Download Towards stability metrics for cloud cover variation under climate

Towards stability metrics for cloud
cover variation under climate change
Rob Wood, Chris Bretherton, Matt Wyant, Peter Blossey
University of Washington
Stability and low clouds, history
• Slingo (1980)
– Used model potential temperature lapse rate between ~700
and ~850 hPa as predictor of subtropical low cloud cover
– Cloud data from satellites (Miller and Feddes 1971)
Stability and low clouds, history
• Klein and Hartmann
(1993)
– Used potential
temperature difference
between 700 hPa and
the surface (“lower
tropospheric stability”)
as predictor of
subtropical low cloud
cover
– Cloud data from volunteer
ship observations (Warren
cloud atlas)
Stability, low cloud, and climate change
• Miller (1997) thermostat hypothesis:
– Static stability expected to increase in warmed
climate
– Cloud amount vs stability relationships indicate
more cloud
– Negative cloud feedback
• Is LTS a suitable predictor of cloud responses to
climate change?
Inversion strength and stability
Wood and Bretherton (2006)
• In the free-troposphere,
d/dz=FT follows a moist adiabat
from 700 hPa to the MBL top.
• Well mixed surface layer below
the LCL, i.e. d/dz=0
• In the cloud layer, d/dz=CL
follows a moist adiabat from the
top of the LCL to the MBL top.
∆
EIS, a measure of inversion strength ∆
∆=(700 – 0) – FT(z700 – zi) – CL(zi – zLCL)
= (700 – 0) + zi(FT – CL) – FTz700+CLzLCL
(a) Neglect term with zi as this term is generally small
(b) Replace FT and CL with a single moist adiabat
850=m([T0+T700]/2, 850 hPa)
(c) Assume surface RH of 80% to estimate zLCL
Then define an estimated inversion strength (EIS) as
EIS = LTS – 850(z700 – zLCL)
EIS solely a function of surface and 700 hPa temperatures for a
reference surface pressure p0=1000 hPa
LTS/EIS and low cloud amount
Subtropical
and tropical
Midlatitude
LTS/EIS and low cloud amount
Subtropical
and tropical
Midlatitude
EIS is a far
better
predictor of
low cloud
amount than
LTS over a
wider
temperature
range
Change in LTS (K)
Low cloud amount in an ensemble
of 2xCO2-control GCM simulations
is poorly estimated using LTS’ (for
which a general increase is
predicted)
Much better agreement with
change in saturated stability
(related to EIS’)
Williams et al. (2006)
Multiscale approach to the problem
• We use a suite of model simulations
– Climate model runs from CAM and GFDL (SST+2K
and 2CO2/SOM, Wyant et al. 2006)
– SP-CAM global run with SST+2K (Wyant et al.
JAMES) and 4CO2/fixed SST change.
– CRM and LES runs using forcings derived from SPCAM simulations for different stability percentiles
(Blossey et al. JAMES)
– Uses only data from tropics (30oS-30oN)
Cloud amount vs LTS (SST+2K)
Klein and Hartmann
control
SST+2K
Cloud amount vs EIS (SST+2K)
Wood and Bretherton
Cloud vs LTS (SST + 2K, with CRM and LES runs)
Cloud vs EIS (SST + 2K, with CRM and LES runs)
EIS and cloud
changes
The three models studied
here have significant
increases in EIS
However, most climate
models show decreasing
SWCF in the tropics in
AR4 runs
Why the discrepancy?
from Bony and Dufresne (2005)
SST+2K vs 2xCO2/SOM
Somewhat weaker low cloud changes for 2xCO2 runs
SST+2K vs 4xCO2/fixed SST (SP-CAM)
Completely different low cloud changes for 4xCO2 runs
MBL depth for control and perturbed runs
SST+2K
MODIS Obs
N×CO2
MBL depth
decreases
despite reduced
subsidence from
CO2 FT warming
 MBL
turbulence
weakens
Conclusions, SST+2K
• CAM3, AM2, and SP-CAM under SST+2K show large LTS increases
while low cloud cover changes increase more slowly than
predicted by LTS
• These models under SST+2K all show increases in EIS too. Cloud
changes in CAM and SP-CAM increase somewhat more rapidly
than predicted by EIS
• CRM driven by SP-CAM output consistent with SP-CAM
• LES driven by SP-CAM output not consistent with SP-CAM
Conclusions
• Cloud responses to changing CO2 very different from those due to
SST changes, even in slab-ocean models. CO2 induces additional
atmospheric radiative forcing at the top of the MBL in addition to
warming the surface.
• CO2 perturbs the relationship between MBL depth and EIS
whereas SST+2K does not
• MBL depth changing for a given EIS consistent with cloud
changes even at constant EIS
• Hypothesize that a single metric may be insufficient to capture
the low cloud changes from radiative forcing by CO2 and from
increased SST.
Doubling CO2 stabilizes lower troposphere
independent of SST changes
• Standard tropical profile, Fu-Liou RT model
free trop. rad. heating/LW down changes
A possible two-metric phase space
N×CO2
with SST
low cloud cover
increasing
EIS, stability-driven changes
The end
Why different sensitivity for SST+2K vs NxCO2?
Models all show
decreasing MBL depth
(defined as 50% RH level)
with increasing stability
Comments on previous
• CRM forced from SP-CAM has larger low cloud amounts, while
LES gives much smaller cloud amounts than parent model.
• SWCF for CRM similar to that from parent model, while LES is
still much weaker
• Climate changes in CRM similar to that in parent model
Comments on previous
• All models show decreasing MBL depth with increasing EIS
• SST+2K runs tend to deepen MBL depth or stay same whereas
NxCO2 runs tend to result in shallower boundary layers.
• The SP-CAM results particularly troublesome to interpret as
4xCO2 run shows huge decreases in MBL depth but much
weaker cloud cover changes. So increased CO2 results in
shallower MBL but with lower cloud cover.
• Could this be explained by methodology for compositing (i.e.
using percentiles)?
SWCF vs LTS and EIS
LTS
EIS
SWCF vs LTS and EIS (with CRM and
LES runs)
Comments on previous
• LTS better predictor of current climate model cloud cover
than EIS
• Increases in LTS are large in all models (1K). Corresponding
increases in low cloud cover are roughly consistent with KH93
for CAM and SP-CAM, but clouds decrease in AM3 despite
large LTS increases
• Increases in EIS are seen in all models but these are much
weaker than LTS increases (order 0.3-0.5K). CAM3 and SPCAM cloud cover increases more strongly than expected from
low cloud-EIS relationship of Wood and Bretherton.
• Similar conclusions drawn for SWCF vs LTS/EIS