1. Marine stratocumulus clouds
E. Pacific
E. Atlantic
 Subtropical high-P regions
Typical cold sea surface temperatures, in conjunction with the subsidence associated with the subtropical high, are responsible for the formation of a shallow cool moist marine boundary layer capped by a strong inversion. Marine stratus clouds form in this marine boundary layer.

2. How do these clouds form?
Over the (warm) ocean, sensible and latent heat fluxes into the BL create buoyant plumes  BL grows by entraining drier air
Eventually deepens above LCL  cloud forms
 shift from dynamics of a dry BL to that of moist BL
(From Houze, 1993)

3. The well-mixed boundary layer
(From Houze, 1993)
The well-mixed boundary layer
This jump at cloud top is maintained by subsidence, which maintains a high potential temperature and low dew point just above the BL
h is the depth of the BL
qT = total water = qV + qL (conserved if BL is not precipitating)
θe = equivalent potential temperature, also conserved in dry adiabatic motion
By conservation of mass, (entrainment velocity, typically > 0, mean vertical velocity at h, i.e. the rate of change of height associated with net horizontal convergence or divergence in the mixed layer)
Typically, w(h) < 0
(divergent)

4. Asides: Equivalent potential temperature / mixing
The potential temperature that a parcel of air would have, if all its water vapor was condensed, and the latent heat converted into sensible heat:
where

5. Cloud-topped boundary later
Entrainment (deepening h) works against the sinking by large-scale subsidence
In dry BL, surface fluxes of heat and moisture are the main source of kinetic energy
In MOIST BL, negative buoyancy occurs at the top of the cloud  radiation cooling and evaporative cooling due to entrainment of dry air drive the motion in the CTBL
Notice importance of radiation cooling at cloud top that has been introduced in this figure!
Radiation continuously DE- stabilizes the laps rate to maintain the cloud as an unstable, turbulent layer
(Cotton, Bryan, van den Heever)

6. Cloud water contents
Consider the cloud liquid water content, starting at cloud base and ignoring the role of the kinetics of CCN activation and drop growth. The adiabatic liquid water content should be condensed out at any height z (i.e., enough water to bring the environment down to saturation at the temperature T(z)). This yields the classic linear profile of liquid water (g water / m3 air). What does vertical profile of CCN or drop number concentration look like? Mean drop size?
As h increases, LWP increases
Stevens, 2006)

7. Aerosol “Indirect Effects” on Climate [Chapter 22, Seinfeld and Pandis; IPCC]
The indirect aerosol effects are hypothesized to be caused by anthropogenic emissions of aerosols and their precursor gases, and their links to cloud formation

8. Postulated indirect effects
First indirect aerosol effect (“Twomey effect”; Twomey, 1977)
for a constant cloud water content, more aerosols lead to  more and smaller cloud droplets  larger optical depth  more reflection of solar radiation
MODIS true-color satellite image (04/03/2009)
(From:

9. Postulated indirect effects
Second indirect aerosol effect (“Cloud lifetime effect”; Albrecht, 1989)
the more and smaller cloud droplets do not collide as efficiently  decrease drizzle formation  increase cloud lifetime  more reflection of solar radiation
MODIS true-color satellite image (04/03/2009)
(From:

10. Postulated indirect effects
“Semi-Direct” aerosol effect (Hansen et al., 1997)
absorption of solar radiation by black carbon within a cloud increases the temperature  decreases relative humidity  evaporation of cloud droplets  more absorption of solar radiation (opposite sign as other indirect effects)
“NASA scientists announced a giant, smoggy atmospheric brown cloud, which forms over South Asia and the Indian Ocean, has intercontinental reach. The scientists discussed the massive cloud's sources, global movement and its implications. The brown cloud is a moving, persistent air mass characterized by a mixed-particle haze. It also contains other pollution, such as ozone.” (AGU, 2004)

