It is known that "one-way" lateral boundary conditions constrain the growth of initial perturbations in limited-area ensemble forecasts, therefore reducing ensemble dispersion. External boundary conditions typically lack fine-scale features, and in the case of ensemble forecasts, also lack consistent perturbations. Perturbations growing on the nested domain become displaced by the coarsely resolved boundary conditions while the domain size itself determines the maximum wavelength attainable by the perturbations. Another aspect of the boundary condition problem that has previously received little attention is the impact of boundary condition update frequency. Commonly used linear interpolation between relatively infrequent boundary condition updates acts as a filter that exacerbates the scale deficiency problem.

To examine the above problem in a controlled and efficient manner, a modified barotropic channel model with periodic east-west boundaries is constructed and shown to produce a rather realistic model atmosphere. Results are accumulated over one hundred independent cases using three nested domain sizes and five different nesting frequencies. Detailed statistics will be presented that demonstrate the lack of growth in error variance, particularly at high wavenumbers. To help restore the error variance lost at small scales, a method is developed to apply coherent perturbations to the boundary forcing at scales unresolved by the external model. The amplitude of such perturbations is chosen to mimic the exponential growth curves generated from simulations in unbounded domains. The use of more frequently updated boundary conditions and the inclusion of small-scale boundary perturbations will be shown to enhance the dispersion for limited-area ensemble forecasts.