entrainment [18]. The multi-plume approach does not require any additional closures that are rather common in other types of convection parameterizations (e.g., [5,6]).…”

“…Of several different diagnostic eddy-diffusivity schemes available in CAM, we chose the Holstag-Boville-Rash (HBR) scheme, which is an extension of [27], as it performs best for the current model configuration. Note that in previous implementations of the multi-plume model [13,18], the ED part was based on the prognostic equation for turbulent kinetic energy, which is not the case for the selected scheme.…”