Sensitivity studies with the physical parametrisations in the BMRC global atmospheric spectral model

A series of 30-day integrations was conducted with the BMRC global spectral model to test the sensitivity of the model's systematic temperature and moisture biases, and the simulated mean climate to changes in the physical parametrisations associated with convection and vertical diffusion. The moisture bias in the model was dramatically improved by the introduction of the Tiedtke shallow convection scheme. There was some decrease in the cold bias for tropical regions with the change to stability dependent vertical diffusion, but a strong cold anomaly persisted in mid-latitudes. The experiment with these refinements in the physical parametrisations generally showed better agreement in the tropics with observed climatologies of precipitation, velocity potential and zonal wind than the control experiment. Another of the options considered was to emulate some aspects of the Kuo scheme as used in the Bureau's operational Australian region model. In this case a relatively smaller percentage of total rainfall was associated with penetrative convection and the systematic biases and mean fields were more similar to the control than the 'refined physics' experiment.