Abstract
In this work, we examine the sensitivity of tropical mean climate and seasonal cycle to low clouds and cloud liquid water path (CLWP) by prescribing them in the NCEP climate forecast system (CFS). It is found that the change of low cloud cover alone has a minor influence on the amount of net shortwave radiation reaching the surface and on the warm biases in the southeastern Atlantic. In experiments where CLWP is prescribed using observations, the mean climate in the tropics is improved significantly, implying that shortwave radiation absorption by CLWP is mainly responsible for reducing the excessive surface net shortwave radiation over the southern oceans in the CFS. Corresponding to large CLWP values in the southeastern oceans, the model generates large low cloud amounts. That results in a reduction of net shortwave radiation at the ocean surface and the warm biases in the sea surface temperature in the southeastern oceans. Meanwhile, the cold tongue and associated surface wind stress in the eastern oceans become stronger and more realistic. As a consequence of the overall improvement of the tropical mean climate, the seasonal cycle in the tropical Atlantic is also improved. Based on the results from these sensitivity experiments, we propose a model bias correction approach, in which CLWP is prescribed only in the southeastern Atlantic by using observed annual mean climatology of CLWP. It is shown that the warm biases in the southeastern Atlantic are largely eliminated, and the seasonal cycle in the tropical Atlantic Ocean is significantly improved. Prescribing CLWP in the CFS is then an effective interim technique to reduce model biases and to improve the simulation of seasonal cycle in the tropics.
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Acknowledgments
In the process of designing and conducting the experiments, we received help from Sarah Lu (EMC/NCEP/NOAA), Dagang Wang (Princeton University), Kathy Pegion (COLA and ESRL/NOAA), Meizhu Fan (COLA, GMU and NOAA), Li Zhang (COLA and NOAA), Larry Marx (COLA), and Julia Manganello (COLA).We also appreciate J. L. Kinter III for his comments and encouragement, and V. Krishnamurthy, B. Cash, and two anonymous reviewers for their comments and suggestions. This work was supported by the NOAA CVP Program (NA07OAR4310310) (Hu and Huang), NSF ATM-0830062 (Huang and Schneider), NSF ATM-0830068, NOAA NA09OAR4310058, and NASA NNX09AN50G (Hu and Stan). Most of the calculation was conducted on the NCAR Bluefire super-computer.
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Hu, ZZ., Huang, B., Hou, YT. et al. Sensitivity of tropical climate to low-level clouds in the NCEP climate forecast system. Clim Dyn 36, 1795–1811 (2011). https://doi.org/10.1007/s00382-010-0797-z
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DOI: https://doi.org/10.1007/s00382-010-0797-z