A new Lagrangian approach to modelling of density currents

Fig 1. Cross-section of a cloud simulated by the Moist Parcel-in-Cell model.


Fig. 2. Simulation of a 2D density current in the Elliptic Parcel-in-Cell model, and evolution of MPIC.

This project will apply the new Moist Parcel-in-Cell (MPIC) model to study the fluid dynamics of density currents (also known as gravity currents) flowing along the ground. The project will exploit a radically new approach to develop fundamental understanding of the dynamics and mixing processes involved. The results will be applicable to meteorology and climate dynamics, and to pollution and dense gas dispersion.

Density currents are widespread in geophysical fluid dynamics. In the atmosphere, they often form due to evaporation of precipitation, but also as a hazard associated with dense gas pollution releases. They can appear as “cold pools”, which spread in a circular way below a precipitating cloud. Cold pools play a role in the rapid growth of deep convective storms via a positive feedback loop. They can also appear as line-like features below convective cloud systems, and work to maintain these systems. The edge of a density current can be visible if it picks up dust, and at this edge, turbulent wind gusts develop, which are a hazard to aviation (microbursts). As a natural hazard, density currents occur when accidental releases of dense gas move along the ground, or when smoke from a fire moves within a building.

The effects of density currents are usually poorly represented in global weather and climate models, where the cloud and rainfall processes that cause them aren’t explicitly represented. The Met Office has recently developed a new method to embed the formation and propagation of density currents into a global model. The role of density currents in triggering new clouds is particularly important, and work is needed to explore how this triggering of new convection depends on environmental conditions, including wind shear. Triggering will commonly occur when density currents collide.

The MPIC method provides a revolutionary approach to modelling small-scale atmospheric dynamics (see Figure 1 above). Rather than modelling the flow on a grid, MPIC represents the atmosphere as a collection of parcels. The parcel-based approach makes it possible to understand the behaviour of clouds and density currents in a fluid-following (Lagrangian) framework. This Lagrangian framework will be ideally suited to look at the circulations around the density current front, as well as the behaviour of parcels above the density current that are lifted by its passage — see Fig 2. MPIC will help to determine the dynamics of the front in a range of ambient wind-shear conditions, in order to characterise the mixing.

Previous work has shown that MPIC is a suitable tool to study cloud updraughts, and a new EPSRC project (https://environment.leeds.ac.uk/faculty/dir-record/research-projects/1575/diabatic-evolution-of-clouds-in-a-lagrangian-framework-turbulence-vorticity-dynamics-and-precipitation-effects) has just started to develop the method into a flexible tool for studying clouds and precipitation in a realistic environment, in collaboration with the University of St Andrews and EPCC. The EPSRC project will make it possible to simulate cold pool case studies from the recent EUREC4A field campaign on tradewind cumulus clouds with MPIC.

The aims of this project are:

  • To establish the use of MPIC to study the fluid dynamics of density current collisions in two and three dimensions. The results from MPIC simulations will be compared against the grid-based MONC model, EUREC4A observations, laboratory experiment data and theoretical models.
  • To analyse the mixing properties of the simulated density currents, exploiting the Lagrangian structure of the model, in a range of ambient environments.
  • To study the propagation and head-structure of density currents, from a Lagrangian vorticity-based perspective, with application to triggering of convective clouds, and to mixing in hazardous flows.
  • To investigate the role of the surface boundary conditions in the behaviour of turbulence near the gust front, and to improve the formulation of the near-surface dynamics in MPIC.