The resolution sensitivity of the Asian summer monsoon and its inter-model comparison between MRI-AGCM and MetUM
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In this study, we compare the resolution sensitivity of the Asian Summer Monsoon (ASM) in two Atmospheric General Circulation Models (AGCMs): the MRI-AGCM and the MetUM. We analyze the MetUM at three different resolutions, N96 (approximately 200-km mesh on the equator), N216 (90-km mesh) and N512 (40-km mesh), and the MRI-AGCM at TL95 (approximately 180-km mesh on the equator), TL319 (60-km mesh), and TL959 (20-km mesh). The MRI-AGCM and the MetUM both show decreasing precipitation over the western Pacific with increasing resolution, but their precipitation responses differ over the Indian Ocean. In MRI-AGCM, a large precipitation increase appears off the equator (5–20°N). In MetUM, this off-equatorial precipitation increase is less significant and precipitation decreases over the equator. Moisture budget analysis demonstrates that a changing in moisture flux convergence at higher resolution is related to the precipitation response. Orographic effects, intra-seasonal variability and the representation of the meridional thermal gradient are explored as possible causes of the resolution sensitivity. Both high-resolution AGCMs (TL959 and N512) can represent steep topography, which anchors the rainfall pattern over south Asia and the Maritime Continent. In MRI-AGCM, representation of low pressure systems in TL959 also contributes to the rainfall pattern. Furthermore, the seasonal evolution of the meridional thermal gradient appears to be more accurate at higher resolution, particularly in the MRI-AGCM. These findings emphasize that the impact of resolution is only robust across the two AGCMs for some features of the ASM, and highlights the importance of multi-model studies of GCM resolution sensitivity.
KeywordsAsian monsoon Orography High resolution AMIP HighResMIP
SJJ and RS were supported by the Joint Weather and Climate Research Programme (JWCRP), a partnership between the Natural Environment Research Council (NERC) and the UK Met Office, under University of Reading Contract R8/H9/37. RS and MED were supported by the National Centre for Atmospheric Science Climate directorate (NCAS-Climate), a collaborative centre of NERC. MED acknowledges NCAS Climate contract R8/H12/83/001 for the High Resolution Climate Modelling programme. The MetUM simulations analysed here were produced as part of the UPSCALE project (information about data access is available from the project website: http://proj.badc.rl.ac.uk/upscale). We thank the UPSCALE team [P.L. Vidale (PI), M. J. Roberts, M. S. Mizielinski, J. Strachan, RS, MED], the large team of model developers, infrastructure experts and all the other essential components required to conduct the UPSCALE campaign, in particular the PRACE infrastructure and the Stuttgart HLRS supercomputing centre, as well as the STFC CEDA service for data storage and analysis using the JASMIN platform. We also acknowledge use of the MONSooN system, a collaborative facility supplied under JWCRP, and HECToR, the UK national supercomputer. Numerical experiments of MRI-AGCM were executed on the Earth Simulator of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). This work was conducted under the SOUSEI Program of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
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