Clouds indirect effects

In addition, aerosol particles have indirect effects. The most important of these is their effect on cloud properties, since clouds obviously also have major effects on climate. In addition, since heterogeneous chemistry can occur on aerosol particles (see Chapter 5), it is possible that such chemistry can alter the concentrations of other contributors to the climate system, such as the greenhouse gases. One example is the formation of N20 from reactions of HONO on the surface of aerosol particles (see Chapter 7.C). 

There are two questions with respect to potential indirect effects of aerosol particles on properties of clouds (1) What are the sources of new particles (2) How do these new particles grow to sufficient size (> 50 nm) to act as CCN ... 

As discussed in Section C.la, sea salt particles in the marine boundary layer have been shown to likely play a major role in backscattering of solar radiation (Murphy et al., 1998), i.e., to the direct effect of aerosol particles. However, they also contribute to the indirect effect involving cloud formation, since they can also act as CCN. Since such particles are a natural component of the marine atmosphere, their contribution will not play a role in climate change, unless their concentration were somehow to be changed by anthropogenic activities, e.g., through changes in wind speed over the... 

The indirect climatic impact of aerosol at the ABL is determined by numerous interactions between aerosol and the dynamics of the microphysical and optical properties of clouds. The input to the atmosphere of anthropogenic aerosol particles functioning as CCN favors an increase in cloud droplet number density. As mentioned above, the related increase in the optical thickness and albedo of clouds, with their constant water content, was called the first indirect effect , which characterizes the climatic impact of aerosol. 

Brenguier J.-L. Pawlowska H. and Schuller L. (2003). Cloud microphysical and radiative properties for parameterization and satellite monitoring of the indirect effect of aerosol on climate. J. Geophys. Res., 108(D15), CMP6/1-CMP6/14. 

The impact of secondary aerosols on indirect radiative forcing is the most variable and is the least understood [3]. The reasons why the indirect effect of secondary aerosols is so difficult to describe is that it depends upon [1] (1) a series of molecular-microphysical processes that connect aerosol nucleation to cloud condensation nuclei to cloud drops and then ultimately to cloud albedo and (2) complex cloud-scale dynamics on scales of 100-1000 km involve a consistent matching of multiple spatial and time scales and are extremely difficult to parameterize and incorporate in climate models. Nucleation changes aerosol particle concentrations that cause changes in cloud droplet concentrations, which in turn, alter cloud albedo. Thus, macro-scale cloud properties that influence indirect forcing result from both micro-scale and large-scale dynamics. To date, the micro-scale chemical physics has not received the appropriate attention. 

Estimating the "indirect effect" is difficult because the mechanisms underlying the chain of microphysical processes connecfing emissions with cloud albedo are uncertain—twice as uncertain as the direct effect as mentioned previously. Clouds form when an air parcel is cooled fhrough vertical liffing and the vapor becomes... 

Atmospheric particles influence the Earth climate indirectly by affecting cloud properties and precipitation [1,2], The indirect effect of aerosols on climate is currently a major source of uncertainties in the assessment of climate changes. New particle formation is an important source of atmospheric aerosols [3]. While the contribution of secondary particles to total mass of the particulate matter is insignificant, they usually dominate the particle number concentration of atmospheric aerosols and cloud condensation nuclei (CCN) [4]. Another important detail is that high concentrations of ultrafine particles associated with traffic observed on and near roadways [5-7] lead, according to a number of recent medical studies [8-11] to adverse health effects. 

Module of feedback mechamisms Direct gas aerosol forcing Cloud condensation nuclei model Other semidirect indirect effects... 

Predecessors (Global-through- urban) GCMOM (247 reactions, 115 species) bulk or size-resolved aqueous-phase sulfate, nitrate, organics, chlorine, oxidant, radical chemistry (64 kinetic reactions) size-resolved, prognostic aerosol/cloud with complex processes feedbacks Direct/indirect effects AQ/health effect 2002, 2004a, b), Jacobson et al. (2004, 2007), Jacobson (2006)... 

For simulation of the aerosol (especially anthropogenic) effects more detailed microphysics in HIRLAM is needed, in the former version it is very difficult to consider all the aerosol indirect effects. The current STRACO (Soft TRAnsition condensation) scheme in HIRLAM (Sass 2002) needs modifications and gives a possibility of developing simpler indirect mechanisms. One of such feedback semi-empirical model was developed in Enviro-HIRLAM by Korsholm et al. (2008b). The new AROMA/HARMONIE cloud scheme (Pinty and Jabouille 1998 Caniaux et al. 1994) is more suitable for implementation of aerosol dynamics and indirect effects of aerosols (CCN) models, but will be more expensive computationally. So, the main focus of our collaboration in this field should be in the improvement of the cloud/microphysics and radiation schemes in HIRLAM. 

At the moment the cloud microphysics-aerosol interaction is included in HIRLAM in a very simple way in the convection schemes, where the cloud condensation nuclei have a lower concentration than over land. Enviro-HIRLAM includes the aerosol dynamics and their indirect effects on meteorology. The use of aerosol may also be prepared by making a 3D field of aerosol that has the characteristics of the currently prescribed values, then the extension to a real 3D distribution of aerosols that can interact with the microphysics is relatively straightforward. Sensitivity studies are needed to understand the relative importance of feedbacks. First experience of Enviro-HIRLAM indicates some sensitivity to effective droplet size modification in radiation and clouds. 

First indirect effect - affect cloud drop size, number, reflectivity, and optical depth via cloud condensation nuclei (CCN) ... 

Both the number concentrations and sizes of aerosol particles directly affect many of their properties and effects. For example, the ability of particles to serve as nuclei for cloud droplet formation depends on their composition as a function of size, although their effectiveness in any given situation depends also on the number of particles present. Knowledge of these aerosol properties is required to evaluate the indirect effects (Section 4.04.7.3) of aerosol particles on climate, i.e., the effect of aerosol particles on cloud reflectivity and persistence. Therefore much attention has been and continues to be focused on determining particle number concentrations and size distributions. 

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