Deep convection initiation: state of the science, limits of understanding, and future directions

A convection initiation (CI) event is understood to represent the initial formation of a deep, moist convective cell that is characterized by a sustained, buoyantly forced main updraft. Since convective storms are invigorated by cloudy updrafts that grow significantly higher than an updraft air parcel's level of free convection (LFC) and thus develop stronger integrated buoyancy forcing, CI may alternatively be referred to as “deep convection initiation”. This talk focuses on three primary aspects of the science regarding CI: 1) discussion of the current state of CI science, 2) discussion of the limits of our current understanding regarding both physical CI-related processes and their application, and 3) assessment of the current open questions and where the science may lead in the next 5 to 10 years. Given the important role of storm observing systems to detect CI events and the role of CI as an initial state condition for subsequent storm development (if any), the present talk has direct relevance to other presentations at this symposium that discuss technology to detect severe storms, storm-scale numerical weather prediction, and operational forecasting for severe convective storms.

To assess the state of the current CI science, a range of observational- and model-based studies of the CI process and CI events are reviewed. A common key finding of the observational and modeling studies of CI is the joint requirement for sufficient convective instability of the mesoscale background environment combined with a source of deep, focused mesoscale lifting of comparable scale in at least one horizontal dimension to the initiated convective updraft. The deep mesoscale lifting that is postulated to precede a CI event has two simultaneous, favorable effects: (i) forced lifting of potentially unstable air through the parcel's LFC to initiate the buoyant updraft growth stage; (ii) deep-layer lifting that weakens or eliminates any inhibiting overlying inversion and lowers the lifted parcel's LFC, thus accelerating the onset of buoyant updraft acceleration.

A key limit of our current understanding regarding physical CI-related processes and their application proceeds from a general inability to adequately define the kinematic and thermodynamic mesoscale structure of the boundary layer and overlying lower atmosphere at relevant scales of CI forcing. Given the localized nature of CI events whose a priori probability may be conceived as somewhat horizontally homogeneous, it may ultimately be necessary to obtain and analyze cloud-scale observations over large contiguous areas to identify those environments that support CI. Another key limit on CI process understanding is the relative paucity of modeling and (particularly) observational studies of CI in which the main contribution of buoyant updraft air parcels is from a lower atmospheric layer that is substantially elevated above the earth's surface.

The presentation concludes with an assessment of current open CI science questions and speculation about progression of CI science and its application over the next 5-10 years.