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.

16 /16

Thank you!

Liang Chen, lchen15@gmu.edu