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

International audienceRadiative-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.Plain Language SummaryThunderstorms dry out the atmosphere since they produce rainfall. However, their efficiency at drying the atmosphere depends on how they are arranged; take a set of thunderstorms and sprinkle them randomly over the tropics and the troposphere will remain quite moist, but take that same number of thunderstorms and place them all close together in a "cluster" and the atmosphere will be much drier. Previous work has indicated that thunderstorms might start to cluster more as temperatures increase, thus drying the atmosphere and letting more infrared radiation escape to space as a result - acting as a strong negative feedback on climate, the so-called iris effect. We investigate the clustering mechanisms using 2km grid resolution simulations, which show that strong turbulent mixing of air between thunderstorms and their surrounding is crucial for organization to occur. However, with grid cells of 2 km this mixing is not modelled explicitly but instead represented by simple model approximations, which are hugely uncertain. We show three commonly used schemes differ by over an order of magnitude. Thus we recommend that further investigation into the climate iris feedback be conducted in a coordinated community model intercomparison effort to allow model uncertainty to be robustly accounted for