Cloud droplet number concentration

Similarly, Martin and co-workers (1994) measured aerosol particles in the size range from 0.05 to 1.5 /rm below the base of stratocumulus clouds, along with cloud droplet number concentrations in maritime and in continental air masses. Figure 14.46 shows the relationship between cloud droplet number concentration and the aerosol particle concentration for a set of flights carried out in the vicinity of the British Isles and in the South Atlantic (Martin et al., 1994). There is an almost linear relationship between the two for maritime air masses. Given that the cutoff for particle measurements was 0.05 /xm, these concentrations may have been underestimated, so that the slope of the line for maritime air masses can be taken as unity. That is, essentially all of the maritime particles at the cloud base could act as CCN under the range of supersaturations in these studies. 

FIGURE 14.46 Average cloud droplet number concentration as a function of subcloud aerosol particle concentration (0.05-1.5 fj.m) in marine ( ) and continental ( ) air masses (adapted from Martin et al., 1994). 

Leaitch, W. R G. A. Isaac, J. W. Strapp, C. M. Banic, and H. A. Wiebe, The Relationship between Cloud Droplet Number Concentrations and Anthropogenic Pollution Observations and Climatic Implications, J. Geophys. Res., 97, 2463-2474 (1992). 

The CCN behavior of ambient particles can be measured by drawing an air sample into an instrument in which the particles are subjected to a known supersaturation, a so-called CCN counter (Nenes et al. 2001). If the size distribution and chemical composition of the ambient particles are simultaneously measured, then the measured CCN behavior can be compared to that predicted by Kohler theory on the basis of their size and composition. Such a comparison can be termed a CCN closure, that is, an assessment of the extent to which measured CCN activation can be predicted theoretically [see, for example, VanReken et al. (2003), Ghan et al. (2006), and Rissman et al. (2006)]. The next level of evaluation is an aerosol-cloud drop closure, in which a cloud parcel model, which predicts cloud drop concentration using observed ambient aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against direct airborne measurements of cloud droplet number concentration as a function of altitude above cloud base. The predicted activation behavior can also be evaluated by independent measurements by a CCN instrument on board the aircraft. Such an aerosol-cloud drop closure was carried out by Conant et al. (2004) for warm cumulus clouds in Florida. 

Indirect climate effects of aerosols are more complex and more difficult to assess than direct effects because they depend on a chain of phenomena that connect aerosol levels to concentrations of cloud condensation nuclei, cloud condensation nuclei concentrations to cloud droplet number concentrations (and size), and these, in turn, to cloud albedo and cloud lifetime. Changes in the number concentration of aerosols are observed to cause variations in the population and sizes of cloud droplets, which are expected to cause... 

If aerosol number concentrations are substantially increased as a result of anthropogenic emissions over that in the absence of such emissions, the number concentration of cloud droplets, which is governed by the number concentration of aerosol particles below cloud, may also be increased. An increased number concentration of cloud droplets leads, in turn, to an enhanced multiple scattering of light within clouds and to an increase in the optical depth and albedo of the cloud. The areal extent of the cloud and its lifetime may also increase. This is the essence of the indirect effect of aerosols on climate. A key measure of aerosol influences on cloud droplet number concentrations is the number concentration of cloud condensation nuclei (CCN). 

Thus xc depends on the liquid water content of the cloud, its thickness, and the mean radius of the cloud droplets. xc can also be expressed in terms of the cloud droplet number concentration N. Since re = (3L/4 r/Vpw) 3, we obtain... 

The essence of the potential effect of aerosols on cloud reflectively is embodied in Figure 24.17 which shows cloud albedo as a function of cloud droplet number concentration at various cloud thicknesses at a constant liquid water content of L — 0.3 g m 3. At constant liquid water content and constant cloud thickness, cloud albedo increases with increasing CDNC. For a cloud 50 m thick with this liquid water content, an increase of CDNC from 100 to 1000 cm-3, corresponding to going from remote marine to continental conditions, leads to almost a doubling of cloud albedo. 

Sensitivity of Cloud Albedo to Cloud Droplet Number Concentration... 

We desire to calculate the sensitivity of cloud albedo Rc to changes in cloud droplet number concentration N(dRcjdN). Rc is a function of N, L, and h, so this derivative can be written as follows ... 

FIGURE 24.17 Cloud albedo as a function of cloud droplet number concentration. Liquid water content L = 0.3 gm-3 (Schwartz and Slingo 1996). (Reprinted by permission of Springer-Verlag.)... 

Twomey termed the quantity dRc/dN, at constant liquid water content, the susceptibility it is a measure of the sensitivity of cloud reflectance to changes in microphysics (Twomey 1991 Platnick and Twomey 1994). The susceptibility is inversely proportional to N such that when N is low, as in marine clouds, the susceptibility is high. For a fractional change in cloud droplet number concentration of AN/N, the discrete version of (24.41) is... 

For numerical values typical of marine stratus clouds, Rc = 0.5, Ac = 0.3 (global average), and Ta = 0.76, a 30% increase in cloud droplet number concentration, AN/N = 0.3, yields... 

Relation of Cloud Droplet Number Concentration to Aerosol Concentrations... 

Leaitch, W. R., Isaac, G. A., Strapp, J. W., Banic, C. M., and Wiebe, H. A. (1992a) Concentrations of major ions in Eastern North American cloud water and their control of cloud droplet number concentrations, in Precipitation Scavenging and Atmosphere-Surface Exchange, Vol. 1, S. E. Schwartz and W. G. N. Slinn, eds., Hemisphere Publishing, Washington, DC, pp. 333-343. 

Leaitch, W. R., and Isaac, G. A. (1994) On the relationship between sulfate and cloud droplet number concentrations, J. Climate, 7, 206-212. 

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