Climate and Atmospheric Science (ICAS) PhD Projects (copy 1)
The dynamics of cumulonimbus storm outflows
Supervisors: Dr Andrew Ross, Professor Doug Parker, Dr John Marsham and Dr Jeff Peakall
This project will bring together laboratory models, numerical models and observational data, to understand the behaviour of the severe surface winds which are produced by cumulonimbus storms. The work will have application to the prediction of severe weather, to the understanding of dust uplift in the climate system and, more deeply, will improve our understanding of the fluid dynamics of the atmosphere.
Cold air outflows from cumulonimbus storms play a key role in the life-cycle of these convective systems. The cold airflows are an important natural hazard in many parts of the world, including the continental USA, Europe, and large parts of the tropical oceans and continents. The severe gust fronts which are often associated with the cold air outflows can cause damage to structures, disrupt aviation and are an important cause of dust uplift. Despite their importance, many unanswered questions remain about these outflows, about the strong winds and turbulence they generate, the transport of dust and water vapour within the outflow and the effects of wind shear and stratification on their development. This project will address some of these fundamental questions using a mixture of high resolution numerical modelling, laboratory experiments and analysis of observational data.
Fig 1: (a) an “arcus” cloud, marking the leading edge of the cold air outflow near the ground (taken from the BAe146 research aircraft August 2006); and (b) scientific instruments destroyed by the high winds in one of these outflows.
Cold air outflows are formed when the downdraft of cold, moist air within a cumulonimbus cloud hits the surface. This colder (and hence denser) air spreads out along the surface. The leading edge of this colder air (often known as a gust front) can be associated with a rapid drop in temperature and a change in wind direction and an increase in wind strength. The rapid changes in wind are a hazard to aircraft landing or taking off, and have led to a number of aircraft being lost over the years, particularly in the USA. The strength of the gust fronts can also cause damage to structures such as off-shore oil rigs. Over arid regions, strong gust fronts are also important for uplifting dust from the surface which can then alter the radiative balance of the Earth. Uplift of moist air by the outflows can also trigger further convection, leading to the regeneration and organisation of storms.
The fact that these outflows are generated by relatively small-scale processes, but can travel large distances (more than 1000 kms in some cases) makes them important, but hard to represent in forecast models. In order to better represent understand their role, to improve their prediction, and to parametrise their impacts for coarse resolution models, we need to better understand the basic fluid mechanics of these outflows and how they interact with the environment.
Fig 2: schematic diagram of a gravity current generated by dense fluid undercutting lighter ambient fluid.
A simple conceptual model for such a cold-air outflow is that of a gravity current (fig 2). By studying this idealised problem using numerical modelling and laboratory experiments it is possible to isolate individual factors such as the effects of a background stratification or temperature inversions, or the effects of wind shear on the development and propagation of a gravity current. Laboratory and numerical modelling approaches also allow detailed study of the turbulence, transport and mixing within a gravity current which is hard to obtain from atmospheric observations. A number of numerical models are used in Leeds for studying this type of problem including the Met Office LEM (large-eddy model). Laboratory experiments will be conducted in the Sorby lab with the school, which has state of the art equipment for generating and measuring this type of gravity current flow (figure 3).
Fig 3: A gravity current generated in the Sorby Laboratory with dense saline fluid undercutting the ambient fresh water. This provides an analogue of the outflow of cold air from a thunderstorm.
Ultimately the ideas developed using such simple models need testing in more complicated real-world situations. Over recent years Leeds has been involved in a number of observational field campaigns in Europe (CSIP, COPS) and in West Africa (AMMA) which have studied convective storms using surface based, remote sensing and aircraft measurements. Case studies from these campaigns have helped to motivate the research questions addressed through these project. The observational campaigns also provide a rich data set with which to test the ideas developed using idealised modelling. Ultimately a better understanding of the fluid dynamics at work in these cold air outflows will allow better interpretation of the limit observations available of these outflow, better representation of them in high resolution models and better parametrisation of their effects in coarse resolution forecast and climate models.
Fig 4: A haboob, or dust storm over Iraq. The dust storm is caused by a cold pool outflow lifting dust up from the surface into the cold pool head. Note the similarity in structure between the atmospheric dust storm and the laboratory gravity current in figure 3.
There is potential for high impact publications from this project, both in improving our understanding and forecasting of severe weather events associated with thunderstorms outflows, and in underpinning the development of new parametrisations of cold pools and dust transport for NWP and climate models.
Although this project is focused on thunderstorm outflows, the underlying idea of a gravity current can be applied to a wide range of other environmental phenomena, from turbidity currents in the oceans to sea breezes and dense gas spillages in the atmosphere. This offers the exciting possibility of this research having wider impacts across a number of fields.
The School of Earth and Environment is a leading centre for this kind of research, with expertise in the fundamental fluid dynamics of gravity currents, in the dynamics of convective storms (both in Europe and Western Africa) and in the use of laboratory experiments to study gravity currents. This studentship would form a part of this active and stimulating research environment. The School has a well-knit group of around 40 atmospheric-science research students. It provides an excellent programme of generic skills training for research students, as well as a number of subject specific activities including the Arran Summer School and a number of Masters-level taught modules.