- How important is the influence of anthropogenic aerosol on droplet and crystal nucleation and solar absorption in the life cycle of clouds, their radiative properties, and precipitation from clouds?
- How important are vertical transport in cloud updrafts and downdrafts, aqueous chemistry in cloud drops, and scavenging by precipitation in the life cycle of the aerosol as a function of particle size?
To structure the CAPI effort, four primary science questions have been identified that will focus the CAPI investigations. All four address Mission Question 1, which is arguably the more important of the two mission questions because it focuses on radiative forcing by anthropogenic aerosol through their influence on clouds, which is a major source of uncertainty in projections of climate change. However, anthropogenic aerosol concentrations that influence clouds depend upon the processes addressed in Mission Question 2, and also drive direct radiative effects of anthropogenic aerosol on the planetary energy balance. Although none of the four primary questions specifically addresses Mission Question 2, the role of cloud processing and removal of aerosol is implicit in all four primary questions.
1. Why do climate models produce a large aerosol indirect effect?
A large fraction of global model estimates of the top-of-atmosphere radiative forcing from the aerosol indirect effect (AIE) are below -1.5 W m-2. Such values are difficult to reconcile with observed 20th century temperature records and with estimates of warming caused by the combination of increasing greenhouse gases and the direct effect of increasing aerosol. The strongly negative AIE values are only obtained when aerosol effects on cloud liquid water content (the second AIE) are included in global models, whereas the aerosol effect on cloud droplet effective radius (the first AIE) may be reasonably well simulated in global models. In those estimates the global simulations produce large increases in cloud liquid water path (LWP) with increasing aerosol. However, cloud-resolving model simulations often find small or (under some common conditions) negative responses of LWP to increasing aerosol, and a recent analysis of satellite retrievals of aerosol and precipitation frequency suggests that the sensitivity of precipitation occurrence and LWP to aerosols is overestimated in most global models. In addition, global models may overestimate the influence of precipitation on wet removal of the aerosol which, combined with the overestimated sensitivity of precipitation occurrence to aerosols, suggests an excessive tendency to produce bifurcations in the cloud-aerosol-precipitation system. The key challenges for ASR are to provide observational constraints on the AIE using ARM observations, to understand why climate models produce a much stronger increase in LWP than do cloud-resolving models, and to identify physically-based ways to produce sensitivities to aerosols in global models that are more in line with high-resolution models and observations.
2. What processes control diversity in the sensitivity of warm low clouds to aerosol perturbations?
Microphysical, structural, and dynamical properties of low, liquid-phase clouds all show sensitivity to aerosol loading, but the responses are not uniform. In general, an increase in aerosol concentration increases cloud droplet concentration and reduces droplet size. These changes impact cloud albedo, precipitation, and cloud dynamics, but the magnitude and even the sign of the response of various cloud-field characteristics (depth, liquid water path, cloud fraction) appear to depend upon cloud type and meteorological regime, cloud field organization (itself a function of precipitation), and upon the aerosol loading in the unperturbed clouds. The precipitation response in turn feeds back on the aerosol loading through wet removal. Understanding which cloud regimes are more or less resilient to aerosol perturbations is fundamental to understanding the attendant radiative forcing. By applying ARM observational facilities and ASR state-of-the-art numerical modeling expertise, DOE is uniquely poised to address the challenge of understanding the diversity of warm low cloud responses to aerosol perturbations.
3. What aerosol-related processes control deep convective cloud properties relevant to climate (precipitation, cloud radiative forcing, latent heating profiles)?
Deep convection powerfully impacts the Earth’s energy balance and water cycle. Recent work suggests that under some conditions atmospheric aerosol loadings can influence the intensity and vertical extent of deep convection, and in so doing modify the precipitation intensity and radiative impact of these clouds. This influence is called the aerosol invigoration effect. Changes in aerosol loading may impact cloud droplet number concentrations in deep convective clouds and thereby modify collision-coalescence, evaporation/sublimation, updraft glaciation, and ice-growth pathways. The net result of a cloud condensation nucleus perturbation on convective strength and precipitation likely depends on cloud base temperature, shear, and environmental relative humidity. A perturbation in ice nucleus concentration could affect anvil size and lifetime and hence cloud radiative forcing. However, the multi-scale dynamical and thermodynamical response of the cloud system might be quite different from the response of individual clouds, as downdraft effects on subsequent cloud development and cloud effects on the aerosol become important on longer time scales.
Only a few global models represent aerosol effects on deep convective clouds. All global estimates of aerosol effects on convective clouds report a negative aerosol indirect forcing. However, a conceptual model, cloud model simulations and observational analysis suggest that increasing aerosol loading could lead to a positive top-of-atmosphere radiative forcing by invigorating convection, raising cloud tops, and expanding anvil area. The key challenges for ARM and ASR are to identify and obtain the extensive observations needed to isolate proposed physical mechanisms involving aerosol from other influences on deep convective clouds, and to constrain cloud-resolving and global simulations across a range of spatial and time scales for systems of convective clouds. The ultimate goal is to observationally constrain estimates of aerosol effects on precipitation rate, cloud radiative forcing and the latent heating profile in deep convective cloud systems.
4. What processes control ice nucleation and its impact on ice-containing clouds (e.g., Arctic stratus, altostratus, cirrus, convective clouds)?
Ice nucleation processes involving aerosols are key to the formation and microphysical and optical properties of ice and mixed-phase clouds. Ice nucleation plays a strong role in determining the ice crystal number concentration and size distribution in ice-containing clouds, the liquid/ice partitioning of mixed-phase clouds, and cloud glaciation, which can significantly impact cloud optical depth, cloud fraction, and precipitation. There are two main aerosol and cloud drop freezing pathways: homogeneous and heterogeneous ice nucleation. Homogeneous nucleation occurs efficiently only at temperatures below about -40 C, where hydrated aerosol or cloud droplets are sufficiently supercooled to freeze spontaneously. It likely plays a dominant role in cirrus clouds, on a global scale, and is fairly well understood. Heterogeneous ice nucleation involves a variety of poorly understood ice nucleation pathways, and much remains unknown about the concentrations and properties of ice nuclei, their dominant modes of action, and competition between them, in part owing to a lack of suitable instrumentation to provide the needed field measurements. The importance of heterogeneous nucleation in cirrus clouds, globally, and in the relatively polluted Northern Hemisphere is still unclear. Regardless of freezing mechanism, the impact of ice nucleation on ice-containing clouds is known to be strongly modulated by ice crystal properties such as habit and fall speed that are not well constrained by field measurements. In addition, the role of ice nucleation in aerosol removal from the atmosphere is poorly understood. ASR laboratory studies of heterogeneous nucleation under controlled conditions and ARM field measurements of the properties of ice nuclei are needed to improve understanding and representation for global models.