Constraints on Cirrus Cloud Seeding - David L. Mitchell Desert Research Institute, Reno, Nevada, USA
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Constraints on Cirrus Cloud Seeding
David L. Mitchell
Desert Research Institute, Reno, Nevada, USA
- ‹#›Photo courtesy of Paul Lawson/J.H. Bain
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lAlbert Einstein: We cannot solve our
problems with the same thinking we
used when we created them.
Radiation management climate engineering might possibly
avert a climate emergency but only if the consciousness
that produced the emergency is quickly changed.
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Role of Cirrus Clouds in Earths Radiation Budget
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WHY CIRRUS CLOUDS?
1. Instead of reflecting sunlight, why not release heat?
a. cirrus-greenhouse gas analogue
2. Cirrus clouds have a net warming effect on climate.
a. optical depth < ~ 4; ubiquitous global coverage ~ 25%
b. The colder the cirrus, the greater the warming effect
3. Climate sensitivity depends on upper tropospheric
water vapor and cirrus cloud coverage (Sanderson et al.
2008). Therefore reducing the amount of the coldest
cirrus & UT water vapor should be an effective climate
engineering strategy.
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Homogeneous Freezing Nucleation:
RHi > 140%, T < - 40°C.
Higher nucleation rates, smaller crystals
Frozen haze Strongest greenhouse effect
droplet Heterogeneous nucl. higher Vi, less
cloud coverage, more OLR
T = -40°C
Heterogeneous
Nucleation
Processes:
Frozen droplet 100% < RHi < 145%,
T > - 40°C
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HOW TO AFFECT UT WATER VAPOR?
1. Seeding occurs in clear sky conditions; cirrus form in
pre-seeded air mass.
2. Higher ice sedimentation rates deplete the UT of water
vapor as a new equilibrium is established.
a. Ubiquitous seeding with efficient ice nuclei may
initially produce cirrus in supersaturated regions
without cirrus, but over time higher Vi dehydrates
the UT as a new water budget is established.
b. RH in GCMs is sometimes “tuned” by adjusting the
ice fall speed.
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GCM study by Lohmann et al. (2008, ERL) lends support to this
climate engineering idea:
Red: homogeneous nucl., RHi > 140% Blue: heterogeneous nucl., RHi = 130%
Green: homo- and heterogeneous nucleation together
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Key results of Lohmann study:
Heterogen. nucl. relative to homogen. nucl. yields:
a. 11% increase in effective size, De , and higher V
b. Reduced cirrus coverage
c. Net change in cirrus cloud forcing = -2.0 W m-2,
due to less cirrus (OLR > albedo effect).
d. A decrease in RH due to higher V, producing an
additional clear-sky net cooling of -0.7 W m-2.
e. This total net cooling of -2.7 W m-2 is comparable
with the net warming due to a doubling of CO2,
which is ~ 3.7 W m-2.
Using more efficient heterogeneous ice nuclei should
produce a stronger total net cooling, perhaps exceeding
the net warming from a CO2 doubling.
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Engineering Constraints
1. Can jet engine safety regulations be satisfied by injecting the
seeding solution (bismuth tri-iodide?) into engine exhaust?
2. Can a fleet of drones be used to disperse seeding material?
3. Seeding aerosol must be well dispersed before cirrus cloud
forms. Therefore seed under clear conditions.
- Atmospheric shear enhances rate of plume dispersion but
aircraft avoid shear.
- Seeding aerosol half-life ~ 1 week. Flight frequency
depends on half-life and plume dispersion rates.
4. Ice in anvil cirrus may be nucleated in convection column,
making seeding strategy uncertain. Cirrus engineering may be
better suited for synoptic cirrus in mid-latitudes and Arctic.
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Using cirrus cloud climate engineering only outside the tropics could cool the
planet where global warming is most severe and restore the temperature
gradient between the topics and polar regions.
