Chaotic behaviour of the regional climate models, CRCM5 and HIRHAM5, in ensemble simulations over an Arctic domain

Abstract

In a chaotic system such as the Earth’s atmosphere, the differences between the members in an ensemble of global climate model simulations launched from different initial conditions initially grow in time until they reach the level of natural variability, indicating that member simulations become uncorrelated. In nested Regional Climate Models (RCMs), however, the growth of inter-member differences is quenched due to the control exerted by the lateral boundary conditions (LBCs), but it nevertheless exhibits episodes of large fluctuations. Earlier work has speculated that this puzzling behaviour may simply reflect remaining chaos allowed by the incomplete control exerted by LBC.In this work, two large ensembles of twenty simulations were performed over an Arctic domain with two different RCMs: the Canadian RCM (CRCM5) and the High-Resolution Limited-Area Model (HIRHAM5). The inter-member variability (IV) of each ensemble was methodically analysed in the framework of the potential temperature IV budget. The study reveals that, despite being simulated by models with entirely different formulation, the two ensembles exhibit nearly identical IV patterns and time evolution, and in both cases baroclinic processes trigger fluctuations of IV. These results confirm earlier speculations that IV in RCMs is not an artefact of specific model nesting technique,but rather a natural phenomenon arising from the chaotic nature of the atmosphere.