The Impact of CIN Depth on Simulated Deep Convection Initiation

Convective inhibition (CIN) reduces the kinetic energy of air rising from a low-level high-θ_e reservoir but, when controlling for the magnitude of CIN, the depth of the CIN layer could impact the likelihood of deep convection initiation (DCI). When ascent is driven by non-thermodynamic forcing beneath cloud base, a condition expected to be true of many instances of DCI (e.g., along airmass boundaries), ascent through the CIN layer is not controlled solely by the integrated buoyancy but by the evolution of the sub-cloud forcing. Thus, whether or not air reaches the LFC is not a function of CIN but on the height of the LFC (i.e., depth of the CIN layer).

Results are presented from idealized numerical experiments aimed to evaluate the sensitivity of DCI to CIN layer depth when controlling for CIN. A generic non-thermodynamic impulsive initiation mechanism is imposed below the LCL with free parameters allowing for tuning of size and strength but constrained to produce a controllable value of vertical motion at the LCL. Experiments reveal a systematic decrease in the likelihood of DCI as CIN layer depth is increased for a given CIN. The physically consistent and dynamically constrained (not prescribed) decay of non-thermodynamic forcing with time is found to be critical for explaining the sensitivity, but is manifested as clouds at the LFC that are too small to support DCI when CIN layer depth is too large. A modified parameter that accounts for both CIN and CIN depth is proposed.