The influence of lateral boundary conditions, fixed surface forcings, and synoptic situations upon predictability estimates using limited area numerical models

The predictability of synoptic scale and mesoscale atmosphere motions is investigated using limited area numerical forecast models. The influence of one-way interacting lateral boundary conditions upon predictability is studied using a 2-D barotropic numerical model and a complete mesoscale limited area model. It is shown that the initial perturbations grow only in domains larger than a critical size and damp in smaller domains. It is found that a main mechanism by which one-way interacting lateral boundary conditions constrain the evolution of initial perturbations is by setting the upper wavelength limit upon horizontal scales that are free to evolve in accordance to model dynamics. A major contributor to the differences between predictability properties of global numerical models and limited area models is the one-way interacting lateral boundary constraint in the latter case. On the other hand, it is shown that large scale growing boundary perturbations contaminate the internal solution rather quickly. This indicates that operational, medium range forecasts with nested limited area numerical models may not show significant improvement relative to the same integrations performed with global models.