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Publication Date
1 November 2012

On the Correspondence between Short- and Long- Timescale Systematic: Errors in CAM4/CAM5 for the Years of Tropical Convection



The correspondence between short and long timescale systematic errors in the Community Atmospheric Model version 4 (CAM4) and version 5 (CAM5) is systematically examined. The analysis is based on the annual mean data constructed from long term "free-running" simulations and short-range hindcasts. The hindcasts are initialized every day with the ECMWF analysis for the Year(s) of Tropical Convection. It has been found that most systematic errors, particularly those associated with moist processes, are apparent in day 2 hindcasts. These errors steadily grow with the hindcast lead time and typically saturate after five days with amplitudes comparable to the climate errors. Examples are the excessive precipitation in much of the tropics and the overestimate of net shortwave absorbed radiation in the stratocumulus cloud decks over the eastern subtropical oceans and the Southern Ocean at about 60°S. This suggests that these errors are likely the result of model parameterization errors as the large-scale flow remains close to observed in the first few days of the hindcasts. In contrast, other climate errors are present in the hindcasts, but with amplitudes that are significantly smaller than and do not approach their climate errors during the 6-day hindcasts. These include the cold biases in the lower stratosphere, the unrealistic double-Intertropical Convergence Zone pattern in the simulated precipitation, and an annular mode bias in extratropical sea-level pressure. This indicates that these biases could be related to slower processes such as radiative and chemical processes which are important in the lower stratosphere or the result of poor interactions of the parameterized physics with the large-scale flow.
“On The Correspondence Between Short- And Long- Timescale Systematic: Errors In Cam4/Cam5 For The Years Of Tropical Convection”. 2012. Journal Of Climate. doi:10.1175/JCLI-D-12-00134.1.
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