Aerosol-cloud interactions in EC-Earth

Changes in cloud properties resulting from aerosol loading can have potentially significant effects on the radiative forcing and cloud and precipitation patterns and amounts (indirect aerosol effects). The Rossby Centre provides global climate predictions and projections using the EC-Earth model (Hazeleger et al., 2012), which features advanced parametrizations of clouds and radiation (including an improved mixed-phase microphysics (Forbes et al., 2011) and McICA radiation transfer (Morcrette et al., 2008)) but has rather crude representation of aerosols. Here we test the benefits of more realistic aerosol distributions and more complete aerosol and cloud representation that accounts for the interactions of aerosols with clouds and radiation.

We present non-coupled (atmosphere only) simulations using EC-Earth version 3 with the existing and the newly introduced aerosol distributions taken from the CMIP5 dataset. The CMIP5 aerosols tend to underestimate aerosol optical depth (AOD), as illustrated against MODIS-MISR observations (Figure 1). An exception are dust affected regions, where AOD tends to be overestimated.

CMIP5 aerosol optical depth and its bias compared to MODIS-MISR observations.
Figure 1: CMIP5 aerosol optical depth and its bias compared to MODIS-MISR observations.

The distribution of cloud condensation nuclei (CCN) density currently used in the radiative scheme for the calculation of droplet liquid effective radius (Martin et al., 1994) is uniform with values 120 cm-3 over land and 60 cm-3 ocean, as global and annual average. The new CCN distribution obtained using the empirical relationship form Menon et al. (2002) and CMIP5 aerosol mass distributions is more realistic with values 200 cm-3 in the aerosol source regions and below 90 cm-3 in pristine oceanic regions, as global and annual average (Figure 2). The higher CCN number density tends to decrease the droplet effective radius and increase the cloud radiative forcing (Figure 3).

RC Feb Irena Fig 2
Figure 2: Distribution of CCN density currently used in the radiative scheme (Martin et al., 1995) and new CCN distribution based on Menon et al. (2001) and CMIP5 aerosol mass distributions
RC Feb Irena Fig 3
Figure 3: Decrease in liquid effective radius (%) and increase in the cloud radiative forcing at the surface (%) when replacing CCN density currently in use (Martin et al., 1995) with the new CCN density obtained using CMIP5 aerosols.

The changes in cloud microphysics due to aerosols is parameterized via the explicit dependence of the critical cloud liquid water mixing ratio for autoconversion on the CCN density. Currently the CCN density in the radiation and microphysics are inconsistent with the latter based on Menon et al. (2002) and aerosol mass distributions from Tegen et al. (1997). We consistently apply the same CCN distributions, based on Menon et al. (2002) and CMIP5 aerosols, in the microphysics and in the radiation. The critical cloud mixing ratio for autoconversion tends to increase when replacing Tegen et al. (1997) with the CMIP5 aerosols (Figure 4). This will modify the cloud and precipitation patterns.

RC Feb Irena Fig 4
Figure 4: Increase in critical cloud mixing ratio for autoconversion (%) when replacing Tegen et al. (1995) aerosols with CMIP5 aerosols.

The impact of the new aerosol representation on the model biases is being currently evaluated against the CFMIP observations for model evaluation (CFMIP-OBS). Of interest is the impact of the aerosol loadings globally and in the Southern ocean, a region greatly influenced by clouds and where the model tends to display a warm surface bias.

References

Forbes, R.M., A.M. Tompkins and A. Untch, 2011: A new prognostic bulk-microphysics scheme for the IFS. ECMWF Tech. Memo. No. 649.
Hazeleger, W. et al., 2012: EC-Earth V2.2: description and validation of a new seamless earth system prediction model. Clim. Dyn., 39, 2611-2629, DOI: 10.1007/s00382-011-1228-5 (2012).
Martin, G. M., D. W. Johnson, and A. Spice (1994), The measurement and parameterization of effective radius of droplets in warm stratocumulus clouds, J. Atmos. Sci., 51, 1823 – 1842, doi:10.1175/1520-0469(1994)
Menon, S., A.D. Del Genio, D. Koch, and G. Tselioudis, 2002: GCM simulations of the aerosol indirect effect: Sensitivity to cloud parameterization and aerosol burden. J. Atmos. Sci., 59, 692-713.
Morcrette, J-J., H. W. Barker, J. N. S. Cole, M. J. Iacono, R. Pincus, 2008: Impact of a New Radiation Package, McRad, in the ECMWF Integrated Forecasting System. Mon. Wea. Rev., 136, 4773–4798. doi: http://dx.doi.org/10.1175/2008MWR2363.1
Tegen, I, P. Hoorig, M. Chin, I. Fung, D. Jacob, and J. Penner, 1997: Contribution of different aerosol species to the global aerosol extinction optical thickness: Estimates from model results. J. Geophys. Res., 102, 23,895-23,915.