An Evaluation of the Boundary Layer Cloud Forecast over the Southern Ocean in an Limited-area Numerical Weather Prediction System with In-situ, Space-borne and Ground-based Observations

Disproportionately large biases of the energy budget over the Southern Hemisphere in both the reanalysis and current climate models have been found to be linked to the poor representation of the Southern Ocean (SO) clouds (Trenberth and Fasullo 2010). A recent study by Bodas-Salcedo et al. (2012) using the UK Met Office Unified Model highlights that the under-prediction of the low- and mid-level post-frontal clouds contributes to the largest bias of the surface downwelling shortwave radiation over the SO. This significant bias is a common feature among the CIMP5 members (Bodas-Salcedo et al. 2013). Williams et al. (2013) suggest that the biases in simulating the SO clouds mainly occur in the environment behind cold fronts and/or on the leading side of transient ridges.

This presented study applies, for the first time, near synchronized in-situ, space-borne (A-Train) and ground-based observations to evaluate the boundary layer clouds (BLCs) over Tasmania and the adjacent SO simulated by the limited-area Numerical Weather Prediction (NWP) version of the Australian Community Climate and Earth System Simulator (ACCESS). This NWP forecast system is largely based on the UK Met Office Unified Model/Variational Assimilation (UM/VAR) system. In this study, two cases associated with postfrontal conditions and the leading side of high pressure ridge are investigated.

Results of the simulations suggest that the operational ACCESS NWP demonstrates a considerable level of skill in forecasting the macrophysical properties of the BLCs in the post-frontal and high pressure environments over the SO, generally consistent with the in-situ and remote sensing observations. However, some notable challenges remain. The fractional cloud cover of the widespread BLCs is under-predicted. There is a positive (negative) bias in the forecast cloud-base and cloud-top heights (cloud-top temperature). The forecast capping inversion is too high and strong compared to the measurements. The secondary inversion constantly present in the observation is not predicted. And the frequently observed large values of liquid water content are not well reproduced.

Sensitivity experimentations of testing the newly developed parameterisations are undertaken. Simulations with the shear-dominated planetary boundary layer (PBL) scheme (Lock et al. 2000) and the autoconversion microphysics scheme (Franklin 2008) show notable improvements in the forecast fractional cloud cover, cloud structure, and the distribution of liquid water content (better average and maximum values), although the lack of enough cloud liquid water is still a discernible shortcoming in the model.

The implication of this study to better understanding the nature of the SO clouds and the regional shortwave radiation bias in the climate model simulations will be discussed. Possible reasons that likely have contributed to the model deficiencies will be presented.