Prediction of large-scale dynamics using unresolved computations

Abstract

We present a theoretical framework and numerical methods for predicting the large-scale properties of solutions of partial differential equations that are too complex to be properly resolved. We assume that prior statistical information about the distribution of the solutions is available, as is often the case in practice. The quantities we can compute condition the prior information and allow us to calculate mean properties of solutions in the future. We derive approximate ways for computing the evolution of the probabilities conditioned by what we can compute, and obtain ordinary differential equations for the expected values of a set of large-scale variables. Our methods are demonstrated on two simple but instructive examples, where the prior information consists of invariant canonical distributions