Stratocumulus clouds

It should be noted that while most studies have focused on the effects of sulfate on stratocumulus clouds, model studies suggest that sulfate may also exert significant effects on convective clouds in tropical systems as well (Andronache et al., 1998). 

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. 

King, M. D., L. F. Radke, and P. V. Hobbs, Determination of the Spectral Absorption of Solar Radiation by Marine Stratocumulus Clouds from Airborne Measurements within Clouds, J. Atmos. Sci, 47, 894-907 (1990). 

Brenguier et al. (2003) discussed the ACE-2 CC project, one of the five projects accomplished within the ACE-2 program of the second field experiment on studies of aerosol characteristics with the aim of understanding the indirect climatic impact of aerosol for marine stratocumulus clouds and to substantiate the strategy of closed aerosol-cloud-radiation experiments. Observations within the CC project were made in June-July 1997 in the Canary Islands using instruments onboard three flying laboratories and installed at a surface network. 

Brenguier et al. (2003) discussed the results of eight series of aircraft measurements of the microphysical characteristics of marine stratocumulus clouds in a broad range of observation conditions (different physico-chemical properties of aerosol, number density values in the interval 50 cm 3-25 cm-3, etc.)- The unique complex of synchronous observations of the microphysical and radiative characteristics of cloud cover obtained can be used to assess the indirect impact of aerosol on clouds and climate based on analysis of the ratio between the cloud optical thickness and effective radius of cloud droplets. Correlation between these values is usually negative, but in a heavily polluted atmosphere it can be positive. From the observational data obtained during ACE-2, the polluted systems of clouds turned out to be somewhat drier and therefore thinner, resulting in the positive correlation between the indirect impact of aerosol on climate and the effective radius of droplets. 

The optical thickness of the fog used in the above-described laboratory experiments is typical for clouds like cumulus or stratocumulus [76]. To extend those results to atmospheric scales, experiments have been performed with... 

The pH dependence of S(1V) oxidation in sea salt aerosols was also investigated by Keene et al. (1998) who found that at a pH of 8, oxidation by O3 dominated, whereas at a pH of 5.5, HOCl was most important. This fink was further investigated with a one-dimensional model by von Glasow et al. (2002b) for the cloud-free and for the stratocumulus-covered MBL. They give values integrated over the whole depth of the MBL and found that in the cloud-free cases between 19% and 57% of the SO2 oxidation within the sea salt aerosol was due to oxidation by HOCl and 0-13% by HOBr. In cloud droplets, however, the relative importance of HOCl and HOBr is reversed compared to that in aerosols in their model. This is caused by the dilution in cloud droplets which makes the backward reaction in the equihbria ... 

Cloud types often are classified based on altitude. High clouds have their bases above 7 km (23,000 ft) and include the wispy mare s tail clouds known as cirrus the cirrocumulus, known as mackerel sky and the layers of cirro-stratus. Middle clouds have altitudes between 2 and 7 km (6500 to 23,000 ft), and are either the rounded altocumulus or the layered altostratus. Low clouds have bases from near Earth s surface to about 2 km (6500 ft), and include stratocumulus, stratus, and nimbostratus. Nimbostratus clouds usually bring rain or snow. Clouds with vertical development extend from about 2 to 7 km or more, and include cumulonimbus (thunderhead clouds) and cumulus. 

The liquid water content of typical clouds, given the symbol L and often abbreviated LWC, varies from approximately 0.05 to 3 g(water) m, with most of the observed values in the 0.1 to 0.3 g(water) m region. A frequency distribution for LWC average values for stratus, stratocumulus, altostratus, and altocumulus clouds is given in Figure 6.2 (Heymsfield, 1993). 

Johnson, D. W., Osborne, S. R., and Taylor, J. P. (1996) The effects of a localised aerosol perturbation on the microphysics of a stratocumulus cloud layer, in Nucleation and Atmo.spheric Aerosols 1996, edited by M. Kulmala and P. E. Wagner. Elsevier, Oxford, pp. 864-867. 

Feingold, G., W. R. Cotton, S. M. Krddenweis, and J. T. Davis (1999) Impact of giant cloud condensation nuclei on drizzle formation in marine stratocumulus Implications for cloud radiative properties. Journal of Atmospheric Science 56, 4100-4117 Feng, J. and D. Moller (2004) Characterization of water-soluble macromolecular substances in cloud water. Journal of Atmospheric Chemistry 48, 217-233 Fenger, J. (2009) Air pollution in the last 50 years - from local to global. Atmospheric Environment 43, 13-22... 

Two clond types, stratocnmnlns and cirras, cover the greatest portion of the earth s snrface and are therefore the most important clond types from the standpoint of the effect of clouds on climate. The focused observations reveal that stratocumulus have a net cooling effect on the planet. However, the net effect of cirrus on climate is still nnclear. The details of the cirrus cloud microphysics, including the mean, maximum, and effective (radiatively important) particle size, crystal shape, temperature, and height in the atmosphere, affect their radiative properties. This is an active area of research. 

Xhe stratocumulus cloud deck over the east Pacific has large cloud variability, on 1-5 km scales. Xhe cloud deck slopes upward from 700 to 1000 m in a northeast-southwest direction over a distance of 120 km. In the examples studied, vertical cloud top distributions were negatively skewed, indicating flat-topped clouds. Xhe dominant spectral peak of the cloud-top variations was found at 4.5 km, which is 5-7 times the depth of the local boundary layer. Xhe cloud layer was stable with respect to cloud-top entrainment instability. Structural properties of stratocumulus clouds observed off the coast of southern California, near San Francisco, and in the Gulf of Mexico are... 

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