11. What is the basis for these hypothesized linkages?
 Aerosol – cloud condensation nuclei (CCN) relationships:
CCN concentrations are observed to be higher in continental airmasses than in the marine atmosphere (e.g., order of 5000 cm-3 vs. 10 cm-3)
Remember [CCN] is reported for a particular supersaturation
Soluble particles are easier to activate to cloud drops
Larger particles are easier to activate to cloud drops
Aerosol – cloud drop number concentrations (CDNC) relationships:
CDNC are observed to be higher in continental clouds than in marine clouds

12. A signal has been looked for in satellite and in-situ data other than the specialized case of ship tracks
The remote sensing studies show that “polluted” clouds are more reflective, and show correlations between aerosol optical depth and cloud optical depth (and negative correlation between AOD and cloud drop effective radius).
The in-situ measurements find relationships between [CCN] and drizzle and effective radius in marine stratocumulus. An example of observations linking cloud reflectivity to pollution is shown below (Brenguier et al., 2000).
IPCC TAR concludes that “these studies leave little doubt that anthropogenic aerosols have a non-zero impact on warm cloud radiative forcing”. This forcing is estimated to be –0.7 to –1.7 W m-2 over oceans.

13. The CLAW Hypothesis R. Charlson, J. Lovelock, M. Andreae and S
The CLAW Hypothesis R. Charlson, J. Lovelock, M. Andreae and S. Warren (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326,

14. The CLAW Hypothesis
we now have evidence that some of the steps within the CLAW hypothesis are correct
but we still don't know whether the system really operates as a negative feedback loop. 
This makes it very difficult to represent the process in climate models and so we are still unsure quite how important DMS is to the cooling of our planet.

15. Modeling indirect effects
Anthropogenic emissions (of SO2, combustion and biomass burning aerosols, and mineral dust aerosols)  CCN
CCN  CDNC
CDNC  cloud optical depth
cloud optical depth  cloud albedo
cloud albedo  changes in short-wave forcing

16. To relate CDNC to cloud optical depth:
First, consider a spatially uniform cloud of depth h with a drop number concentration distribution n(r) based on drop radius, r. Extinction of radiation by the cloud is given by, The optical depth of the cloud, tc, is the product bexth. If the cloud drop size distribution is monodisperse, with number concentration N and radius re (effective radius), then since for large size parameters (satisfied by 10 micron drops with visible wavelengths), Q approaches 2.

17. (continued)
The cloud liquid water content, in g water per cubic meter air, is (where rho is the density of water), Combining, So we can see that the cloud optical depth depends on the liquid water content, physical thickness, and mean radius of the cloud drops.

18. (continued)
We could also express the optical depth in terms of N, by replacing effective radius with its definition in terms of L (see above): So cloud optical depth is proportional to liquid water content to the 2/3 power, physical thickness, and number concentration of cloud drops to the 1/3 power.

19. Relating cloud albedo (reflectivity) to optical depth:
(Seinfeld and Pandis)
cloud becomes completely reflective if it has an optical depth much bigger than 7.7

20. Sample calculated relationships
For a costant liquid water content and constant cloud thickness (isolines), cloud albedo increases with increasing CDNC.
For example, for a 50 m thick cloud, changing CDNC from 100 cm-3 to 1000 cm-3 (like a change from clean marine to continental), the cloud albedo doubles.

21. Cloud Susceptibility to Meteorology versus Aerosol
FIRE July 2009 case study  = 287 K, qt = 10 g/kg
zt=600 m, zb=200 m, H=400 m, W=220 gm-2, dzb/dqt=150 m/gkg-1
A CDNC doubling is balanced by a decrease of the cloud thickness of H=H/5=80 m, i.e. qt=-0.5 g/kg, equivalent T=0.8 K.
(qt=-0.12 g/kg or T=0.2 K for a 100 m thick cloud layer).

22. Cloud Susceptibility to Meteorology versus Aerosol
In MBL clouds, the susceptibility to the meteorology is two orders of magnitude greater than to the aerosols.
Assuming we observe a pristine and a polluted cloud system, and detect different trends in LWP, it is difficult to attribute these differences to the aerosols because the small differences in meteorological forcings that could also explain the LWP changes are not measurable !
Statistical approaches are necessary to filter out the variability of the meteorology (same methodology as in weather modification assessments)