Clouds and Aerosols – Part 2
Jón Egill Kristjánsson
Scattering of Solar Radiation
Absorption of Solar Radiation
Aerosol Influence on Liquid Clouds
Aerosol-Cloud-Precipitation processes
Influence of Pollution on Convective
Clouds
Rosenfeld et al. (2008: Science)
Influence of Pollution on Clouds, Radiation
and Precipitation
Europe
East Asia
Smith et al. (2011: ACP)
Simulated sulfate burden 2005 minus
1975
µg m-2
Europe: - 59%
Black Triangle: - 73%
East Asia: + 71%
Stjern & Kristjánsson (2015: J.Climate)
Simulated Changes in Cloud Droplet
Number Concentration at ~870 hPa
Non-interactive run
Interactive run
cm-3
cm-3
Europe: - 35%
Black Triangle: - 52%
East Asia: + 32%
Coherent and statistically significant signals in all three regions
Stjern & Kristjánsson (2015: J.Climate)
Simulated Change in Precipitation
Stratiform Precipitation –
linked to aerosols via
cloud microphysical
parameterizations
Convective Precipitation –
no explicit link to aerosols
mm
day-1
mm
day-1
Europe: + 4.3%
Black Triangle: + 7.9%
East Asia: - 4.4%
Not statistically significant
signals in ANY of the three
regions
Statistically significant
signals in ALL three
regions
Simulated Precipitation Susceptibility
for Stratiform Precipitation
 d ln( R) 

S 0  
 d ln( N ) 
Largest over remote ocean regions of lowlatitudes (clean air, warm rain important)
R: Rate of precipitation
(mm day -1)
N: Cloud droplet number
concentration (cm-3)
Sorooshian et al. (2009: GRL)
Stjern & Kristjánsson (2015: J.Climate)
RFari (‘aerosol direct effect’) vs
latitude
Global Average: - 0.35 ±0.5 W m-2
- Why negative on average?
- Why positive near the Poles?
- Why stronger in NH than SH?
RFari (‘aerosol direct effect’) for
different aerosol species
Estimates of RFari, ERFaci and ERFari+aci
Estimates of RFari, ERFaci and ERFari+aci
<RFari>:
- 0.35 ±0.5 W m-2
<ERFari+aci>: - 0.9 ±0.5 W m-2
Direct and Indirect Forcing in NorESM
- 0.08 W m-2
- 1.20 W m-2
Kirkevåg et al. (2013: GMD)
Papers referenced
Estimates of ERFari+aci
Modeling Groups
‘Atmospheric Overturning’ vs Global
Warming
As the climate warms, the
atmospheric overturning
circulation in the tropics weakens
How does global warming influence
precipitation?
• If Relative Humidity in the atmosphere is
constant, water vapor amount will increase by
6-10% per degree warming
• Will precipitation increase correspondingly?
• In fact, with CO2-induced warming, precip.
only increases by 1-3% per degree warming
• However, for each individual rain event, there
is a 6-10% increase per degree
• How is that possible?
How will the Statistical Behavior of
Precipitation change?
Stratospheric Sulfur Injections
Robock et al. (2009: GRL)
Marine Cloud Brightening
Wood (2012: MWR)
Salter et al. (2008: PTRSA)
Cirrus Cloud Thinning
Storelvmo et al. (2013: GRL)
Efficient ice nuclei are introduced into an upper troposphere depleted of these ice
nuclei so that instead of forming cirrus clouds by homogeneous ice nucleation
(hom), cirrus are formed by heterogeneous ice nucleation (het). Het cirrus have
larger ice crystals that fall faster, thinning the cirrus to allow more thermal radiation
to escape to space, cooling the planet (Mitchell & Finnegan, 2009: ERL)
Climate Engineering
• At the L1 Lagrangian
point, 1.5·106 km from
Earth (cf. SOHO
satellite)
Angel (2010: PNAS)
• The main GeoMIP
experiment G1 is based
on this idea
Kravitz et al. (2011: ASL)
Response to 4xCO2
Response to 4xCO2 plus reduced solar
radiation (‘G1’)
Precipitation is reduced (by ~5%)
even though the temperature is
slightly increased.
Why?
Uncontrolled global warming vs
engineered climate: Temperature
Uncontrolled global warming vs
engineered climate: Precipitation
What happens if we do geoengineering
for 50 years, then turn it off?
What happens if we do geoengineering for
50 years, then turn it off?