An evaluation of the boundary conditions for nested domains for idealized hurricane simulations

A recent study completed for an idealized hurricane simulation over water at a constant surface temperature using the PSU/NCAR mesoscale model version 5, MM5, showed that the storm developed very differently as a function of the vertical spacing of the model sigma levels. In addition, all the simulations, regardless of the sigma distribution, eventually filled and lost their hurricane characteristics, an unexpected result. The time and spatial resolutions had been carefully chosen to avoid stability bounds, yet the simulations became unstable.

This new study evaluates the boundary conditions at the outer boundaries of the primary domain. Preliminary results show that many of the physical parameters such as the winds and many of the precipitation variables are convected outward to the edge of the domain, where the boundary values are held constant for an idealized simulation. In spite of the relaxation boundary conditions applied to the variables near the boundary, these parameters are seen to ‘stack' up at the boundary of the domain, causing new recirculation regions that eventually rebound back into the simulation to affect the hurricane development. Even when the domain is enlarged, similar results can be seen, although the impact is muted.

An investigation into the way the boundary conditions are addressed on the primary domain has been started. One possibility that is being explored is to build in non-reflective boundary conditions for some of the parameters to allow the variables to be convected out of the domain. Another prospect may be to apply more damping at the domain boundaries, coupled with recommendations for size of the primary domain as a function of the size of the expected hurricane to minimize the adverse affect.