Aerosol-induced changes of convective cloud anvils produce strong climate warming

Abstract

Abstract. The effect of aerosol on clouds poses one of the largest uncertainties in estimating the anthropogenic contribution to climate change. In contrast, even small human-induced perturbations in cloud coverage, lifetime, height or optical properties can change the instantaneous radiative energy flux by hundreds of watts per unit area, and this forcing can be either warming or cooling. Clouds and aerosols form a complex coupled system that, unlike greenhouse gases, have relatively short lifetime (hours to days) and inhomogeneous distribution. This and the inherent complexity of cloud microphysics and dynamics, and the strong coupling with meteorology explain why the estimation of the overall effect of aerosol on climate is so challenging. Here we focus on the effect of aerosol on cloud top properties of deep convective clouds over the tropical Atlantic. The tops of these vertically developed clouds consist of mostly ice and can reach high levels of the atmosphere, overshooting the lower stratosphere and reaching altitudes greater than 16 km. We show a link between aerosol, clouds and the free atmosphere wind profile that can change the magnitude and sign of the overall climate radiative forcing. This study demonstrates the deep link between cloud shape and aerosol loading and that the overall aerosol effect in regions of deep convective clouds might be warming. Moreover we show how averaging the cloud height and optical properties over large regions may lead to a false cooling estimation.

Comparing results from a physical model with satellite and in situ observations to determine whether biomass burning aerosols over the Amazon brighten or burn off clouds

“…Furthermore, convective anvils, which may have a large impact on the radiative balance, are not resolved in this study. Cloud invigoration over the Atlantic and Pacific Oceans has been observed to result in an increase in the total and anvil cloud fraction, a decrease in the tower size to anvil size ratio, and consequently an increase in the TOA downward irradiance [Koren et al, 2010b]. However, the majority of clouds that form in this region and season are shallow cumuli that do not contain anvils.…”

Aerosol-Cloud Interactions from Urban, Regional, to Global Scales

“…The near-shore areas closer to the Asian continent experience larger increase of AOD compared to the remote maritime areas, and the consequent zonal extension of AOD enhancement just reflects the transportation process of particulate pollution from East Asia to West Pacific. Averaged over the analysis region, AOD is elevated by 0.03 in PD case, which accounts for 46% relative increase defined as the ratio of mean difference between PD and PI over the aerosols are consistent with the previous reports from satellite measurements [Koren et al, 2010] and CRM simulation results [Fan et al, 2012]. The response of the surface precipitation to the different aerosol scenarios is not uniform over the NW Pacific region (Figure 5.19g).…”

The cloud condensation nuclei and ice nuclei effects on tropical anvil characteristics and water vapor of the tropical tropopause layer

“…Since convective transport may affect aerosols and hydrometeors in the upper troposphere, these changes in convective strength could significantly alter cloud anvil properties and WVC in the TTL. A recent study by Koren et al (2010) found that aerosol-induced invigoration of convection resulted in expanded anvil spatial coverage and reduced optical depth. Satellite analysis indicates that aerosols from forest fires result in smaller particles and much longer anvil lifetime (Lindsey and Fromm 2008) and biomass burning aerosols increase water vapor entering the stratosphere by decreasing ice particle size (Sherwood 2002).…”