EGU2020-9470
https://doi.org/10.5194/egusphere-egu2020-9470
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Exploring the aerosol-cloud-radiation relationships in deep marine stratocumulus layers

Anna Possner1, Ryan Eastman2, Frida Bender3, and Franziska Glassmeier4
Anna Possner et al.
  • 1Goethe University, Institute for Atmospheric and Environmental Sciences, Frankfurt/Main, Germany (apossner@iau.uni-frankfurt.de)
  • 2Department of Atmospheric Sciences, University of Washington, Seattle, USA
  • 3Department of Meteorology and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
  • 4Department of Environmental Sciences,Wageningen University, Wageningen, the Netherlands

Marine stratocumuli cover around a fifth of the worlds oceans and are a key contributor to Earth’s radiative balance at the surface. Their sensitivity to changes in anthropogenic aerosol concentrations remain a key uncertainty in the climate system. Our current understanding of their sensitivity and the plausible range of the aerosol-cloud radiative forcing is largely based on the process understanding obtained from field campaigns, high-resolution modelling, and satellite records of aerosol-induced phenomena such as volcano or ship tracks.

Yet, a large fraction of these records is only applicable to relatively shallow planetary boundary layers (PBLs). Ship tracks are only found in boundary layers up to a depth of 800m. Field campaigns and high-resolution modelling studies of aerosol-cloud-radiation interactions in marine stratocumuli have been restricted to a similar range of PBL depths in the past. Meanwhile over 70% of marine boundary layers reside in deeper PBLs.

The liquid water path (LWP) adjustment due to aerosol-cloud interactions in marine stratocumuli remains a considerable source of uncertainty for climate sensitivity estimates. An unequivocal attribution of LWP adjustments to changes in aerosol concentration from climatology remains difficult due to the considerable covariance between meteorological conditions alongside changes in aerosol concentrations.

Here, we combine a range of space-born remote sensing retrievals to investigate the relationship of cloud-radiative properties for different boundary layer depths and aerosol concentrations. As done in previous studies we utilise the susceptibility framework, i.e. the relative change in LWP scaled by the relative change in cloud droplet number concentration, to quantify the change in LWP adjustment with PBL depth. We show that the susceptibility of LWP adjustments triples in magnitude from values of -0.1 in PBLs shallower than 0.5 km to -0.33 in PBLs deeper than 1 km.

We further argue that LWP susceptibility estimates inferred from deep PBL climatologies are poorly constrained due to a lack of process-oriented observations. Meanwhile, susceptibilities inferred from climatology in shallow PBL regimes are consistent with estimates obtained from process modelling studies, but are overestimated as compared to pollution track estimates.

How to cite: Possner, A., Eastman, R., Bender, F., and Glassmeier, F.: Exploring the aerosol-cloud-radiation relationships in deep marine stratocumulus layers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9470, https://doi.org/10.5194/egusphere-egu2020-9470, 2020

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Presentation version 2 – uploaded on 15 Apr 2020
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  • CC1: Comment on EGU2020-9470, Andrew Gettelman, 04 May 2020

    Hi Anna,

    Nice talk. Two questions.

    1. If the gradient due to BL height might be due to entrainment strength, Would that be more LTS or EIS dependent? And you could see that? 

    2. And have you thought about testing this in a model (either a global large scale or a small scale model)?

    Thanks!

    Andrew

  • AC1: Comment on EGU2020-9470, Anna Possner, 06 May 2020

    Hi Andrew,

    you raise some good points.

    1. We tested for LTS dependence (at least in terms of the broad categories of stable and unstable) and did not find a strong dependence. We did construct EIS from the reanalysis.

    2. I would love to verify this by contrasting cloud-resolving simulations in deep and shallow boundary layers. As of yet, I have not found a suitable “deep” (BL depth >= 1.5 km) with a stable stratocumulus deck (CF>80%) to dig into this further. I could test whether this relationship exists within large-scale models or not, but first it would be nice to confirm it in ground-base observations and if its real attribute the driving process. If the latter is the case, it would then be nice to see if large-scale models obey this relationship.

    Thanks,
    Anna

    • CC2: Reply to AC1, Andrew Gettelman, 06 May 2020

      Thanks for the reply Anna.

      Happy to provide some large scale model output to test this if you want....

Presentation version 1 – uploaded on 10 Apr 2020 , no comments