A three-dimensional numerical model of the sea breeze for the Plymouth region
A three-dimensional, hydrostatic, primitive equation model is developed to simulate the Plymouth sea breeze. The equations are integrated forward in time on a staggered mesh with a domain of 64km X 64km X 3km, using a combination of central and upstream differencing. The ground surface is assumed to be smooth and the heat input to the atmosphere transferred vertically without the explicit use of diffusion coefficients. This results in a less stringent stability condition on the timestep, thus reducing the computational cost of the simulations. Sensitivity tests for a two-dimensional version are presented, examining the influence of atmospheric stability, the synoptic scale flow and the magnitude of the surface heat flux. The major features of the raesoscale circulation are well represented including the strong overland updraughts associated with the sea breeze front. Frontal propagation rates are estimated in each simulation. and are found to be in general agreement with available data for Southern England. The preliminary three-dimensional results concern the sensitivity of the model to variations in the synoptic scale flow and the coastal configuration. The former tests show a more vigorous system developing with an offshore synoptic flow and a much weaker circulation for the onshore. The second test illustrates the development of a bay-induced landward bulge in the temperature gradient resulting in an asymmetric distribution of onshore convergence zones. The final simulations represent two case studies of Plymouth sea breeze events during August 1983 and May 1984. The major features of the system are again well simulated, however several key problem areas are identified. These involve the influence of topographic variations, the numerical grid resolution, the heat flux parameterisation and the role of turbulent transfer. Recommendations for further research are proposed and include the application of a terrain-following coordinate scheme and a new observational initiative. In addition, the need for an improved heat flux parameterisation and turbulence closure are identified.