CGD's Dr. James Hack

Abstract

In this study, it is shown that the NCAR and GFDL GCMs exhibit a marked difference in climate sensitivity of clouds and radiative fluxes in response to doubled CO₂ and ±2-K SST perturbations. The GFDL model predicted a substantial decrease in cloud amount and an increase in cloud condensate in the warmer climate, but produced a much weaker change in net cloud radiative forcing (CRF) than the NCAR model. Using a multiple linear regression (MLR) method, the full-sky radiative flux change at the top of the atmosphere was successfully decomposed into individual components associated with the clear sky and different types of clouds. The authors specifically examined the cloud feedbacks due to the cloud amount and cloud condensate changes involving low, mid-, and high clouds between 60°S and 60°N. It was found that the NCAR and GFDL models predicted the same sign of individual longwave and shortwave feedbacks resulting from the change in cloud amount and cloud condensate for all three types of clouds (low, mid, and high) despite the different cloud and radiation schemes used in the models. However, since the individual longwave and shortwave feedbacks resulting from the change in cloud amount and cloud condensate generally have the opposite signs, the net cloud feedback is a subtle residual of all. Strong cancellations between individual cloud feedbacks may result in a weak net cloud feedback. This result is consistent with the findings of the previous studies, which used different approaches to diagnose cloud feedbacks. This study indicates that the proposed MLR approach provides an easy way to efficiently expose the similarity and discrepancy of individual cloud feedback processes between GCMs, which are hidden in the total cloud feedback measured by CRF. Most importantly, this method has the potential to be applied to satellite measurements. Thus, it may serve as a reliable and efficient method to investigate cloud feedback mechanisms on short-term scales by comparing simulations with available observations, which may provide a useful way to identify the cause for the wide spread of cloud feedbacks in GCMs.

Figure caption: Zonally averaged change in annual mean cloud amount, CWP, and net CRF in response to doubled CO₂ and ±2-K SST perturbations simulated by CAM3 and AM2. In the doubling CO₂ experiments, the two models are coupled to an SOM.

Support: The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Zhu, P., J. J. Hack, J. T. Kiehl, and C. S. Bretherton, 2007: Climate sensitivity of tropical and subtropical marine low cloud amount to ENSO and global warming due to doubled CO₂. Journal of Geophysical Research, 112, D17108, doi:10.1029/2006JD008174.

Abstract

In this study, we systematically analyzed the sensitivity of tropical and subtropical marine low cloud amount to the short-term climate anomaly associated with the 1997-1998 El Niño and the long-term climate change caused by doubled CO₂ using the International Satellite Cloud Climatology Project (ISCCP) cloud measurements, European Centre for Medium-Range Weather Forecasting (ECMWF) reanalyses, and the sea surface temperature (SST) forced and coupled simulations performed by the latest version of the National Center for Atmospheric Research (NCAR) and Geophysical Fluid Dynamics Laboratory (GFDL) climate models. It is found that the changes in low cloud amount associated with the 1997-1998 El Niño and the doubled CO₂ induced climate change have different characteristics and are controlled by different physical processes. Most reduction in low cloud amount related to the 1997-1998 El Niño occurs in the eastern tropical Pacific associated with an upward large-scale motion and a weak atmospheric stability measured by the 500 hPa vertical velocity and the potential temperature difference between 700 hPa and the surface, and is negatively correlated to the local SST anomaly. In addition to the other mechanisms suggested by the previous studies, our analyses based on the ISCCP observations indicate that the change in atmospheric convective activities in these regions is one of the reasons responsible for the change in low cloud amount. In contrast, most increase in low cloud amount due to doubled CO₂ simulated by the NCAR and GFDL models occurs in the subtropical subsidence regimes associated with a strong atmospheric stability, and is closely related to the spatial change pattern of SST consistent with previous studies. The increase in low cloud amount appears to favor the location where SST is less increased. After removing the background mean SST increase due to doubled CO₂, the results show a clear negative correlation between the change in low cloud and the SST change. An analysis based on the simple atmospheric mixed layer model demonstrates a thermodynamic reason for such a change. The increase in the above-inversion atmospheric stratification due to doubled CO₂ tends to reduce the mixed layer depth in the areas with a small temperature increase, which helps to trap the moisture within the mixed layer, thus, favors low cloud formation.

Figure caption: Binned change in low cloud amount based on the SST change after removing the mean SST change over the ocean basin between 40°S and 40°N associated with the 1997-1998 El Niño (observations) and doubled CO₂ simulated by the NCAR and GFDL GCMs. Solid lines scaled to the right indicate the PDF of each temperature bin. CAM3-SOM and AM2-SOM represent the standard NCAR and GFDL atmospheric models coupled with a slab ocean model. CAM3-UW-SOM is a branch version of CAM3-SOM that uses a new moist turbulent mixing scheme and shallow convection scheme

Support: The authors wish to acknowledge the support for this work from the National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory (NOAA/GFDL). Hack and Kiehl also acknowledge the support from the Department of Energy Office of Science Climate Change Prediction Program. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.