Master's Defence | Kwan Tsaan Lai, Sensitivity of Idealized Squall Line Simulation to Resolution and Subgrid Turbulence Mixing

MC 5417

Candidate

Kwan Tsaan Lai | Applied Math, University of Waterloo

Title

Sensitivity of Idealized Squall Line Simulation to Resolution and Subgrid Turbulence Mixing

 Abstract

In this project, the sensitivity of idealized squall line simulation to horizontal grid spacing and turbulent mixing parameterization will be discussed.
Inconsistent results from numerical simulations of convective system have suggested that there are issues with the behavior of the subgrid turbulent mixing parameterizations with increasing resolution that still need to be resolved.  
The Weather Research and Forecasting Model (WRF) will be used to perform large eddy simulations of idealized squall lines with horizontal grid spacing of 4 km, 1 km, 500 m and 250 m.  A common background sounding profile is used so that simulations from this project can also be compared with other studies in the literature.  
The dependence of various squall line characteristics on resolution, including cloud top height and mass flux, turbulence kinetic energy, energy spectra and backscatter will be discussed.  In particular, we find that 4 km grid spacing is not recommended for simulations with explicit convection as it contains an unreasonable amount of subgrid turbulence kinetic energy and it is not able to resolve the large energy containing eddies.  A significant amount of backscatter exists in simulations with $\Delta x=$ 4 km and even 1 km, which is ignored by the eddy viscosity model.