Contrasting Shallow and Deep Convection over Bankhead National Forest

Principal Investigator: Thijs Heus (Cleveland State University)

Co-Investigators: John Peters (Pennsylvania State University) and Girish Nigamanth Raghunathan (Cleveland State University)

It is commonplace in the atmospheric sciences and climate modeling communities to delineate moist atmospheric convection (and often, the communities themselves) into “shallow” and “deep” categories.  Shallow convection is confined to the lowest few kilometers of the free troposphere (usually below the freezing level), whereas deep convection extends most of the way through the free troposphere to the tropopause and usually contains ice.   Indeed, both of these categories are typically parameterized separately in global climate models.  Shallow and deep convection are often studied by separate research teams with very limited literary crossover between the two groups. 

Deep and shallow convection are typically treated in distinct ways, both in research questions and in parameterization schemes. However, recent studies have shown that shallow convection precipitates and organizes in a manner reminiscent of deep convection, and deep convection undergoes entrainment processes in a similar manner to shallow convection. In this project, we propose to study the similarities and differences between shallow and deep convection. As a result, we will work towards a unified conceptual model that improves our fundamental understanding of all processes governing these clouds at a continuous spectrum.  This improved conceptual model will serve as the basis for a future unification of shallow and deep convective parameterization schemes.

To perform our studies, we will use a mix of observations and high resolution (Large Eddy Simulation) computer modeling over the Bankhead National Forest (BNF) in Alabama to address the following research questions:
1. What are the primary factors that drive shallow cloud width and organization over land?  How do these factors differ from that of oceanic shallow convection?
2. What are the primary factors that drive shallow cloud width and organization in the absence of surface precipitation over land?  How do these factors differ from that of oceanic shallow convection?
These observations include scanning and vertically pointing radar observations of clouds, observed thermodynamic and wind profiles, precipitation, surface flux measurements, and spatially distributed surface observations. The climatology and the long-term data collection at BNF will guarantee a rich dataset, while keeping environmental factors like orography and land use largely identical in the comparison. 

While it is not within the scope of the work we have proposed here, it is useful for us to lay out ideas for how the knowledge we gain from this project will guide the future development of a unified parameterization scheme.  The basis for such a scheme would be a bin-like multi-plume scheme with a cloud size distribution. Such schemes feature many narrow clouds and fewer wide clouds, with the entrainment rate inversely proportional to updraft width.  Hence, the narrow clouds in the scheme represent shallow cumulus and the wide clouds represent congestus and deep convection.  A critical element of such a scheme is knowing when to “trigger” the deeper clouds, thereby emulating a deep convective scene, or exclude these deeper clouds, thereby emulating a cloud seen that only has shallow cumulus.  This is where the knowledge we gain from the proposed work will come in handy.  In particular, we plan to develop a future “triggering scheme” based on physical properties such as the depth of the boundary layer, the onset of precipitation, or the presence of sub-grid scale land-surface variability, which tell the scheme whether to include the wider clouds or not.  The knowledge gained through our hypothesis evaluation will be central to developing this triggering scheme.