Revealing the hidden turbulence in atmospheric simulations

High-resolution models run on our platforms help researchers study the energy exchange behaviours of tornadoes and wildfires.

Research background

Understanding the energy exchange between the atmosphere and Earth’s surface is critical to many scale-dependent and high impact phenomena (tornadoes, extreme precipitation from atmospheric rivers, wildfires, etc).

The energy exchange takes place via three-dimensional (3D) coherent structures (roll cells, helical plumes, etc) that vary in space and time. This is an active research area and new methods for identifying these turbulent structures are likely to emerge.

University of Canterbury researchers Tamara Pletzer and Marwan Katurji contacted REANNZ to help them acquire the skills to implement these methods as they arise.

One such method, the Finite Time Lyapunov Exponent (FTLE), seeks to describe the material boundary in the flow such as regions of greatest attracting or repelling. This highlights process scale effects of the plume in complex topography and can provide guidance for action within these high impact phenomena.

Project challenges

The FTLE method involves integrating velocity trajectories starting from gridpoints. Some high resolution models can have up to billions of trajectories and this comes at a high computational cost.

What was done

REANNZ Research Software Engineer Alex Pletzer worked with Tamara and Marwen to develop a Python module (pv_ftle), which computes the finite time Lyapunov exponent (FTLE) for a velocity field on a rectilinear grid.

The module implements a standalone script, or can be invoked through as a Paraview source plugin. The FTLE computation is based on mimetic interpolation of the velocity, which was recently developed as part of another consultancy.

Mimetic interpolation respects the physical properties of the flow, such as zero divergence for example, which is critical when interpolating around buildings and topography.

Main outcomes

A more accurate implementation of the 3D FTLE – It now takes into account the staggering of the velocity field.
Ease of use – The mimetic interpolation implemented no longer requires a preprocessing step where velocity components need to be averaged to cell centres.
Integration into the Paraview visualisation tool – This makes it easier to explore the locations of transport barriers.
Optimised for performance – The module runs in parallel via OpenMP threads.

Paraview visualisation of FTLE at 10m height for a wildfire simulation. Red values represent chaotic regions of high attraction. Pictured on the left: Original FTLE computation at the onset of the consultancy. Pictured on the right: Improved FTLE field shows crisp boundaries around convective cells. The influence of the urban layout, including streets and buildings, can be seen in the lower part of the figure on the right.

"We are extremely satisfied with the support received through this Consultancy project. REANNZ brought exceptional expertise in scientific computing to a complex project combining visualization (VTK and Paraview) and interpolation techniques that allowed us to obtain the most realistic results from our data. The plugin they created has directly enhanced our teaching materials and will contribute to an upcoming publication on coherent structures in fire plumes."

– Tamara Pletzer, School of Earth and Environment, Faculty of Science, University of Canterbury

This case study shares some of the technical details and outcomes provided through our Consultancy Service. This service supports projects across a range of domains, with an aim to lift researchers’ productivity, efficiency, and skills in research computing. Get in touch to discuss how our Research Software Engineers and specialist support could help advance your project.