Evaluation of ACCESS model clouds
Evaluation of ACCESS model clouds over the Southern Ocean using synthetic brightness temperature program and COSP package
Abstract
Sea Surface Temperature (SST) warm bias in the Southern Ocean (SO) is a common problem in most climate models. We investigate the causes of this problem by performing comparisons between the modeled cloud and radiative properties and satellite observations. We use the Australian Community and Climate Earth System Simulator (ACCESS) model to perform short period NWP experiments and long-term climate model simulations. The radiation budgets at the earth’s surface and the top of the atmosphere determined by these simulations are compared with corresponding observations determined by satellites. The cloud properties are further evaluated using a synthetic brightness temperature (BT) code and the Cloud Feedback Model Intercomparison Project (CFMIP) Observational Simulator Package (COSP). The BT code uses the ACCESS model atmosphere to determine the brightness temperature that can be seen by satellite and the results are compared directly with instantaneous satellite measurements. CloudSat and CALIPSO simulators have been implemented into the ACCESS model and these observations are used to evaluate the modeled cloud fraction and microphysical properties.
From these comparisons we have found that the SO warm biases exist in both long term climate simulation and short time NWP forecasting. The net SW radiation around the Southern Ocean is overestimated at both the TOA and surface and these occur in both NWP and climate model simulations. There are seasonal variations in the warm biases, with maximum bias occurring in summer. The excess solar radiation is mainly due to poorly simulated model clouds. The middle and low level cloud amounts as well as cloud LWP/IWP are all underestimated by the ACCESS model while high level cloud amounts are overestimated. These are the main reasons causing SO warm biases in the ACCESS model.