# Land surface parameterization

Mécanique

22 févr. 2014 (il y a 4 années et 4 mois)

133 vue(s)

Land surface in climate models

Land surface parameterization
schemes in climate models

Bart van den Hurk

(KNMI/IMAU)

Land surface in climate models

The global energy budget

Trenberth, 2009

Land surface in climate models

The global hydrological cycle

Peixoto & Oort, 1992

residence time

in atmosphere: ~10 days

in ocean: ~3000 yrs

land: ~ 1
-
5 yrs

Land surface in climate models

The global carbon budget

IPCC, 2007

Land surface in climate models

General setup of General Circulation
Models (1)

What determines the evolution of the atmosphere?

Motion

Equation of motion

U, V = f (pressure gradient, friction)

Temperature

Conservation of energy:

Moisture content

Conservation of mass

q = f (evaporation, condensation)

Land surface in climate models

General setup of General Circulation
Models (2)

Basic equations are solved on a
grid

Computational constraints

Typically 100
-
500 km horizontal

1

1
°
: 65,000 surface points

Typically 20
-
50 vertical layers

1

5
million

grid points

Numerical constraints

Numerical stability limits time step of
integraton

10

60 minutes/time step

One year = 10
5

time steps, one century = 10
7

Land surface in climate models

Land treatment in GCMs

General setup of General Circulation
Models (3)

Land surface in climate models

General setup of General Circulation
Models (3)

Many processes are sub
-
grid, and need to be
parameterized

Fine scale processes (fluxes) expressed in terms of
resolved variables (mean state) using (semi
-
)
empirical, observation based equations

Example: turbulent sensible heat flux

a
s
H
p
UC
c
H

s

a

H = Sensible heat flux [W/m
2
]

= air density [kg/m
3
]

c
p

= specific heat [J/kg K]

U = wind speed [m/s]

C
H

= exchange coefficient [
-
]

s

-

a

s

a

H

Land surface in climate models

Parameterizations in GCMs

Examples

Condensation/cloud formation

Convection

Turbulent mixing

Land surface processes

Land surface in climate models

Landprocesses in atmospheric models

Energy
-
budget

Albedo

Surface

Albedo

Dark forest

9
-
12%

Grassland

15
-
20%

Bare soil

20
-
30%

Snow in forest

15
-
25%

Open snow

50
-
85%

Land surface in climate models

Landprocesses in atmospheric models

Energy
-
budget

Albedo

Evaporative fraction

Q*

H

LE

G

Surface

LE/Q*

Boreal forest

25%

Forest in temperate climate

65%

Dry vineyard

20%

Irrigated field in dry area

100%

Land surface in climate models

Landprocesses in atmospheric models

Energy
-
budget

Albedo

Evaporative fraction

Water budget

Runoff
-
fraction

P

LE

Infiltration

Direct runoff

Drainage

Land surface in climate models

Landprocesses in atmospheric models

Energy
-
budget

Albedo

Evaporative fraction

Water budget

Runoff
-
fraction

Soil water reservoir

Season

Shallow

rootzone

Deep

rootzone

Land surface in climate models

Landprocesses in atmospheric models

Energy
-
budget

Albedo

Evaporative fraction

Water budget

Runoff
-
fraction

Soil water reservoir

Carbon budget

CO
2

H
2
O

Land surface in climate models

Fluxnet data analysis

Fluxnet
: collection of ground stations worldwide
over various surface types

Teuling et al, 2010

Land surface in climate models

General form of land surface schemes

Energy balance equation

K

(1

a
) +
L

L

+

E

+
H

=
G

Water balance equation

W
/

t

=
P

E

R
s

D

Q*

H

LE

G

P

E

Infiltration

R
s

D

Land surface in climate models

General form of land surface schemes

Energy balance equation

K

(1

a
) +
L

L

+

E

+
H

=
G

Water balance equation

W
/

t

=
P

E

R
s

D

Coupled via the evaporation

Q*

H

LE

G

P

E

Infiltration

R
s

D

Land surface in climate models

Development history of land schemes

Late 1960’s: bucket scheme (Manabe, 1969) with
depth of the reservoir = 15cm

P

E

Direct runoff

E = (W/W
max
) E
pot

R = 0

(W<W
max
)

R = P

LE

(W

W
max
)

Land surface in climate models

Development history of land schemes

Mid 1970’s: explicit treatment of vegetation
(Penman
-
Monteith ‘big leaf’)

