Why can nighttime convection occur despite strong convective inhibition?

Over continental plains, precipitation tends to peak in the late afternoon or during nighttime. The accurate simulation of the land precipitation diurnal cycle in GCMs has been a long-standing challenge. Nighttime surface cooling tends to yield a stable layer with large convective inhibition (CIN). However, CIN arises from traditional parcel considerations—measuring the inhibition for an infinitesimal parcel. Here, we argue that the CIN layer is less effective in inhibiting convection than previously thought for convective entities of typical horizontal cloud size.

A time-dependent process model for anelastic convective entities (ACE) is formulated to consistently include dynamic entrainment/detrainment as well as a representation of nonhydrostatic perturbation pressure. Spatially nonlocal effects mediated by the pressure field imply that horizontal feature size becomes a factor in the vertical conditional instability problem. ACE simulations using nighttime GoAmazon soundings with strong surface inversion demonstrate that the vertically nonlocal pressure response and its interaction with the surface boundary condition make the CIN layer ineffective for convective features of substantial horizontal size. Within the convective column, buoyancy of different signs offset each other via the nonlocal interaction over vertical scales comparable to the typical horizontal scale. Furthermore, the interaction with the surface tends to downweight the effectiveness of negative buoyancy contributions at low levels. This implies that a much smaller vertical velocity perturbation (or more generally, nonlocal buoyancy forcing from neighboring disturbances) can tunnel through the CIN layer. The same effect yields smaller magnitude for the mass flux above the CIN layer compared with steady plume models. 

A related implication of including spatially nonlocal interactions is that the vertical acceleration due to deep-convective buoyancy tends to extend above the level of neutral buoyancy (LNB). This results in cloud top much higher than the LNB, exhibiting the convective cold-top feature previously noted in observations. Results here point to revision for convective parameterizations.