Emission Reductions Significantly Reduce the Hemispheric Contrast in Cloud Droplet Number Concentration in Recent Two Decades

Abstract

Cloud droplet number concentration (N d ) plays an essential role in understanding cloud physics and quantifying the effective radiative forcing associated with aerosol-cloud interactions. N d is the bridge intimately connecting aerosols and cloud properties. Changes in aerosols, including the aerosol number and physicochemical properties, modify cloud microphysical and macrophysical properties for a given dynamical condition. Specifically, Twomey (1977) pointed out that more (larger N d ) and smaller droplets caused by more aerosols increases the cloud albedo under a constant liquid water path. More (larger N d ) and smaller droplets slow down the collision-coalescence process (Rosenfeld et al., 2012), hence increasing the cloud lifetime (Albrecht, 1989).

Reducing Aerosol Forcing Uncertainty by Combining Models With Satellite and Within‐The‐Atmosphere Observations: A Three‐Way Street

“…The product of AOD and ANG (derived from AOD retrieved at several wavelengths), designated the aerosol index (AI), yields a metric that weights AOD toward smaller (CCN‐like) sizes (Nakajima et al., 2001). As an estimate of CCN, AI is a qualitative indicator at best, in part because particle hygroscopicity is generally unconstrained by remote‐sensing measurements but can dominate the relationship between retrieved AOD and particle number, especially for small particles (Cao et al., 2023; Kapustin et al., 2006), and in part because extrapolating the observed part of the aerosol spectrum to smaller sizes engenders additional assumptions. However, AI is often treated as quantitative for lack of other observational constraints.…”

Aggressive Aerosol Mitigation Policies Reduce Chances of Keeping Global Warming to Below 2C

“…As aerosol loadings increase, the Twomey effect always produces a negative forcing but cloud adjustments can be positive or negative depending upon the background meteorology and properties of the clouds and boundary layer. Marine low clouds downstream of major industrialized regions have seen declines in cloud droplet concentration (D. T. McCoy et al., 2018) that indicate a reduction in the Twomey effect, and the hemispheric contrast in cloud droplet concentration between the polluted Northern and more pristine Southern Hemispheres has decreased significantly since 2000 (Cao et al., 2023). We are now in an era where the rate of change of aerosol radiative forcing is positive, which ceteris paribus must increase the rate of global warming (Dvorak et al., 2022).…”