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Computational Fluid Dynamics Modelling of Tornadoes along Quasi-Linear Convective Systems


Project Description

The UK experiences as many tornadoes as the USA as a whole: 1.2 vs 1.3 tornadoes per 10,000 km2 per year on average (Mulder and Schultz 2015). UK tornadoes, however, tend to be less violent than their American cousins. Also, most Tornado Alley tornadoes occur during a few weeks in spring, whereas UK tornadoes are spread throughout the year. These differences in intensity and seasonality appear to be linked to a difference in the parent storms that produce the tornadoes.

The parent storms of 79% of USA tornadoes appear on radar imagery as cells. In the British Isles, only about 37% of tornadoes develop from cells (Mulder and Schultz 2015). These cells are typically supercells, isolated rotating storms with updrafts and downdrafts that favour both the tilting and stretching of vorticity needed to produce storm-scale rotation (known as the mesocyclone) and to generate tornadic-strength vorticity at the ground. In contrast, the most common category of parent storm for tornadoes in the British Isles is quasi-linear convective systems (QLCSs), accounting for 42% of all tornadoes (Mulder and Schultz 2015). Although QLCS tornadoes also occur in the USA, they are less common there (18% of all tornadoes). The mechanisms forming QLCS tornadoes are less well understood than for supercell tornadoes, and fewer resources have been put into their study.

Supercell tornadoes have been successfully simulated in models with horizontal grid spacings of 30–100 m, but fewer simulations of QLCS tornadoes have been performed. This PhD project will investigate the mechanisms of QLCS tornado formation using computational fluid dynamics (CFD). We will use methods developed and applied by the group for these cases and will employ nested simulations using the open source code OpenFOAM or similar. We will start with a systematic study of an isolated tornado-like vortex, varying dimensionless quantities to develop an appreciation of their role in the overall dynamics: aspect ratio, swirl ratio and Reynolds number. Simulations will commence with Reynolds Averaged Navier Stokes (RANS) for a set up corresponding to the reference work of Baker (1981) or similar. Following this, we will simulate QLCS misocyclone initiation and development based on simplified domains similar to that proposed in Lee and Wilhelmson (1997). Our nested simulation approach makes use of synthetic turbulence approaches developed at the University of Manchester (e.g. Skillen et al 2016). We will use these approaches to include turbulence in atmospheric boundary layers and investigate the role of turbulence in the formation and evolution of fronts, misovortices, and tornadoes.

We estimate that large eddy simulation (LES) with 10–20 m horizontal grid spacing will provide potential for detailed analysis of tornado formation, maintenance, and dissipation; through diagnosis of the three-dimensional vorticity-tendency equation, trajectories, and depiction of vortex tubes. These diagnostics will address the importance of terms in the vorticity-tendency equation (especially stretching and tilting) to tornadogenesis.


With tornadogenesis simulated, we can test the sensitivity of tornadogenesis to various environ-ments and surface drag. We will incorporate uncertainty quantification methods to investigate sensitivity to perturbations in initial and boundary conditions. One of the experiments to be performed is to vary the strength and direction of the flow on either side of the front and see the effect on tornadogenesis. In doing so, we will be able to determine whether the prefrontal or postfrontal winds are more important to misocyclogenesis and tornadogenesis.

References

Mulder, K. J., and D. M. Schultz, 2015: Climatology, storm morphologies, and environments of tornadoes in the British Isles: 1980–2012. Mon. Wea. Rev., 143, 2224–2240, doi: 10.1175/MWR-D-14-00299.1.
Baker, G.L., 1981. Boundary Layers in Laminar Vortex Flows. PhD thesis, Purdue University,
Lee, B.D. and Wilhelmson, R.B., 1997. The numerical simulation of non-supercell tornadogenesis. Part I: Initiation and evolution of pretornadic misocyclone circulations along a dry outflow boundary. J. Atmos. Sci., 54, 32–60.
Skillen, A., Revell, A. and Craft, T., 2016. Accuracy and efficiency improvements in synthetic eddy methods. International Journal of Heat and Fluid Flow, 62, 386–394.

How good is research at The University of Manchester in Earth Systems and Environmental Sciences?

FTE Category A staff submitted: 42.13

Research output data provided by the Research Excellence Framework (REF)

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