This is the third in a series of articles on spectral dispersion. This paper is a little off-topic, but it’s important for understanding the next post.
Lu and Seinfeld in the Journal of the Atmospheric Sciences, Study of the Aerosol Indirect Effect by Large-Eddy Simulation of Marine Stratocumulus [PDF].
Abbreviated Abstract: A total of 98 three-dimensional large-eddy simulations (LESs) of marine stratocumulus clouds covering both nighttime and daytime conditions were performed to explore the response of cloud optical depth (Ï„) to various aerosol number concentrations (Na = 50–2500 cm−3) and the covarying meteorological conditions (large-scale divergence rate and SST)… The second indirect effect is found to enhance (reduce) the overall aerosol indirect effect for heavily (lightly) drizzling clouds; that is, Ï„ is larger (smaller) for the same relative change in Na than considering the Twomey (first indirect) effect alone. The aerosol indirect effect (on Ï„) is lessened in the daytime afternoon conditions and is dominated by the Twomey effect; however, the effect in the early morning is close but slightly smaller than that in the nocturnal run.
As usual, since this is a modeling study, I will gloss over the specifics of the model. Again, not because they aren’t important, but because I don’t fully understand them. This model is RAMS, the Regional Atmospheric Modeling System. Lu and Seinfeld wrote, “In the basic droplet activation scheme implemented in the RAMS modle the aerosols are assumed to have a constant size distribution is space and time.” This is describing the non-liquid water aerosols, or the potential CCN particles. While this is technically untrue, the size distribution of CNN changes with respect to space and time, I am unsure what the effects this would have on the outcome of this particular study. In fact, it’s probably a good thing to keep the CCN distribution constant to isolate the effects of the other changed variables. But I would find it interesting to see the effects of the changes in CCN sizes in both space and time.
The results reported seem to be contradictory. In the beginning of the second paragraph in the results section it is stated that “cloud LWP varies only slightly with aerosol concentration, particularly at high aerosol loadings. There is a slight decrease of LWP as Na increases from 50 to 500 cm-3 in each of the cases. The effect of high pollution loadings in suppressing drizzle formation (Albrecht 1989) is evidenced as precipitation disappears when Na exceeds 500 cm-3.” Later, at the end of that same paragraph is the following “the result shows cloud LWP is slightly higher for the clean clouds in contrast to the typical secondary aerosol effect.” How do they resolve this? Not very well in my opinion, if at all. From their figures, it appears that at aerosol loading under 500 cm-3 (clean clouds), there is an accumulation of precipitation at the surface. And that at aerosol loadings of 500 cm-3 or greater (dirty clouds), there is no accumulation of precipitation. Thus, showing the traditional second indirect effect. However, in the figure right next to that one, for aerosol loadings less than 500 cm-3, the liquid water path (LWP) is greater than for aerosol loadings equal or greater than 500 cm-3. To first order, it doesn’t make sense why precipitation should decrease and LWP decrease at the same time. Though there are at least two possible solutions. First, there could be more precipitation from the cloud, but that it is being evaporated below the cloud deck. Also, the favorite cause of entrainment; dry air from above cloud top can be mixed with the moist air to lower LWP. Both hypotheses are posited, but its not articulated which one is the primary cause, if any.
The effective radius of the cloud size distribution, re, is defined as the third moment of the size distribution divided by the second moment. The spectral shape parameter, k, is inversely proportional to effective radius. But, effective radius is proportional to the LWP. Therefore, it is important to properly understand the effects of aerosol loading on LWP because it has implications for the calculations of spectral dispersion (shape).