On the sensitivity of deep-convection initiation to horizontal grid resolution

Idealized numerical simulations are used to study the sensitivity of deep-convection initiation to horizontal grid spacing (Delta h). In a conditionally unstable but strongly inhibited environment, a localized surface heating function gives rise to low-level convergence and a vigorous subcloud updraught that breaches the level of free convection (LFC). The vertical cloud development is sensitive to Delta h: the clouds reach 8-9 km for Delta h=250 m (CTRL) and Delta h=500 m (DX500) but only 6-7 km for Delta h=125 m (DX125). This trend is not associated with major differences in midlevel cloud vigour (e.g., buoyancy and vertical velocity), but can be explained on the basis of cloud-core mass flux (Mc). The Mc profile is regulated by its value at the LFC, largely set by subcloud processes, and its core-layer gradient, set by entrainment and detrainment. The former is the largest in CTRL and weakens as the grid is coarsened (DX500) or refined (DX125), both due to a widening and weakening of the subcloud updraught. Whereas in DX500 this widening stems from a failure to adequately resolve the 1-km-wide updraught on the model grid, in DX125 it results from stronger total (subgrid plus resolved) turbulent mixing, which increases monotonically as Delta h is decreased. The wider and more diffuse updraughts in DX125 and DX500 are also more hydrostatic and generate weaker buoyancy-driven accelerations. Within the cloud layer, the entrainment is very similar in the three cases, as is the detrainment for the higher-resolution cases (DX125 and CTRL). However, the midlevel detrainment is relatively weak in DX500, which facilitates slightly deeper ascent than in CTRL. By contrast, the small core-base Mc and strong midlevel detrainment yields the shallowest clouds in DX125.