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.
Highlights
Systematic variations in aerosol amount and composition over time give rise to a forcing on the climate system, through aerosol interaction with clouds and with radiation
We find that R(L, Nd) varies with larger amplitude with varying upper tropospheric vertical velocity in models than in observations, but that reanalysis has yet weaker dependence on dynamical regime, again using SO4 to approximate Nd variations, and again indicating that the lack of aerosol–cloud coupling in the reanalysis can underestimate the observed correlations between L and Nd
We study relations between several macro- and microphysical cloud and aerosol properties, in a range of regions with different characteristics in terms of meteorology and aerosol loading
Summary
Systematic variations in aerosol amount and composition over time give rise to a forcing on the climate system, through aerosol interaction with clouds and with radiation. In lack of long-term observations of variations in aerosol and cloud, it is customary to relate present-day, short-term relations between aerosol and cloud properties to those on the time scale of anthropogenic forcing. It has been shown, that present-day relations and their uncertainties may not be representative of those dominating the PI–PD changes, and the use of present-day variability to constrain.
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