The Role of CIN Depth in Regulating Deep Convection Initiation

Convective inhibition (CIN) reduces the kinetic energy of rising air but, when controlling for the magnitude of CIN, the depth of the CIN layer could impact the likelihood of deep convection initiation (DCI).  Considering thermodynamics alone, CIN depth should not impact vertical velocity at the LFC, but it would impact the transit time of air passing through the CIN layer.  This could impact entrainment within this layer.  Moreover, when ascent below the LFC is driven by non-thermodynamic forcing imparting kinetic energy to air passing through the CIN layer, ascent through the CIN layer is not controlled solely by the integrated buoyancy but also by the evolution of this forcing.  During transit of air through the CIN layer it is expected that non-thermodynamic forcing will evolve significantly.  Thus, DCI is likely to depend on transit time through the layer and, by extension, the CIN layer depth. 

Results will be presented from analysis of environments near more than 60,000 observed DCI points.  Vertical profiles of the atmospheric state are approximated using RAP/RUC analyses and are compared to vertical profiles at the same time but away from initiation points (referred to as Null points).  Results show that CIN depth is among the most important distinguishing parameters and is more important than CIN in differentiating DCI environments from Null environments. 

Idealized numerical experiments were also conducted to explain the importance of CIN depth while controlling for CIN magnitude.  A generic non-thermodynamic impulsive initiation mechanism is imposed below the LCL.  Experiments reveal a systematic decrease in the likelihood of DCI as CIN layer depth is increased for a given CIN.  Specifically, for deeper CIN depth, a longer traverse of parcels through the inversion along with natural relaxation of dynamic forcing means that air reaching the LFC encounters upward forcing that is insufficient to carry it vertically even as thermal instability is released.  Unexpectedly, clouds at the LFC in environments with deeper CIN layers are actually larger.  Ordinarily, larger clouds would be expected to be more likely to yield DCI but, in these simulations, are not well correlated with the likelihood of DCI.