Details of Award
NERC Reference : NE/M009882/1
Severe convective storms
Training Grant Award
- Lead Supervisor:
- Professor PF Linden, University of Cambridge, Applied Maths and Theoretical Physics
- Grant held at:
- University of Cambridge, Applied Maths and Theoretical Physics
- Science Area:
- Atmospheric
- Overall Classification:
- Atmospheric
- ENRIs:
- Environmental Risks and Hazards
- Global Change
- Science Topics:
- Boundary Layer Meteorology
- Tropospheric Processes
- Regional & Extreme Weather
- Abstract:
- Deep, precipitating convective clouds can produce localised downdraughts of cold air, in which the cooling arises through evaporation from precipitation. The associated negative buoyancy, and/or momentum transfer from precipitation, accelerates the downward motion. When such a localised downdraught strikes the earth, it forms a so-called "cold pool", which is diverted horizontally into a current of air spreading along the ground. The touchdown of these flows has long been associated with the creation of hazardous gust fronts. More recently, incorporation of downdraught and cold-pool processes into the parametrisation of convection has helped to address the long-standing temporal error in model simulation of peak convective activity (e.g. Grandpeix and Lafore, J. Atmos. Sci., 2010) The structure of these flows has been the subject of some previous studies. Fujita (U.Chicago, 1986) performed qualitative lab experiments to compare with a conceptual model of downbursts. Lundgren et al. (J.Fluid Mech., 1992) also carried out experiments in which they found that the flow evolution could be described in terms of characteristic length and time scales. Anabor et al. (Atmos. Chem. Phys., 2011) performed large-eddy simulations of a microburst flow generated by an imposed cooling of prescribed strength and duration. They found an empirical self-similarity in the gust-front profile on the ground, and also reported flow features such as the formation of a strong vortex at the leading edge of the cold outflow. Vortex formation has also been noted as a characteristic of the initial phase of axisymmetric lock-exchange gravity currents (Patterson et al., Phys. Fluids, 2006). A recent US observational study by Giangrande et al. (J. Appl. Meteor. Climatol., 2013) indicated that approximately half of the observed downdraughts were buoyancy-driven. This project aims to investigate the dynamics of the descent and spread of localised, negatively buoyant flows, in parameter ranges relevant to convective downdraughts. This will be carried out by a combination of laboratory, analytical and numerical techniques. Water-based experiments will be performed in the G.K.Batchelor Laboratory at DAMTP. Many aspects of meteorological (and oceanographic) parametrisation have been inspired and guided by idealised, controlled studies in laboratory conditions. Numerical simulations will be carried out using the state-of-the-art Met Office LEM. Recent work (Rooney 2014, in submission) has shown that this model can produce results comparable to those of laboratory studies. Using both methods allows the validation of the LEM against the lab experiments, and gives confidence in the use of diagnostics from the LEM which are harder to access in the lab. Particular aspects to consider are the entrainment processes and the influence of vortex stretching in the impact region, and the effect of source characteristics on the speed and structure of the spreading flow. The influence of surface characteristics (e.g. varying roughness) will also be investigated. Detailed experiments using novel methods of flow visualisation will allow features such as secondary-vortex generation to be examined in detail. Studies will concentrate firstly on the case of release in a still environment. The influence of an ambient wind will, however, also be considered. This is a case in which the use of numerical modelling can bridge the gap between experimental results and realistic, full-scale environmental flows. The transition between wind-free and wind-influenced intrusion flow in a stratified fluid has recently shown that LEM data corroborated the analytical model of upwind spread by an axisymmetric intrusion. However, much is still to be understood about the interaction of wind with these types of buoyancy-driven flows.
- NERC Reference:
- NE/M009882/1
- Grant Stage:
- Completed
- Scheme:
- DTG - directed
- Grant Status:
- Closed
- Programme:
- Industrial CASE
This training grant award has a total value of £85,122
FDAB - Financial Details (Award breakdown by headings)
| Total - Fees | Total - RTSG | Total - Student Stipend |
|---|---|---|
| £16,587 | £11,000 | £57,538 |
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