Modifications to atmospheric physical parameterisations aimed at improving SST simulations in the ACCESS coupled model

Abstract

The Australian Community Climate and Earth System Simulator (ACCESS) has been developed at the Centre for Australian Weather and Climate Research. It is a coupled modeling system consisting of ocean, atmosphere and land surface. The ACCESS atmospheric component is the UK Met Office Unified Model (UM). The initial results from the ACCESS coupled model had significant errors in the sea surface temperature (SST). It has been identified that the SST bias is largely due to errors in the representation of clouds. We have found that the use of the homogenous cloud distribution within model grid-boxes produced an underestimation of solar radiation reaching the surface, causing a cooling effect. The model cloud scheme PC2 does not produce enough high cloud cover, which also led to a cooling effect. These two deficiencies have been largely remedied by the implementation of the triple-cloud scheme and a modification to the ice cloud fraction parameterisation in the PC2 cloud scheme. We have also modified the air–sea flux exchange scheme to improve the simulation of ocean currents. These modifications have led to significant improvements in the simulation of SST in ACCESS.