The roles of diurnal forcing and large-scale moisture transport for initiating rain over northwest Australia in a GCM

The representation of rainfall, and in particular its diurnal cycle, is generally poor in general circulation models (GCMs). Nonetheless, studies make use of GCMs in future climate projections in regions where precipitation has a strong diurnal cycle. In this study we evaluate whether one GCM (ACCESS1.3) can represent the rainfall in such a region (Australia) where the diurnal cycle of rainfall is produced as a result of both the destabilisation of the boundary layer (convection) and a larger-scale reorganisation of the low-level flow, the latter of which may be resolved by a GCM. In northern and eastern Australia, where the diurnal cycle of rainfall is controlled by convective processes, the GCM produces rain 3 to 6 h too early in the day. Nevertheless, the model represents the continental-scale reorganisation of the low-level circulation that results from the diurnal cycle of surface heating and cooling. A nocturnal low-level jet forms over the western half of the continent with strong convergence at the jet exit, which initiates rain overnight in the continental northwest, in agreement with previous work. The model also captures the change in the air-flow direction, from a southeasterly to a northeasterly, responsible for bringing the necessary moisture for precipitation to occur. Thus, while the model may have a tendency to initiate convection too early, it is able to represent the larger-scale nocturnal reorganisation of the flow and the associated rainfall.