Use of the Parcel Buoyancy Minimum (Bmin) to Diagnose Simulated Thermodynamic Destabilization. Part I: Methodology and Case Studies of MCS Initiation Environments

A method based on parcel theory is developed to quantify mesoscale physical processes responsible for the removal of inhibition energy for convection initiation (CI). Convection-permitting simulations of three mesoscale convective systems (MCSs) initiating in differing environments are then used to demonstrate the method and gain insights on different ways that mesoscale thermodynamic destabilization can occur. Central to the method is a thermodynamic quantity B-min, which is the buoyancy minimum experienced by an air parcel lifted from a specified height. For the cases studied, vertical profiles of B-min using air parcels originating at different heights are qualitatively similar to corresponding profiles of convective inhibition (CIN). Though it provides less complete information than CIN, an advantage of using B-min is that it does not require vertical integration, which simplifies budget calculations that enable attribution of the thermodynamic destabilization to specific physical processes. For a specified air parcel, B-min budgets require knowledge of atmospheric forcing at only the parcel origination level and some approximate level where B-min occurs. In a case of simulated daytime surface-based CI, destabilization in the planetary boundary layer (PBL) results from a combination of surface fluxes and upward motion above the PBL. Upward motion effects dominate the destabilizing effects of horizontal advections in two different simulated elevated CI cases, where the destabilizing layer occurs from 1 to 2.5 km AGL. In an elevated case with strong warm advection, changes to the parcel at its origination level dominate the reduction of negative buoyancy, whereas for a case lacking warm advection, adiabatic temperature changes to the environment near the location of B-min dominate.