High-resolution Climate Modeling
GFDL’s CM2.1 climate model, incorporating 1o ocean and 2o atmospheric components, was developed to produce output and science for the IPCC 4th assessment report. When evaluated over a broad suite of metrics this model was found to produce a high quality simulation of the present-day climate and its variability. In addition to other applications, GFDL uses the CM2.1 model for experimental seasonal predictions as part of the North American Multi-Model Ensemble (NMME).
Two subsequent development pathways took CM2.1 as their starting point, in order to produce climate models with enhanced representation of atmospheric chemistry, aerosols and clouds (CM3), and of biogeochemical processes (ESM2M and ESM2G).
A third stream of development attempted to improve CM2.1’s simulation by increasing the resolution of the atmospheric and oceanic components. This effort produced CM2.5 and its variants CM2.5FLOR and CM2.6. The resolution of the atmospheric and oceanic components of CM2.5 is increased by a factor of four compared to CM2.1, along with a 33% increase in the number of vertical levels in the atmosphere. A further refinement of the ocean component to 1/10o produced CM2.6 which can claim to resolve ocean eddies based on agreement with an observational estimate of the eddy kinetic energy.
Because seasonal predictions need large ensembles of model runs, the CM2.5 model
is not practical for seasonal prediction at this time due to limits on computational resources. Therefore, a variant of CM2.5 was produced with a coarse (1o) ocean that both doubled the model speed and permitted the use of existing ocean assimilation systems. This version, called CM2.5_FLOR (where FLOR stands for “Forecast version Low Ocean Resolution”) is now also used as part of the NMME, producing seasonal predictions on a real-time basis.
Nominal Model Resolution
Precipitation biases are significantly improved in CM2.5 relative to CM2.1. The double ITCZ problem is ameliorated and orographic precipitation is better represented. The simulation of ENSO variablity is also improved, as is seasonal variablity in the tropical Atlantic.
CM2.5’s higher resolution atmosphere allows simulation of tropical cyclone activity. To exploit this and other opportunities, CM2.5FLOR has been incorporated into the GFDL seasonal forecasting system and is showing improvements in tropical cyclone forecasts over the CM2.1-based system. Seasonal forecasts of temperature and precipitation over land are also improved.
CM2.5 has been applied to the problem of the future of global snow cover in a warming climate where its resolution allows improved simulation of changes in mountainous regions. The sign of projected snowfall changes was reversed in some regions due to the better representation of orographic effects.
The CM2.5/CM2.5FLOR/CM2.6 suite is currently being used to evaluate the role of ocean component resolution in the simulation of climate sensitivity and natural variability. The ability to explicitly simulate ocean eddies allows us to assess whether the eddy parameterizations that have been used in the current generation of climate models have introduced biases into the simulation of sensitivity and thereby biased projections of future climate change. Higher resolution simulations also help assess whether there may be biases in model simulations of natural internal climate varibility which are used in
detection/attribution and climate prediction studies.
The CM2.5 suite is currently being used to study ongoing and projected changes of drought on a regional basis, as well as other aspects of decadal climate variability.
Although it is essential for addressing certain climate problems, the increased realism of high resolution modeling comes at a cost. When resolution is increased, grid cell sizes are reduced and more of them are needed to cover the globe. The computational expense of a model increases with the number of such cells. Consequently, high resolution climate models are best used in conjunction with other tools that are more amenable to exploring long time scales or focus on incorporating comprehensive climate processes. Judicious use of a variety of models with differing resolutions allows robust results to
- Biases in the Atlantic ITCZ in seasonal-interannual variations for a coarse and a high resolution coupled climate model
- Controls of Global Snow Under a Changed Climate
- Doi, T., G.A. Vecchi, A. Rosati, and T.L. Delworth, 2013: Response to CO2 doubling of the Atlantic Hurricane Main Development Region in a High-Resolution Climate Model. Journal of Climate, 26(12), DOI:10.1175/JCLI-D-12-00110.1.
- Kapnick, S., and T.L. Delworth, 2013: Controls of global snow under a changed climate. J. Climate, 26 (15), 5537-5562. DOI: 10.1175/JCLI-D-12-00528.1
- Delworth, T.L., A. Rosati, W. Anderson, A.J. Adcroft, V. Balaji, R. Benson, K. Dixon, S.M. Griffies, H-C. Lee, R.C. Pacanowski, G.A. Vecchi, A.T. Wittenberg, F. Zeng, R. Zhang, 2012: Simulated Climate and Climate Change in the GFDL CM2.5 High-Resolution Coupled Climate Model, J. Climate, vol. 25, issue 8, pp. 2755-2781. DOI 10.1175/JCLI-D-11-00316.1