Using large-scale forcing data taken from observations over the TOGA-COARE intensive flux array, the UK Met Office Cloud Resolving Model is used to simulate six days into a westerly wind burst event starting from 00Z 20 December 1992. The aim of the experiments is to study the development and lifecycle of mesoscale convective systems (MCSs) which are simulated in the domain. The model is run in three-dimensions with prognostic variables for cloud ice, snow and graupel along with interactive long-wave and short-wave radiation schemes. Most simulations are performed using a 256km x 256km x 20km domain size with 2km horizontal resolution and 60 vertical levels, increasing in spacing from 120m near the surface to 330m in the free troposphere to 500m in the stratosphere. Shorter experiments investigating the sensitivity to domain size and horizontal resolution are also performed. A number of time-averaged diagnostics are specified in terms of common core properties or column classifications and these are used to define the convective and stratiform regions of the simulated MCSs and to investigate their properties.
Preliminary results show that the net mass flux of the MCSs are fairly constant above the freezing level with large net detrainment in the convective cores being balanced by an increase in the net stratiform mass flux. Towards the top of the system the mesoscale component becomes a significant fraction of the total mass flux of the system. The anvil region is shown to be mostly positively buoyant above the freezing level, and condensate production within the mesoscale updraughts is roughly constant between the freezing level and the anvil top. This compares to the convective updraughts in which maximum latent heating occurs just below the freezing level. At anvil top both the convective and mesoscale updraughts provide similar domain scale latent heating rates of about 4K/day. A sensitivity run in which the radiative forcing is fixed rather than interactive with the cloud fields demonstrates the importance of radiation in the MCS development.
The momentum transport in the convective cores and mesoscale updraughts is also studied. When the horizontal wind field is allowed to vary freely with the forcing, rather than be relaxed to observed values, the effect of convective momentum transports on reducing the low level shear can be clearly seen. This reduced shear hinders the development of organized convection within the model. The momentum flux is shown to be primarily downgradient in the convective cores and the core shear profile is dominated by the pressure gradient term and agrees well with current parametrizations. The pressure gradient term does not seem to dominate in the mesoscale updraughts.
Applications of these results to the representation of tropical mesoscale convective systems in global forecast and climate models shall be discussed.