The radiative influence of aerosol effects on liquid-phase cumulus and stratiform clouds based on sensitivity studies with two climate models

Abstract

Aerosol effects on warm (liquid-phase) cumulus cloud systems may have a strong radiative influence via suppression of precipitation in convective systems. A consequence of this suppression of precipitation is increased liquid water available for large-scale stratiform clouds, through detrainment, that in turn affect their precipitation efficiency. The nature of this influence on radiation, however, is dependent on both the treatment of convective condensate and the aerosol distribution. Here, we examine these issues with two climate models—CSIRO and GISS, which treat detrained condensate differently. Aerosol–cloud interactions in warm stratiform and cumulus clouds (via cloud droplet formation and autoconversion) are treated similarly in both models. The influence of aerosol–cumulus cloud interactions on precipitation and radiation are examined via simulations with present-day and pre-industrial aerosol emissions. Sensitivity tests are also conducted to examine changes to climate due to changes in cumulus cloud droplet number (N _c); the main connection between aerosols and cumulus cloud microphysics. Results indicate that the CSIRO GCM is quite sensitive to changes in aerosol concentrations such that an increase in aerosols increases N _c, cloud cover, total liquid water path (LWP) and reduces total precipitation and net cloud radiative forcings. On the other hand, the radiative fluxes in the GISS GCM appear to have minimal changes despite an increase in aerosols and N _c. These differences between the two models—reduced total LWP in the GISS GCM for increased aerosols, opposite to that seen in CSIRO—appear to be more sensitive to the detrainment of convective condensate, rather than to changes in N _c. If aerosols suppress convective precipitation as noted in some observationally based studies (but not currently treated in most climate models), the consequence of this change in LWP suggests that: (1) the aerosol indirect effect (calculated as changes to net cloud radiative forcing from anthropogenic aerosols) may be higher than previously calculated or (2) lower than previously calculated. Observational constrains on these results are difficult to obtain and hence, until realistic cumulus-scale updrafts are implemented in models, the logic of detraining non-precipitating condensate at appropriate levels based on updrafts and its effects on radiation, will remain an uncertainty.

Appendix A

1.1 Aerosol processes

Below, we briefly describe the aerosol processes treated in both models. For CSIRO, the transport of aerosols and other trace quantities occurs by advection, vertical turbulent mixing and vertical transport inside deep convective clouds as described in Rotstayn and Lohmann (2002b). Large-scale wet scavenging processes are linked to the warm-rain and frozen precipitation processes in the stratiform cloud microphysical scheme (Rotstayn 1997) and the convection scheme (Gregory and Rowntree 1990). Below-cloud scavenging is proportional to the area swept out by precipitation, based on the assumed raindrop or snowflake size distribution. In-cloud scavenging is proportional to the amount of precipitation removed, divided by the liquid water (or ice-water) content. Also included is the re-evaporation of aerosol due to evaporation (sublimation) of rain (snow). Further details are in Rotstayn and Lohmann (2002b). Prognostic variables in the sulfur-cycle model are dimethyl sulfide (DMS), sulfur dioxide (SO₂) and sulfate. The treatment of the sulfur chemistry is based on that in ECHAM4 (Feichter et al. 1996). The carbonaceous aerosol module follows the approach of Cooke et al. (1999), with an e-folding time of 1.15 days used for the conversion of black carbon (BC) and particulate organic matter (POM) from their hydrophobic to hydrophilic forms.

For the GISS aerosol chemistry and transport model, prognostic species for the sulfur cycle include SO₂, DMS, sulfate, and hydrogen peroxide (H₂O₂) and are described in detail in Koch (2001) and Koch et al. (1999). The sulfate chemistry scheme treats both gas-phase and aqueous-phase chemistry. Dry deposition of aerosols and gases is through a resistance in series scheme. Large-scale wet scavenging processes uses a first-order removal mechanism and is dependent on the autoconversion process. For moist convection, scavenging is applied to species that get dissolved with cloud updrafts, and is then treated similarly as raindrops. Both below-cloud scavenging and evaporation of species is included. Aerosol tracers are advected via a highly non-diffusive quadratic upstream scheme as described in Schmidt et al. (2006). Source terms for carbonaceous aerosols are obtained from emissions, and these get transported similar to sulfates as described in Koch (2001). The hydrophobic to hydrophilic conversions for carbonaceous aerosols follow an e fold time of 1 day for the industrial (fossil-fuel and biofuel) component and for the biomass component partial solubility for BC (20%) and POM (50%) are assumed.

1.2 Aerosol emissions

The CSIRO model includes anthropogenic emissions of sulfur dioxide (Smith et al. 2001) and carbonaceous aerosols (Ito and Penner 2005), both for the year 2000. The carbonaceous aerosol emissions include primary sources of BC and POM from the burning of fossil fuel, open vegetation, and biofuel. Since secondary sources of POM were not considered by Ito and Penner, the fossil-fuel POM source for each year was multiplied by 11.2, so that the global emission for 1985 matched that from Liousse et al. (1996), as used in the model intercomparison by Penner et al. (2001). The total anthropogenic aerosol burden for the year 2000 in the CSIRO model are 1.18 TgS as sulfate, 1.18 TgC as OC, and 0.17 TgC as BC. The CSIRO model also includes natural sources of sulfur (Rotstayn and Lohmann 2002b) and natural organic carbon from terpenes (Guenther et al. 1995), with a yield of 13% assumed for rapid conversion of terpenes to POM. In addition, number concentrations of two modes of sea salt aerosol (film-drop and jet-drop) are diagnosed as a function of 10-m wind speed above the ocean surface, following O’Dowd et al. (1997). Sea salt aerosols are assumed to be well mixed in the marine boundary layer, and are set to zero above the top of the boundary layer. Aerosol emissions in the GISS model are specified by the AEROCOM project (An aerosol model intercomparison project: http://www.nansen.ipsl.jussieu.fr/AEROCOM/aerocomhome.html) and are described in Menon and Del Genio (2006). The total anthropogenic aerosol burden for the year 2000 in the GISS model are 0.86 TgS as sulfate, 0.87 TgC as OC and 0.18 TgC as BC. Although the industrial SO₂ emissions have been increased, the relatively low values of sulfate compared to CSIRO are due to less aqueous phase sulfate production (Koch et al. 2003).

The conversion of aerosol mass to aerosol number, used for the cloud droplet number prediction, is described as follows: For tropospheric sulfate, the size distribution (effective radii = 0.05 μm, standard deviation = 1.9, density = 1.77 g cm⁻³) follows the fossil-fuel size distribution given in Penner et al. (2001). For carbonaceous aerosols–organic matter (1.3× OC) and black carbon—the size distribution for an internal mixture (effective radii = 0.08 μm, standard deviation = 1.65, density = 1.25 and 1.5 g cm⁻³, for the mixture and for BC, respectively) follows the biomass burning size distribution given in Penner et al. (2001). For sea salt distributions, the aerosol number used in the CSIRO GCM is as described earlier. In the GISS GCM sea salt mass in the 0.1–1.0 μm range is converted to an aerosol number following Lohmann et al. (1999) (volume radii = 0.44 μm, and density = 2.169 g cm⁻³).