Key land surface coupling processes over
semi-arid region analyzed in the DICE case.
S. Ait Mesbah, F. Cheruy, J.L. Dufresne,F. Hourdin,
JY Grandpeix, M.P. Lefebvre
Outline
• 2 PBL and LSM schemes developed at LMD/IPSL
and used for the 1D DICE case
• The DICE simulations
• A simple conceptual model to analyze the results
•
3D simulations and generalization
• conclusion
2 PBL and LSM schemes developed at LMD
and used for the 1D DICE cases
CASE DICE : Diurnal Land-atmosphere Coupling Experiment
ST
- Eddy diffusivity Kz,
- Counter gradient (1K/km),
- Surface PBL : Louis 1979
PBL
NP
-
Yamada (1983)
Thermal plume
2 PBL and LSM schemes developed at LMD
and used for the 1D DICE cases
CASE DICE
ST
PBL
- Eddy diffusivity Kz,
- Counter gradient (1K/km),
- Surface PBL : Louis 1979
ORCHIDEE2 (OR2)
- Moisture : Conceptual model.
( 2m depth. 2 layers, Top layer can
vanish, No drainage)
- Temperature : heat diffusion eq
(7 layers, 5m), thermal properties
depend on moisture.
SOIL
NP
-
Yamada (1983)
Thermal plume
ORCHIDEE11 (OR11)
- Moisture : Solves Richards
diffusion equation. 2m depth. 11
layers, Free drainage)
-Temperature heat diffusion eq
(7 layers, 5m), thermal properties
depend on moisture
PLAN
• 2 PBL and LSM schemes developed at
LMD and used for the 1D DICE cases
• The DICE simulations
• A simple model to analyze the results
•
3D simulations and generalization
• conclusion
DICE simulation
80% of C3 grassland, LAI = 0.4, 20 % of bare soil, loam, surface roughness=0.01m
-
KANSAS : 37.65N,263.265E
23 oct 1400 to 26 oct 1999.
Dry, no rain, no clouds.
DICE simulation
Différence OR2 – OR11
• Different moisture profiles ( OR2 ,OR11 )induce different thermal inertia at the
surface.
LMDZOR - ORC2 et ORC11
IT variable, due to differences in soil
moisture parameterization
LMDZ-Bucket
Beta = 0.03, IT variable
PhyA IT 946
PhyB IT 946
DTROR11>DTROR2
ST et NP similar during the day
NP amplifies the differences:
colder Ts during the night
OBS
LMDZOR - ORC2 et ORC11
Imposed IT : 1080 (J/(K.m².s-0.5)
PhyA IT 2190
PhyB IT 2190
LMDZ-Bucket , IT variable The differences in nocturnal Ts
are due to the
qualitatively similar to
LMDZOR-ORC2 et ORC11. parameterization of the BL
DICE simulation
• DTR weaker with ORC2 , due to an higher G;
 A bucket LSM with prescribed b coupled with LMDZ reproduces the behavior
•
DTR is less affected than expected ;
is less than expected (G divided 2, same DF> DTR divided 2)
 DF (atmospheric processes) are impacted
• Strong day/night dissymmetry in the DTR sensitivity to G
•
The atmospheric physics modulates the temperature differences (lower with ST /
NP)
DICE simulations:
Questions
1- How to explain the greater sensitivity to the thermal inertia
during the night ?
2- What is the role of the boundary layer scheme in the
asymmetry?
PLAN
• 2 PBL and LSM schemes developed at
LMD and used for the 1D DICE cases
• The DICE simulations
• A simple model to analyze the results
•
3D simulations and generalization
• conclusion
SIMPLE MODEL, ENERGY
BALANCE
SWdn
LWup
LWup
LWdn
LE H
LE H
SWup
G
Swdown – Swup + Lwdown – Lwup = H +LE + G
LWdn
G
Swdown – Swup + Lwdown – Lwup = H +LE + G
SIMPLE MODEL, ENERGY
BALANCE
ATMOSPHERIC FORCING
LWup
SWdn
LWdn
SWup
Swdown – Swup + Lwdown – Lwup = H +LE + G
Swdown – Swup + Lwdown – Lwup = H +LE + G
SIMPLE MODEL, ENERGY
BALANCE
Response of the surface to atmospheric forcing
LWup
LWup
LE H
LE H
G
Swdown – Swup + Lwdown – Lwup = H +LE + G
G
Swdown – Swup + Lwdown – Lwup = H +LE + G
Simple model
Equilibrium
a Duration of the day
fs Surface balance
Sensitivity of X to the thermal inertia :
From 3days DICE
SP
NP
Bucket , b constant
G2 = 2140 J.m-2.K-1.s -.5 ,
G1= 940 J.m-2.K-1.s -.5
Simple model
SP
NP
• If a = .5 , sensitivity of atmospheric flux to I identical
changes without impacting Tmean
(DICE, a =0.36)
= 1 and DTR
•
day/night differences arises from H’ and LE’
Both are significantly higher during the day
• DICE dry LE’ << H’ : Dissymmetry arises from the sensible heat flux
H’day and H’night function of stability of the PBL
 Day= PBL unstable, small variation of Ts translates into turbulence
 Night = stable PBL, decoupled surface, small turbulence,
The energy stored in the soil released, only radiation responds to change
of Ts , a small change of G is changed in a change of Ts
Sensitivity to the PBL scheme
• The larger the contrast of PBL stability between day and night,
the higher the dissymmetry of the sensitivity of Ts to the
thermal inertia.
• SP produces more mixing than NP during the night, the
day/night stability contrast is lower
 Ts(SP) is less sensitive to the thermal inertia than Ts(NP)
PLAN
• 2 PBL and LSM schemes developed at
LMD and used for the 1D DICE cases
• The DICE simulations
• A simple model to analyze the results
•
3D simulations and generalization
• conclusion
From DICE to arid and semi-arid regions
Increase G
NP
Low thermal inertia
High thermal inertia
Conclusions
• The thermal inertia (parameterization of soil
moisture and thermal properties of the soils)
impact DTR and Tmean
• It impacts the atmospheric fluxes as well
• The diurnal variation of the stability of the
atmosphere is essential
• The sensitivity to the thermal inertia is higher
with PBL schemes which produces less mixing
during the night
• Its parameterization in GCM’s can lead to biases
in the surface temperature