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2014 .

(6 publications)

S. Fermepin and S. Bony. Influence of low-cloud radiative effects on tropical circulation and precipitation. Journal of Advances in Modeling Earth Systems, 6:513-526, September 2014. [ bib | DOI | ADS link ]

Low-level clouds, which constitute the most prevalent cloud type over tropical oceans, exert a radiative cooling within the planetary boundary layer. By using an atmospheric general circulation model, we investigate the role that this cloud radiative cooling plays in the present-day climate. Low-cloud radiative effects are found to increase the tropics-wide precipitation, to strengthen the winds at the surface of the tropical oceans, and to amplify the atmospheric overturning circulation. An analysis of the water and energy budgets of the atmosphere reveals that most of these effects arises from the strong coupling of cloud-radiative cooling with turbulent fluxes at the ocean surface. The impact of cloud-radiative effects on atmospheric dynamics and precipitation is shown to occur on very short time scales (a few days). Therefore, short-term atmospheric forecasts constitute a valuable framework for evaluating the interactions between cloud processes and atmospheric dynamics, and for assessing their dependence on model physics.

S. Bony, G. Bellon, D. Klocke, S. Sherwood, S. Fermepin, and S. Denvil. Addendum: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation. Nature Geoscience, 7:547, July 2014. [ bib | DOI | ADS link ]

A. Voigt, S. Bony, J.-L. Dufresne, and B. Stevens. The radiative impact of clouds on the shift of the Intertropical Convergence Zone. Geophysical Research Letters, 41:4308-4315, June 2014. [ bib | DOI | ADS link ]

Whereas it is well established that clouds are important to changes in Earth's surface temperature, their impact on changes of the large-scale atmospheric circulation is less well understood. Here we study the radiative impact of clouds on the shift of the Intertropical Convergence Zone (ITCZ) in response to hemispheric surface albedo forcings. The problem is approached using aquaplanet simulations with four comprehensive atmosphere models. The radiative impact of clouds on the ITCZ shift differs in sign and magnitude across models and is responsible for half of the model spread in the ITCZ shift. The model spread is dominated by tropical clouds whose radiative impact is linked to the dependence of their cloud radiative properties on the circulation. The simulations not only demonstrate the importance of clouds for circulation changes but also propose a way to reduce the model uncertainty in ITCZ shifts.

Y. Li, D. W. J. Thompson, G. L. Stephens, and S. Bony. A global survey of the instantaneous linkages between cloud vertical structure and large-scale climate. Journal of Geophysical Research (Atmospheres), 119:3770-3792, April 2014. [ bib | DOI | ADS link ]

The instantaneous linkages between cloud vertical structure and various large-scale meteorological parameters are investigated using 5 years of data from the CloudSat/CALIPSO instruments. The linkages are systemically explored and quantified at all vertical levels and throughout the global ocean in both the long-term mean and on month-to-month timescales. A number of novel large-scale meteorological parameters are used in the analysis, including tropopause temperatures, upper tropospheric stability, and storm track activity. The results provide a baseline for evaluating physical parameterizations of clouds in GCMs and a reference for interpreting the signatures of large-scale atmospheric phenomena in cloud vertical structure. In the long-term mean, upper tropospheric cloud incidence throughout the globe increases with (1) decreasing tropopause temperature (at a rate of 2-4% K-1), (2) decreasing upper tropospheric stability (5-10% per K km-1), and (3) increasing large-scale vertical motion (1-4% per 10 hPa d-1). In contrast, lower tropospheric cloud incidence increases with (1) increasing lower tropospheric stability (10% per K km-1) and descending motion (1% per 10 hPa d-1) in regions of subtropical regime but (2) decreasing lower tropospheric stability (4% per K km-1) and ascending motion (2% per 10 hPa d-1) over the Arctic region. Variations in static stability and vertical motion account for 20-35% of the month-to-month variance in upper tropospheric cloudiness but less than 10% of the variance in lower tropospheric clouds. Upper tropospheric cloud incidence in the storm track regions is strongly linked to the variance of large-scale vertical motion and thus the amplitude of baroclinic waves.

H.-Y. Ma, S. Xie, S. A. Klein, K. D. Williams, J. S. Boyle, S. Bony, H. Douville, S. Fermepin, B. Medeiros, S. Tyteca, M. Watanabe, and D. Williamson. On the Correspondence between Mean Forecast Errors and Climate Errors in CMIP5 Models. Journal of Climate, 27:1781-1798, February 2014. [ bib | DOI | ADS link ]

S. C. Sherwood, S. Bony, and J.-L. Dufresne. Spread in model climate sensitivity traced to atmospheric convective mixing. Nature, 505:37-42, January 2014. [ bib | DOI | ADS link ]

Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

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