FIGURE 4.6 Portion of an orbit showing the cloud mask that combines CALIPSO lidar and CloudSat radar (upper panel) and the CloudSat radar reflectivity (lower panel). This example of monsoonal convection illustrates how precipitation (easily identified as regions of high reflectivity above the surface) falls from mixtures of deep and shallow convection. Shallow precipitating convection is often concealed from above by thick overlying cirrus clouds as apparent in the middle portion of this cross section. SOURCE: Image courtesy of G. Stephens.

the vertical structure of clouds and aerosols. The vertical structure revealed by CloudSat, for instance, offers deeper insights into the key processes that shape clouds and precipitation. For example, the image shown in Figure 4.6 is a cross section of the vertical distribution of radar reflectivity measured along a portion of one orbit. Also shown is the matching cloud mask information obtained from the combination of lidar and radar data. This example shows observations of clouds and precipitation associated with an active monsoon over southern China.

Observations such as these provide a way of observing the cloud structures with embedded precipitation and begin to provide hints about the way precipitation is organized. When accumulated over the entire tropics, these observations are now beginning to reveal that not all precipitation falls from deep convective clouds, as has generally been assumed, but that significant accumulations of water come from precipitation that falls from shallower clouds, as highlighted in this one example. This result has further implications for the nature of the vertical distribution of latent heating by precipitating cloud systems in the atmosphere, with ramifications on the way such clouds add (latent) heat to the atmosphere. The latter is essential for understanding the dynamic envelope of monsoons as well as the topic of the prediction of medium-and longer-term variability of the tropical atmosphere.

AEROSOLS FROM NATURAL PROCESSES AND HUMAN ACTIVITIES

An aerosol is a suspension of tiny liquid or solid particles in the atmosphere. Aerosol particles are distinguished from clouds by requiring that aerosol particles be stable in unsaturated air. Examples include dust, sulfuric acid particles, sea salt, organic particles, and smoke. Aerosols play important roles in the energy budget of Earth, in the formation of clouds, and in the chemistry of the atmosphere. Aerosol particles are produced naturally through biological emissions or elevation of particles by wind, but human activities provide a substantial enhancement to the natural aerosol loading of the atmosphere through agricultural and industrial activities. Aerosol particles can be produced either directly or by the chemical conversion of precursor chemicals that exist in solid or liquid form. Aerosols influence climate in several ways. Because aerosol particles reflect and absorb radiation, they can directly influence the energy balance of Earth. For many aerosols their primary effect is to reflect solar radiation and thereby cool the climate. Aerosols may also warm the atmosphere directly by absorption of radiation, however, and this is particularly important for highly absorbing aerosols such as soot (Figure 4.7).

Space measurements have succeeded in depicting aerosols associated with human activity over the oceans by isolating fine-mode from coarse-mode aerosols such as dust and sea salt that arise from natural processes. Plumes of fine aerosols are shown to result from biomass burning and from industrial activities (Tanré et al. 2001). The ability to distinguish fine from coarse aerosols has led to efforts to characterize the anthropogenic contribution to the aerosol direct forcing of climate (Bellouin et al. 2005, Kaufman et al. 2005).

INDIRECT EFFECTS OF AEROSOLS

Another way that aerosols can influence climate is through their role as the small particles on which clouds form (cloud condensation nuclei). An important contribution



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