SIO 209 Special Topics: Cloud Dynamics and Climate
Winter 2013
Homework #4
Due February 13
Various geoengineering projects have been proposed to offset global warming due to
increased carbon dioxide in the atmosphere. One idea is to increase the number of cloud
particle nuclei so that scattering area will be increased and precipitation will be
suppressed. Both of these factors will promote increased cloud optical thickness.
For the following problem assume that a technology exists that will increase cloud optical
thickness by a factor of one (∆τ = 1) in both water and ice clouds. For simplicity, assume
these clouds do not occur over cold, ice-covered surfaces.
1) a) How could increased cloud optical thickness (∆τ = 1) affect the amount of
reflected shortwave radiation at the top of the atmosphere?
b) Would this impact be smaller, larger, or about the same in optically thin clouds
(τ ~ 1) or optically thick clouds (τ ~ 20)? Why? Assume the clouds are otherwise
the same.
c) Would this impact be smaller, larger, or about the same in low-level clouds (top at
1.5 km) or high-level clouds (top at 10 km)? Why? Assume the clouds are
otherwise the same.
2) a) How could increased cloud optical thickness (∆τ = 1) affect the amount of
outgoing longwave radiation at the top of the atmosphere?
b) Would this impact be smaller, larger, or about the same in optically thin clouds
(τ ~ 1) or optically thick clouds (τ ~ 20)? Why? Assume the clouds are otherwise
the same.
c) Would this impact be smaller, larger, or about the same in low-level clouds (top at
1.5 km) or high-level clouds (top at 10 km)? Why? Assume the clouds are
otherwise the same.
3) Specify what kind of clouds would be the best target for the hypothetical
geoengineering program: low-level optically thin clouds, high-level optically thin
clouds, low-level optically thick clouds, or high-level optically thick clouds. Why?
Bonus points: Describe what surface type and clear-sky atmospheric conditions
would be most favorable for a large geoengineering cloud impact.