Zhao F.

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Zhao
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  • Article
    On the exchange of momentum over the open ocean
    (American Meteorological Society, 2013-08) Edson, James B. ; Jampana, Venkata ; Weller, Robert A. ; Bigorre, Sebastien P. ; Plueddemann, Albert J. ; Fairall, Christopher W. ; Miller, Scott D. ; Mahrt, Larry ; Vickers, Dean ; Hersbach, Hans ; Zhao, F.
    This study investigates the exchange of momentum between the atmosphere and ocean using data collected from four oceanic field experiments. Direct covariance estimates of momentum fluxes were collected in all four experiments and wind profiles were collected during three of them. The objective of the investigation is to improve parameterizations of the surface roughness and drag coefficient used to estimate the surface stress from bulk formulas. Specifically, the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk flux algorithm is refined to create COARE 3.5. Oversea measurements of dimensionless shear are used to investigate the stability function under stable and convective conditions. The behavior of surface roughness is then investigated over a wider range of wind speeds (up to 25 m s−1) and wave conditions than have been available from previous oversea field studies. The wind speed dependence of the Charnock coefficient α in the COARE algorithm is modified to , where m = 0.017 m−1 s and b = −0.005. When combined with a parameterization for smooth flow, this formulation gives better agreement with the stress estimates from all of the field programs at all winds speeds with significant improvement for wind speeds over 13 m s−1. Wave age– and wave slope–dependent parameterizations of the surface roughness are also investigated, but the COARE 3.5 wind speed–dependent formulation matches the observations well without any wave information. The available data provide a simple reason for why wind speed–, wave age–, and wave slope–dependent formulations give similar results—the inverse wave age varies nearly linearly with wind speed in long-fetch conditions for wind speeds up to 25 m s−1.
  • Article
    Historical and idealized climate model experiments : an intercomparison of Earth system models of intermediate complexity
    (Copernicus Publications on behalf of the European Geosciences Union, 2013-05-16) Eby, Michael ; Weaver, Andrew J. ; Alexander, K. ; Zickfeld, K. ; Abe-Ouchi, A. ; Cimatoribus, A. A. ; Crespin, E. ; Drijfhout, Sybren ; Edwards, N. R. ; Eliseev, A. V. ; Feulner, G. ; Fichefet, T. ; Forest, Chris E. ; Goosse, H. ; Holden, P. B. ; Joos, Fortunat ; Kawamiya, M. ; Kicklighter, David W. ; Kienert, H. ; Matsumoto, K. ; Mokhov, I. I. ; Monier, Erwan ; Olsen, Steffen M. ; Pedersen, J. O. P. ; Perrette, M. ; Philippon-Berthier, G. ; Ridgwell, Andy ; Schlosser, A. ; Schneider von Deimling, T. ; Shaffer, G. ; Smith, R. S. ; Spahni, R. ; Sokolov, Andrei P. ; Steinacher, M. ; Tachiiri, K. ; Tokos, K. ; Yoshimori, M. ; Zeng, Ning ; Zhao, F.
    Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO2, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions.