The strong impact of weak horizontal convergence on continental shallow convection

Idealized large-eddy simulation (LES) is a basic tool for studyingthree-dimensional turbulence in the planetary boundary layer. LESis capable to provide benchmark solutions for parameterizationdevelopment efforts. However, real small-scale atmospheric flows developin heterogeneous and transient environments with locally-varyingvertical motions inherent to open multi-scale interactive dynamicalsystems. These variations are often too subtle to be detected bystate-of-the-art remote and in-situ measurements, and are typicallyexcluded from idealized simulations. The present study addressesthe impact of weak (i.e., $\mathcal{O}(10^{-6})$~s$^{-1}$) short-livedlow-level convergence/divergence perturbations on LES of the continentalshallow convection diurnal cycle.The results show a strong response of shallow nonprecipitatingconvection to the applied weak dynamical forcing. Evolutions ofCAPE, mean liquid water path, and cloud top heights are significantlyaffected by the imposed convergence/divergence. In contrast, evolvingcloud base properties, such as the area coverage and mass flux, areonly weakly affected. To contrast those impacts with microphysicalsensitivity, the baseline simulations are perturbed assuming differentobservationally-based cloud droplet number concentrations and thusdifferent rainfall. For the tested range of microphysical perturbations,the imposed convergence/divergence provides significantly largerimpact than changes in the cloud microphysics. Simulationresults presented here provide a stringent test for convectionparameterizations, especially important for large-scale modelsprogressing towards resolving some nonhydrostatic effects.