reconciling the disagreement between observed and
TRANSCRIPT
Boulder, CO
Reconciling the Disagreement between Observed and Simulated Temperature Responses to Deforestation
Liang Chen, Paul Dirmeyer
George Mason University
06/19/2018
2018 CESM WORKSHOP
Motivation
• Most of the LUCID and CMIP5 didn’t capture the “observed” JJA TX warming by deforestation.
(Lejeune et al. 2017)
land use only – piControlall forcings – all forcings but without land use
TX90P
(Li et al. 2018)
2 /16
Research question
Space-for-time analogy vs. coupled deforestation simulations
• Is it a fair comparison?
• Does scale matter?
space-for-time
3 /16
Data and methods
• Observations• MODIS LST, land cover type, albedo, NDVI and ET• Global Forest Cover (Hansen/UMD)• CERES surface fluxes analysis• Other satellite (and reanalysis) LST products
less treehigher tmp.
more treelower tmp.
more treehigher tmp.
less treelower tmp.
sensitivity of Ts to deforestation (-1*TreeCover)
Tree Cover
Daytime Ts
~ * -1
assume the pixels receive the similaratmospheric forcingswithin the analysis window…
Ts tree cover
4 /16
Data and methods
• CESM large-scale deforestation simulations (CAM4 and CLM4)
Replace all the trees PFTs with grass.
The type of grass is determined by its latitude (arctic C3, C3, or C4 grass)
• low-res offline (2 deg)
• Low-res subgrid (2 deg)
• high-res explicit (0.5 deg)
• high-res regular (0.5 deg)
low-res(1.9x2.5)
high-res explicit(0.47x0.63)
tree: 37.5%
grass: 62.5%
high-res regular(0.47x0.63)
slope (Ts~deforestation) grass–tree grass–tree
5 /16
Observations MODIS
Within the bigwindow (1° x 1° )
6 /16
Offline vs. coupled experiments low-res
7 /16
Ts change at PFT level
Local impacts:same atmosphere
different land cover
Non-local impacts:same land cover
different atmosphereCombined effects
before: the coupled run before deforestationafter: the coupled run after deforestation
8 /16
T, Q, RH, and cloud fraction in coupled simulations
a b
c d
9 /16
“Fair” comparisons
SW
offline experiments, PFT-level comparisons, or observationally
based assumptions
LW
share the same atmospheric background
coupled experiments (mid/high latitudes in the CESM
deforestation experiments)
different atmospheric background
coupled experiments (clear sky)
different atmospheric background
coupled experiments (clear sky condition
and the same LWdown)
“same” atmospheric background
10 2 3
10 /16
Filtered Ts changes in the coupled experiments
1 2
3
11 /16
Is it a valid assumption if at a broader scale?clear-sky downward LW
atmosphericbackground can bedifferent
cloud can alsoplay a role
Within the bigwindow (3° x 3° ):
cloud radiative effects net SW
clear-sky downward SW
cloud radiative effects net LW
CERES
W/(m2%)*10
12 /16
Does scale matter?MODIS
satellite observations and reanalysis
0.05 deg 0.05 deg 0.05 deg
25 km 0.5 deg 0.5 deg
0.75 deg 1 deg
1 deg window 1 deg window 1 deg window
125 km window 3 deg window 3 deg window
3 deg window 3 deg window
1 deg
3 deg window
13 /16
k/% * 10
Does scale matter? CESM simulations
“local” effects through different ways
“coupled” effects
14 /16
K
model-dependent?
subgrid PFT-level comparison in GFDL(courtesy of Li and Liao)
JJA temperature
Local vs. non local impacts of LCC in MPI-ESM
(Winckler et al. 2017)
(Lejeune et al. 2017)
15 /16
Conclusion
• CESM (CLM) agrees with the observations in a “fair” comparison.
• More attention should be paid to atmospheric models and the coupling between land and atmosphere.
• The scale matters, and summer cooling by deforestation is possible.
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