Potentially the best method for cooling
the Arctic to prevent runaway
global warming due to loss of
sea-ice and possibly increasing
methane release. Reduced
solar insolation in Polar
Regions may make SRM
methods less effective.
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-Closing Comments
• Cirrus climate engineering may buy time for transitioning to a non-carbon
energy system, but it is not a solution to global warming.
• Cirrus engineering does not address ocean acidification.
• Geo-engineering carries the risk of changing climate in unforeseen
ways. If negative side-effects occurred in this approach, the process
could be terminated and conditions could return to normal within months
since the residence time of the seeding aerosol is ~ 1 week.
• Relative to stratospheric sulfur injections, this approach appears to
have the following advantages:
a. Potentially less impact on the hydrologic cycle
(releasing heat rather than reflecting solar energy)
b. Does not destroy ozone
c. Better suited for regional applications (e.g. Arctic)
d. Sky color unaffected
• Research is needed to explore the this idea in terms of ice nucleation
processes, plume dispersion rates, GCM simulations and limited field
campaigns.
• Recent findings published by InTech (open access) in “Planet Earth 2011
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– Global Warming Challenges and Opportunities for Policy and Practice”.
-EXTRA SLIDES
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-Does homogeneous nucleation dominate for T < -40°C?
Results from SPARTICUS: Note changes in ice crystal number
concentration and size near - 40°C …
Ice Particle Mean Size (µm)
N / IWC (g-1)
Temperature (°C) Temperature (°C)
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-… and changes in ice particle shape and fall velocity.
Mass-weighted fall speed (cm/s)
Mean PSD Area Ratio
Temperature (°C) Temperature (°C)
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-In situ and satellite results suggest De is sensitive to seeding
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Courtesy of Dr. Naomi Vaughan, Univ. East Anglia
Decrease cirrus
Decrease cirrus clouds
to increase OLR
and water vapor
Outgoing
longwave
radiation
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-Seeding Material and Delivery Mechanism:
• Bismuth tri-iodide and perhaps other substances nucleate ice
as efficiently as silver iodide for T < -20°C, are relatively
inexpensive and are non-toxic (e.g., Pepto Bismol = bismuth
subsalicylate).
• The delivery mechanism may already exist if the seeding
substrate can be injected and vaporized in the exhaust of
commercial airliner engines. The engines themselves may not
need to be exposed to seeding material.
• Alternatively a fleet of drone aircraft could remain in flight for
many hours and seed large regions of atmosphere.
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-HOW TO AFFECT CIRRUS CLOUDS?
1. Cirrus cloud coverage is affected by the concentrations of
small ice crystals through their affect on ice fall speeds.
(Mitchell et al. 2008, GRL)
a. Higher fall velocities (V) shorter cirrus lifetimes
2. Change the concentrations (and sizes) of cirrus ice crystals
by changing their nucleation rates.
a. T < - 40 oC: Homogeneous nucleation dominating
higher concentrations, smaller sizes, lower V
b. High supersaturation wrt ice (RHi > 140%) required for
homogeneous nucleation
c. Introduce ice nuclei that activate at much lower RHi
less competition for vapor larger crystals, larger V
3. This reduces cirrus coverage for T < - 40°C where their
greenhouse effect is largest.
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-Similar results were found in GCM (CAM3) study by Mitchell et al. 2008, GRL.
As fall speeds increase, net cooling occurs where global warming effects
are greater (mid-latitudes & polar regions). Key difference from Lohmann
et al. is that fall speed change is greater in the tropics.
Lower fall speed Higher fall speed
SWCF LWCF
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-A fundamental question is whether homogeneous nucleation
dominates for T < - 40°C. A recent field campaign sampling
mid-latitude cirrus for 6 months provides evidence for this:
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-Problems with injecting
sulfur into stratosphere:
1. Affects hydrologic
cycle (less pptn.)?
2. O3 destruction
3. Sky color turns white
Figures from Trenberth & Dai, GRL, 2007
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-Release more heat
to space (new idea)
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-SUMMARY & END OF TALK
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