Assessment of aerosol–cloud–radiation correlations in satellite observations, climate models and reanalysis

Abstract

Representing large-scale co-variability between variables related to aerosols, clouds and radiation is one of many aspects of agreement with observations desirable for a climate model. In this study such relations are investigated in terms of temporal correlations on monthly mean scale, to identify points of agreement and disagreement with observations. Ten regions with different meteorological characteristics and aerosol signatures are studied and correlation matrices for the selected regions offer an overview of model ability to represent present day climate variability. Global climate models with different levels of detail and sophistication in their representation of aerosols and clouds are compared with satellite observations and reanalysis assimilating meteorological fields as well as aerosol optical depth from observations. One example of how the correlation comparison can guide model evaluation and development is the often studied relation between cloud droplet number and water content. Reanalysis, with no parameterized aerosol-cloud coupling, shows weaker correlations than observations, indicating that microphysical couplings between cloud droplet number and water content are not negligible for the co-variations emerging on larger scale. These observed correlations are, however, not in agreement with those expected from dominance of the underlying microphysical aerosol-cloud couplings. For instance, negative correlations in subtropical stratocumulus regions show that suppression of precipitation and subsequent increase in water content due to aerosol is not a dominating process on this scale. Only in one of the studied models are cloud dynamics able to overcome the parameterized dependence of rain formation on droplet number concentration, and negative correlations in the stratocumulus regions are reproduced.

Deconvolution of boundary layer depth and aerosol constraints on cloud water path in subtropical stratocumulus decks

“…N d is estimated based on the relationship established by Boers et al (2006) and Bennartz (2007) for marine boundary layer clouds: where ρ w denotes the density of water, eff = f ad ad is the effective rate of increase in adiabatic liquid water content with increasing height, and R eff|top denotes the effective radius at cloud top. All assumptions regarding the degree of adiabaticity and the proportionality constant k between the true and effective N d are the same as in Eastman and Wood (2016).…”

Linking large-scale circulation patterns to low-cloud properties

“…These cloud types are widespread (coverage on the order of one-third of the globe at any given time; e.g., Hartmann et al, 1992) in the subsiding branch of the Hadley circulation (e.g., Wood, 2012) due to a separation of the cool, moist MBL and the warm, dry free troposphere by a strong (∼ 10 K) and sharp O(100-500 m) thermal inversion (e.g., Parish, 2000). Despite their substantive role in the radiation budget (global shortwave cloud radiative effect (CRE SW ) of ∼ 60-120 W m −2 ; e.g., Yi and Jian, 2013), MBL clouds and their radiative response to changes in the climate system are not simulated accurately by global climate models (e.g., Palmer and Anderson, 1994;Delecluse et al, 1998;Bachiochi and Krishnamurti, 2000;Bony and Dufresne, 2005;Webb et al, 2006;Lin et al, 2014;Bender et al, 2016Bender et al, , 2018Brient et al, 2019); however, results from regional climate models are more encouraging (e.g., Wang et al, 2004Wang et al, , 2011.…”

Did the Roman Empire affect European climate? A new look at the effects of land use and anthropogenic aerosol emissions

“…Last but not least, aerosol-radiation and aerosol-cloud interactions from aerosol-climate models are uncertain. As an example, some studies show that the models typically overestimate the effect of aerosols on the cloud liquid water content, at least in some regions (Bender et al, 2018). The aerosol effective radiative forcing also depends on the minimum CDNC value (Hoose et al, 2009); since we lowered the minimum CDNC to 1 cm −3 , the aerosols have a stronger impact on radiation than with the standard setup of ECHAM-HAM-SALSA where the minimum CDNC is 40 cm −3 .…”