EGU2020-15422
https://doi.org/10.5194/egusphere-egu2020-15422
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Radiative feedbacks in a 1D radiative-convective equilibrium model

Lukas Kluft1, Sally Dacie1, Stefan A. Buehler2, Hauke Schmidt1, and Bjorn Stevens1
Lukas Kluft et al.
  • 1Max-Planck-Institut für Meteorologie, Hamburg, Germany (lukas.kluft@mpimet.mpg.de)
  • 2Universität Hamburg, Faculty of Mathematics, Informatics and Natural Sciences, Department of Earth Sciences, Meteorological Institute, Hamburg, Germany

Equilibrium climate sensitivity (ECS), the change in surface temperature in response to a doubling of atmospheric CO2, is arguably one of the most important quantities when discussing climate change. Despite major improvements in climate modelling over the last decades, ECS estimates lie within a rather constant range between 1.5-4 K. The cause of this spread is not obvious as the comparison of comprehensive climate models is difficult due to the complexity of their formulations.

 

We are revisiting one of the simplest climate models, one-dimensional radiative-convective equilibrium (RCE). Despite their simple and concise model formulation, RCE models include the most dominant clear-sky radiative feedbacks. In our study, we quantify the strength of the Planck, water-vapor, and lapse-rate feedback by turning them on or off using different model configurations. This method allows us to compare the effect of different model assumptions, e.g. the vertical distribution of water vapor, on the decomposed radiative feedbacks. We find that the interplay of the water-vapor and the lapse-rate feedback is especially affected by the relative humidity in the upper troposphere.

 

The RCE model is run with a state-of-the-art radiation scheme, that is also used in comprehensive  Earth system models. A line-by-line radiative transfer model is used to both verify the performance of the fast radiation scheme, and to attribute changes in the radiative feedbacks to specific regions in the electromagnetic spectrum.

 

In a further step, conceptual rectangular clouds are added to investigate possible cloud masking effects on both the radiative forcing and feedback. A large Monte Carlo ensemble is used to tune the cloud optical parameters in a way that the resulting cloud radiative effect matches satellite observations. Preliminary results suggest a near zero long-wave feedback, in contrast to previous studies.

How to cite: Kluft, L., Dacie, S., Buehler, S. A., Schmidt, H., and Stevens, B.: Radiative feedbacks in a 1D radiative-convective equilibrium model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15422, https://doi.org/10.5194/egusphere-egu2020-15422, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 04 May 2020
  • AC1: Comment on EGU2020-15422, Lukas Kluft, 06 May 2020

    > Franziska Winterstein DLR (co-author) (14:58) Do you find variations in the climate feedback parameter with respect to the forcing agent?

    In our first publiaction [0] we did experiments using conescutive doublings of the CO2 concentration. The equilibirum surface warming increased due to a strengthening of the effective forcing (more than the usual ln(CO2) law). However, the climate feedback itself was almost unchanged.

    But the simulations only covered a small temperature range (~8K), that's why we decided to investigate this in more depth.

    [0]

  • CC1: Comment on EGU2020-15422, Miklos Zagoni, 06 May 2020

    Dear Lucas, you commented this afternoon, @15:00:
     “full line-by-line model represents our best physical understanding of radiative transfer in the atmosphere”.
    I absolutely agree. May I call your kind attention to my presentation, where I use the simplest textbook form of the spectrally and globally integrated radiative transfer equations (in two-stream approx), and their evident all-sky extensions, in my Eq. (1) - (4). I check them on two decades of CERES data, and found them valid within 3 Wm-2. (Display D3436, EGU2020-1). The consequences are far-reaching;  I do hope you will find them interesting!
    Many thanks, Miklos