The Influence of One-Way Interacting Lateral Boundary Conditions upon Predictability of Flow in Bounded Numerical Models

Abstract

The influence of one-way interacting lateral bounday conditions upon the predictability of flows in boundeddomains is studied using the barotropic nondivergent model in global and local domains. Past studies haveattempted to reconcile the apparent contradiction between pessimistic forecast of predictability theory and thehigh predictability actually found in regional models. Those investigations have emphasized the rather differentspectra and forcing mechanisms that are not considered in the theoretical estimates. We demonstrate that thepredictability remains high in an unforced, inertially driven local flow characterized by a typical synoptic scalespectrum, and constrained only by lateral boundary specification. We also offer a possible reconciliation ofthese results with the classical theory. It is shown that one-way interacting boundary conditions enhance thepredictability of flow in a local region which, without the boundary constraints, has limited predictabifity. Thedegree of this boundary constraint is dependent on the size of the domain, on the nature of flow in the domainand on the scale structure of the error field. The boundary constraint is particularly strong when a substantialportion of the larger scale flow in the domain is imposed through the boundary condition. In that case, smallscale initial uncertainties have limited interaction with the basic flow field because of scale separation andbecause the largest scales in the domain do not react to internal dynamics.