Cloud properties

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). 

Another piece of evidence for anthropogenic emissions leading to increased CCN and hence effects on cloud properties such as albedo and extent is found in ship tracks. These are lines of clouds that trace ship movements, either in initially cloud-free regions (Conover, 1966 Platnick and Twomey et al., 1994) or superimposed on preexisting clouds (Coakley et al., 1987). Emissions associated with the ship exhausts serve as CCN. This allows clouds to form where the background CCN concentration is too small for cloud formation. Alternatively, the CCN can modify existing cloud properties in the exhaust plume by changing the number and size distribution of the cloud droplets as well as the liquid water content (e.g., Ferek et al., 1998). 

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. 

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. 

Table 2.3 Treatments of cloud properties of on-line models... 

Precollapse cloud cores are composed of cold molecular gas with ternperamres in the range 7 -15 K, and with gas densities —10 -10 mol cm (Figure 1). Some clouds may be denser yet, but this is hard to determine because of the limited density ranges for which suitable molecular tracers are abundant (typically isotopes of carbon monoxide and ammonia). Masses of these clouds range from roughly a solar mass to thousands of solar masses, with the distribution of clump masses fitting a power-law such that most of the clumps are of low mass, as is also true of stars in general. The cloud properties described below are used to constrain the initial conditions for hydrodynamic models of the collapse of cloud cores. 

Schaefer (1966) reported the activation of large numbers of ice nuclei on the addition of trace levels of iodine vapor to car exhaust (containing lead oxide nanoparticles) at temperatures from —3 to —20°C in the laboratory. The formation of lead iodide was concluded to have a seeding effect similar to that of silver iodide particles (Vonnegut, 1947), which had been used in an attempt to artificially modify cloud properties and enhance precipitation. Consequendy this method was proposed as a means to remove harmful aerosol formed in polluted urban areas, and also in artificial weather modification. However, the development of unleaded fuels, for which no similar ice nucleating ability was shown to occur in the presence of iodine (Hogan, 1967), provided a better long-term solution to this problem. 

B) Once these particles have grown beyond about 50 nm in particle diameter they will affect radiation budgets due to direct scattering of solar radiation as well as due to changing cloud properties (indirect effects). 

Over the past five years, there have been a number of studies which have sought to examine the effect of man s activities on clouds and climate. Modification of the microphysical stractuie of clouds as a result of biomass burning and the potential impact of the affected clouds on rainfall has been a topic of major interest. Field programs in the tropics have shown that biomass burning modifies the cloud condesation nuclei, and therefore there will be a change in the microphysical properties of clouds in the area. Increases in the CCN concentration results in more cloud droplets produced, with associated increases in cloud reflectivity. Assessing the impact of man s activities on cloud properties and reflectivity is an area of intense activity. 

The shallow-water method is based on vertical integration of the cloud properties, reducing the problem to a two-dimensional set of conservation equations (mass, species, downwind and cross-wind momentum, and energy). Some models consider only downwind distance as the independent variable. Physical aspects of interest are (see also Table 19.3 for box models) ... 

CCN closure studies, where observations of ambient aerosol CCN activity are compared to the CCN concentration predicted via Kohler theory based on the particle size distribution and composition, often show discrepancies that can be attributed to kinetic limitations to droplet growth. Besides affecting cloud properties, kinetic limitations to droplet growth in the atmosphere may also impact the lifetime of aerosol particles. Changes in particle lifetime may influence the aerosol composition due to photochemical aging or heterogeneous processing of the aerosol. 

In Chapter 6 we consider instrumental effects, such as spectral resolution and signal-to-noise ratio, and discuss data from the terrestrial and the giant planets in a qualitative manner. In Chapter 7 we examine methods for interpreting spectroscopic and radiometric data produced by real instruments in terms of physical properties of atmospheres and surfaces. Emphasis is placed on the retrieval of thermal stmcture, gas composition and cloud properties of the atmospheres, and thermal properties and texture of surfaces. Limitations on the information content inherent in measured quantities are assessed. 

Strom, J., J. Heintzenberg, K. J. Noone, K. B. Noone, J. A. Ogren, F. Albers and M. Quante. Small Crystals in Cirrus Clouds Their Residue Size Distribution, Cloud Water Content, and Related Cloud Properties. J. Atmos. Res. 32, 125-141. 1994. 

S. A. Ackerman, W. L. Smith, A. D. Collard, X. L. Ma, H. E. Revercomb and R. O. Knuteson. Cirrus Cloud Properties Derived from High Spectral Resolution IR Spectrometry during FIRE II. Part II Aircraft HIS Results. J. Atmos. Sci. 52, 4246-4263. 1995. 

Vogelmann, A. and T. Ackerman, 1995 Relating Cirrus Cloud Properties to Observed Fluxes A Critical Assessment. J. Atmos. Sci. 52, 4285-4301. 

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