Modelling of elevated mesoscale convective systems

Elevated convection occurs when convection originates from above the boundary layer. The interaction of an elevated storm with the stable layer beneath it often generates
features such as waves and bores that maintain the convection.

The Convective Storm Initiation Project (CSIP) took place in the UK in 2005. Only one case of elevated convection was observed during CSIP, in which several mesoscale convective
systems (MCSs) formed. One MCS remained elevated and wave-lifted throughout the observation period. Another elevated MCS observed during IOP 3 was associated with Kelvin-Helmholtz billows. The billows and the elevated convection appeared to interact.

The aim of this thesis is to use high-resolution numerical models to investigate the processes occurring in the elevated MCSs observed during CSIP. The thesis is presented in two parts. In the first part a simulation is performed using the Weather Research and Forecasting (WRF) model. The model reproduces the wave-lifted elevated convection in the early stages of the simulation but, unlike the observations, the simulated convection becomes surface-based and gravity current-lifted. The sensitivity of the simulated MCS to surface heat fluxes and diabatic cooling processes is explored. Surface heating and advection are
shown to increase the buoyancy of the boundary layer air and enhance the transition to surface-based convection. Diabatic cooling processes are shown to maintain the simulated MCS in two ways: they strengthen the descent of the rear-inflow jet, generating a wave, and they also strengthen the undercurrent via cold outflow from the north of the storm. In the second part of this thesis the Met Office Large Eddy Model is used to investigate the
interaction between Kelvin-Helmholtz billows and elevated convection. It is shown that there is a strong coupling between the updraughts and downdraughts in the billows and
convective clouds.