This is the fourth article in the spectral dispersion series. I’ll be summarizing / reviewing the 2006 paper by Lu and Seinfeld in the Journal of Geophysical Research, Effect of aerosol number concentration on cloud droplet dispersion: A large-eddy simulation study and implications for aerosol indirect forcing.
Abstract: Through three-dimensional large-eddy simulations of marine stratocumulus we explore the factors that control the cloud spectral relative dispersion (ratio of cloud droplet spectral width to the mean radius of the distribution) as a function of aerosol number concentration and the extent to which the relative dispersion either enhances or mitigates the Twomey effect. We find that relative dispersion decreases with increasing aerosol number concentration (for aerosol number concentrations less than about 1000 cm−3) because smaller droplets resulting from higher aerosol number concentrations inhibit precipitation and lead to (1) less spectral broadening by suppressed collision and coalescence processes and (2) more spectral narrowing by droplet condensational growth at higher updraft velocity because reduced drizzle latent heating at cloud top results in increased boundary layer turbulent kinetic energy production by buoyancy and thereby stronger turbulence. Increased spectral broadening owing to increased cloud-top entrainment mixing, also as a result of increased boundary layer turbulence, is relatively insignificant compared with outcomes 1 and 2. The coefficient k, an important parameter that relates cloud droplet effective radius and volume mean radius in large-scale models, is a function of skewness and relative dispersion of the distribution and is negatively correlated with relative dispersion. Increasing k with increasing aerosol number concentration leads to maximum enhancement of the cloud susceptibility (the change of cloud optical depth due to change of cloud droplet number concentration) over that attributable to the Twomey effect alone by about 4.2% and 39% for simulated FIRE and ASTEX cases, respectively.
As with the last paper I talked about in this series, this one is again a modeling study. The good news is that it is by the same authors as the last one, and uses the same model. That was good because I didn’t need to wade through all the modeling talk, that I only vaguely understand. As the title of this paper says, it is about the effect of aerosol number concentration on cloud droplet dispersion. Remember that relative dispersion is just a measure of the width of the size distribution of the liquid water droplets. They attempt to show the factors that control dispersion between clean and polluted conditions, and to what extent this change has on the Twomey effect.
The paper presents a flow chart representing the physical mechanisms that result from an increase in Na, the aerosol concentration and potential cloud condensation nuclei (CCN). An increase in Na results in more numerous, smaller droplets. This is because there are more CCN for the water to condense onto. This in turn leads to suppressed drizzle (Albrecht, 1989). Less drizzle means that there are fewer large droplets that fall into and collect the smaller droplets, called collision and coalescence. This leads to a smaller relative dispersion. Suppressed drizzle also leads to less latent heat caused by the condensation of water at cloud top, leading to more turbulent kinetic energy (TKE) production. This leads to stronger updrafts, and condensational spectral narrowing, or a smaller relative dispersion. Increased TKE production also implies a larger entrainment mixing, which leads to a larger relative dispersion, but they note that the effect is small.
The results of the model experiment is that an increase in Na does in fact lead to a decrease in the relative dispersion. They note that this is consistent with several observations (Pruppacher and Klett, 1997; Miles et al., 2000; Yum and Hudson, 2005). However, they also point out that other observations noticed the exact opposite effect, an increase in Na leads to an increase in the relative dispersion (Martin et al., 1994, Ackerman et al., 2000; McFarquhar and Heymsfield, 2001; Liu and Daum, 2002).
As can be seen, there is no general agreement in the literature what effect an increase in aerosol has on the spectral dispersion of the liquid water droplets. There seems to be an equal number of studies that find opposite results. Part of this could be contributed to experimental design, as the paper points out. Most of the past studies of this nature were done within the size range of the Forward Scattering Spectrometer Probe (FSSP) instrument. As a result, the relative dispersion was only calculated over the cloud droplet spectrum, and did not include the drizzle. The addition of the drizzle tends to increase the relative dispersion, according to Lu and Seinfeld. This was a result from their model, and as far as I know, has not been shown in field studies.
References:
Ackerman, A. S., O. B. Toon, J. P. Taylor, D. W. Johnson, P. V. Hobbs, and R. J. Ferek, Effects of aerosols on cloud albedo: Evaluation of Twomey’s parameterization of cloud susceptibility using measurements of ship tracks, J. Atmos. Sci., 57, 2684–2695, 2000.
Albrecht, B., Aerosols, cloud microphysics, and fractional cloudiness, Science, 245, 1227-1230, 1989.
Liu, Y. G., and P. H. Daum, Anthropogenic aerosols: Indirect warming effect from dispersion forcing, Nature, 419, 580–581, 2002.
Martin, G. M., D. W. Johnson, and A. Spice, The measurement and parameterization of effective radius of droplets in warm stratocumulus clouds, J. Atmos. Sci., 51, 1823–1842, 1994.
McFarquhar, G. M., and A. J. Heymsfield, Parameterizations of INDOEX microphysical measurements and calculations of cloud susceptibility: Applications for climate studies, J. Geophys. Res., 106(D22), 28, 675–28, 698, 2001.
Miles, N. L., J. Verlinde, and E. E. Clothiaux, Cloud droplet size distributions in low-level stratiform clouds, J. Atmos. Sci., 57, 295–311, 2000.
Pruppacher, H. R., and J. D. Klett, Microphysics of Clouds and Precipitation, 976 pp., Springer, New York, 1997.
Yum, S. S., and J. G. Hudson, Adiabatic predictions and observations of cloud droplet spectral broadness, Atmos. Res., 73, 203–223, 2005.