Mechanisms of improved rainfall simulation over the Maritime Continent due to increased horizontal resolution in an AGCM
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The General Circulation Models experience a significant challenge in realistically simulating rainfall over the tropical Maritime Continent (hereafter, MC). Here, we investigate the mechanisms of an improvement in monthly rainfall simulation over the MC region in the UK Met Office Unified Model (version Global Atmosphere 6.0), which occurs when the horizontal resolution is increased from N96 (grid spacing ~135 km) to N216 (~60 km). The increased resolution enhances the area-averaged rainfall rate over the MC, thereby reducing the dry rainfall bias seen in the model at the N96 resolution. We find that the enhanced area-averaged rainfall is mostly due to an increase in the medium rainfall rates that occurs over the MC islands in the N216 experiment. The rainfall change is predominantly associated with changes in the atmospheric convective circulation and the related horizontal moisture flux convergence. The vertical profiles of convective circulation show a strong sensitivity to the increased horizontal resolution over the MC islands, but not over the surrounding oceans. It is shown that a significant underestimation of the deep convection (as opposed to the shallow convection) in the N96 experiment is primarily responsible for the stronger dry bias in this experiment. We present evidence that the dry bias, and the associated weaker deep convection, are in part caused by the strongly smoothed orography used in the N96 experiment, which provides a weaker orographic lifting of the moist surface air (in a conditionally unstable atmosphere) than that in the N216 experiment.
KeywordsRainfall bias Maritime Continent GCMs Tropical convection Orographic lifting
We thank two anonymous reviewers for providing useful comments, which have helped improve this paper. We also thank Ian Watterson and Matthew Wheeler for providing useful feedbacks on an earlier version of this manuscript. Thanks are also due to Martin Dix for setting up the AMIP-style experiments analysed in this work and Faina Tseitkin for performing one of the experiments. This work has been undertaken as part of the Australian Climate Change Science Program, funded jointly by the Australian Government Department of the Environment, the Bureau of Meteorology and CSIRO. The simulations were performed at the NCI National Facility at the Australian National University. We thank the members of the Maritime Continent Process Evaluation Group (MC-PEG) for discussions during the course of this work. GPCP and CMAP Precipitation data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.esrl.noaa.gov/psd/.
- Gregory D, Allen S (1991) The effect of convective scale downdrafts upon NWP and climate simulations. In: Ninth conference on numerical weather prediction. Denver, Color. Amer Met Soc, pp 122–123Google Scholar