University of Cambridge
On convective penetration of a buoyant plume into a stably stratified layer
Abstract
dc:description.abstractThe convective penetration of a buoyant plume from a uniform layer into an overlying stably stratified layer is relevant to a wide variety of geophysical and industrial flows. An example we focus on is the hydration of the tropical lower stratosphere (TLS) by overshooting deep convection in the tropics. In these flows, turbulent mixing between the plume and environment results in significant diapycnal transport of tracers carried by the plume. Internal gravity waves are also generated within the stratified region which can transport horizontal momentum in the vertical, thereby influencing atmospheric winds. Despite its relevance, convective penetration of an individual buoyant plume remains an understudied problem. This thesis serves to address the turbulent mixing, wave generation, and tracer transport occurring in this fluid dynamical problem using large-eddy simulations (LES) of the flow at laboratory scale. We first examine the turbulent mixing between the plume and environment, using a representation of the plume in buoyancy-tracer phase space to consider the mixing in more detail than has previously been achieved. We develop a new method for objectively partitioning plume fluid in buoyancy-tracer space into three regions, each of which corresponds to a coherent region in physical space. This enables quantification of different measures of turbulence and mixing within each of the three regions. Using simulations in which the stratification strength ranges over two orders of magnitude, we then examine the structure and source of internal waves that appear to emanate from the top of the plume. Internal waves are generated with frequencies in a relatively narrow band that is moderately smaller than the buoyancy frequency, despite the broad-banded frequency spectrum of turbulence in the plume and oscillations of the plume top. We provide evidence that the waves originate from within the turbulent flow rather than at the turbulent/non-turbulent interface between the plume top and the surrounding stratified fluid, and explain the frequency selection using a linear viscous decay model. Finally, we formulate a parameterisation of moisture which retains the essential processes involved in convective hydration of the condensation/sublimation of vapour/ice and sedimentation of ice. Using this model, we explore the interaction between transport, microphysical processes and mixing in convective hydration of a stratified layer and examine the influence of large-scale vertical shear. We find that hydration is controlled by mixing which is modulated by sedimentation and directly influenced by convective intensity. Vertical shear enhances hydration by promoting small-scale mixing via internal wave breaking and shear instabilities. In summary, this thesis presents three results: a novel understanding of the stages of mixing in convective penetration of a stably stratified layer, the first evidence that internal waves are generated within the turbulent plume rather than at the interface between the plume and environment, and a more fundamental understanding of the role of microphysics, mixing, transport and large-scale shear in controlling convective hydration of a stratified layer. The latter study offers insight on the physical processes which are most important in convective hydration of the TLS and may aid in the interpretation of more comprehensive studies and development of convective parameterisations in climate models.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2025
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Powell, Charles
- Advisors dc:contributor.advisor
-
- Haynes, Peter
- Taylor, John
Subjects
dc:subject × 4Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.118911
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/385189