The effect of indiscriminate nudging time on large and small scales in regional climate modelling: Application to the Mediterranean basin

摘要

A regional climate model (RCM) is driven by the ERA-40 reanalyses produced by the ECMWF general circulation model (GCM) to simulate the winter 1998 climate over the Mediterranean basin. In this article, we consider the effects on internal variabilityof temporal nudging. This technique consists of relaxing the ROM's prognostic variables towards the GCM values within a predetermined time-scale, with the aim of disallowing large and unrealistic departures between driving and driven fields. To interpretthe significant effect of time nudging on the regional climate prediction, we develop a 'toy model' basically consisting of resolving a linear transport equation with a Newtonian relaxation term. This model predicts the existence of an optimal nudging time which depends on the time-scale over which numerical errors affect significantly the accuracy of the 'regional' solution at the large spatial scales, and the typical time-scale of the small-scale phenomena that are not resolved by the GCM.