To be combined with submodel for soil
infiltration/runoff

P

E

Direct runoff

a
c
a
p
r
r
D
r
c
G
Q
LE
/
/
*

Land surface in climate models

First Soil
-
Vegetation
-
Atmosphere Scheme
(SVAT)

Deardorff (1978) combined

Penman
-
Monteith

Partial vegetation coverage, but
still one energy balance equation
(lumped surface types)

‘effective’ surface resistance
(interpolating between canopy
value for full vegetation, and
large value for bare ground)

Land surface in climate models

First Soil
-
Vegetation
-
Atmosphere Scheme
(SVAT)

Deardorff (1978) combined

Interception of snow and precipitation by leaves

(small) bucket equation

Prognostic equation for soil temperature and
moisture (‘force restore’)

dW
l
/dt = P

E

(W
l

< W
lmax
)

dW
l
/dt = 0

(W
l

W
lmax
)

W
lmax

= c LAI

(c = 0.2 mm)

E = E
pot

I = P

E

dW
l
/dt

dW
1
/dt = (C
1
/z
1
) I

C
2

(W

W
equ
)/

dW/dt = I/z
2

Land surface in climate models

Explicit multi
-
component SVATs

Separate treatment of vegetation and
understory/bare ground (Shuttleworth et al,
1988)

canopy resistance

evap. resistance for bare ground

Complex rewriting of PM, involving

components

solution of T,q “within canopy” (at
network node)

separate aerodynamic coupling of two
components

Evaporation at bare ground affects canopy
transpiration and vice versa

Land surface in climate models

Tiled scheme

For instance ECMWF (2000)

Multiple fractions (“tiles”)

vegetation (transpiration)

bare ground (evaporation)

interception/skin reservoir
(pot. evaporation)

snow (sublimation)

Multi
-
layer soil

diffusion

gravity flow

Explicit root profile

Land surface in climate models

More on the canopy resistance

Active regulation of evaporation via
stomatal aperture

Two different approaches

Empirical (Jarvis
-
Stewart)

r
c

= (r
c,min
/LAI) f(K

) f(D) f(W) f(T)

(Semi)physiological, by modelling photosynthesis

A
n

=

f(W)

CO
2

/ r
c

A
n

= f(K

, CO
2
)

CO
2

= f(D)

Land surface in climate models

Summary of development

Soil hydrology

single bucket

two
-
layer force restore

multi
-
layer diffusion/gravity flow

Evaporation from surface

E = b E
pot

PM ‘big leaf’ (effective r
c
)

PM ‘multi
-
source’

Tiling

Canopy resistance

constant

empirical dependence on environment

photosynthesis
-
based

Land surface in climate models

Some other developments

Replace lat/lon grid by sub
-
catchment as spatial
unit (Koster et al, 1996)

Explicit parameterization of surface runoff (Dumenil
& Todini, 1992)

Infiltration curve

(dep on W and

orograpy)

Surface runoff

Land surface in climate models

Carbon allocation

Carbon allocation

distribution over leaf, stems, roots

decay and cycling through soil

GPP = Gross Primary
Production

NPP = Net Primary
Production

AR = Autotrophic
Respiration

HR = Heterotrophic
Respiration

C = Combustion

GPP

120

AR

60

HR

55

NPP

60

C

4

Land surface in climate models

Other biochemical processes

Nitrogen cycle

Land use change

http://www.visionlearning.com/library/module_viewer.php?mid=98

Land surface in climate models

International comparison/
evaluation experiments

Project for Intercomparison of Land
-
surface
Parameterization Schemes (
PILPS
; Pitman et al)

Observed atmospheric forcing

Comparison between partitioning of energy and
water

Single site (e.g. Cabauw)

2D catchments (e.g. Sweden)

Land surface in climate models

International comparison/ evaluation
experiments

Global Soil Wetness Project (
GSWP
)

Global 2D

Forcing from satellite, in situ and meteorological
(re)analysis data

-
up

Land surface in climate models

International comparison/ evaluation
experiments

Atmospheric Model Intercomparison Project
(
AMIP
)

Comparison of land surface processes in multiple
GCM’s

Forcing is not similar for all models

abs soil moisture content

soil moisture anomaly

Land surface in climate models

Orders of magnitude

Estimate the energy balance of a given surface type

What surface?

What annual cycle?

What is the Bowen ratio (H/LE)?

How much soil heat storage?

Is this the complete energy balance?

The same for the water balance

How much precipitation?

How much evaporation?

How much runoff?

How deep is the annual cycle of soil storage?

And the snow reservoir?

Land surface in climate models