Organization of tropical convection in low vertical wind shears: Role of updraft entrainment

Abstract

Radiative‐convective equilibrium simulations with a 2 km horizontal resolution are conducted to investigate the impact on convective organization of different parameterizations for horizontal and vertical subgrid turbulence mixing. Three standard approaches for representing horizontal diffusion produce starkly differing mixing rates, particularly for the entrainment mixing into updrafts, which differ by more than an order of magnitude between the schemes. The simulations demonstrate that the horizontal subgrid mixing of water vapor is key, with high mixing rates a necessary condition for organization of convection to occur, since entrainment of dry air into updrafts suppresses convection. It is argued that diabatic budgets, while demonstrating the role of spatially heterogeneous radiative heating rates in driving organization, can overlook the role of physical processes such as updraft entrainment. These results may partially explain previous studies that showed that organization is more likely to occur at coarser resolutions, when entrainment is solely represented by subgrid‐scale turbulence schemes, highlighting the need for benchmark simulations of higher horizontal resolution. The recommendation is for the use of larger ensembles to ensure robustness of conclusions to subgrid‐scale parameterization assumptions when numerically investigating convective organization, possibly through a coordinated community model intercomparison effort.

Observing Convective Aggregation

“…However, numerous studies (e.g., Stephens et al 2008;Muller and Bony 2015;Wing and Cronin 2016;Holloway and Woolnough 2016;Silvers et al 2016;Tompkins and Semie 2017) demonstrate that the behavior of convective aggregation in models can be sensitive to aspects of the experimental setup (such as the size of the domain) and/or to the models themselves (e.g., horizontal resolution, the representation of diabatic processes or the parameterization of subgrid-scale mixing).…”

Nonrotating Convective Self‐Aggregation in a Limited Area AGCM

“…Until recently, there had been little direct evidence that the interaction between convection and tropospheric humidity plays a role in aggregation. Tompkins and Semie () found a significant impact from the choice of subgrid mixing scheme in a CRM, with schemes that produced enhanced mixing around updraft cores also producing stronger aggregation, implying that convective entrainment processes are important. Here we conduct an analogous experiment in a model with parameterized convection, where it is possible to homogenize the humidity field “seen” by the parameterization without actually altering the humidity.…”

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

“…To address this question using numerical models is a challenging task as the simulations required for this purpose would have to be conducted in a large enough domain, and, at the same time, with a spatial grid spacing capable of properly representing turbulent mixing at the gust front (see Grant and van den Heever (2016) for such an attempt). Such efforts will also improve parameterizations of the boundary layer turbulent transports towards representing clear-air entrainment and up/downdrafts correctly (see also Tompkins and Semie 2017).…”