Slide 1
PA Surface III of IV - training course 2013 Slide 1
Introduction
General remarks
Model development and validation
The surface energy budget
The surface water budget
The surface CO2 budget
Soil heat transfer
Soil water transfer
Snow
Initial conditions
Conclusions and a look ahead
Layout
Slide 2
PA Surface III of IV - training course 2013 Slide 2
Soil science miscellany (1)
The soil is a 3-phase system, consisting of
- minerals and organic matter soil matrix
- water condensate (liquid/solid) phase
- moist air trapped gaseous phase
Texture - the size distribution of soil particles
Hillel 1982, 1998
Slide 3
PA Surface III of IV - training course 2013 Slide 3
Soil science miscellany (2)
Structure - The spatial organization of the soil particles
Porosity - (volume of maximum air trapped)/(total volume)
Composition
Water content
Hillel 1982,1998
Reference:Hillel 1998 Environmental Soil Physics, Academic Press Ed.
Slide 4
PA Surface III of IV - training course 2013 Slide 4
Soil properties
Rosenberg et al 1983
Arya 1988
Slide 5
PA Surface III of IV - training course 2013 Slide 5
Skin layer at the interface between soil (snow) and atmosphere; no thermal inertia, instantaneous energy balance
At the interface soil/atmosphere, each grid-box is divided into fractions (tiles), each fraction with a different functional behaviour. The different tiles see the same atmospheric column above and the same soil column below.
If there are N tiles, there will be N fluxes, N skin temperatures per grid-box
There are currently up to 6 tiles over land (N=6)
TESSEL
N,...,1i
i
TTG i,sksi,ski
for tileindex
Slide 6
PA Surface III of IV - training course 2013 Slide 6
TESSEL skin temperature equation
Grid-box quantities
4,
,
,
,
, , ,
, ,
(1 )
( )
( , )
( ) 0
i S g T g
h i L
sk i
skp L p
h i L L i L s i sat s
sk i s
i
sk i
sk i
R R
C u C T gz C
C
T
T
u a q a q pT
TT
fraction Tile
,
i
iiskisk
iii
iii
C
TCT
ECE
HCH
Slide 7
PA Surface III of IV - training course 2013 Slide 7
Ground heat flux
2
2s
g
T
T
g
Ts
g
z
Tk
t
T
k
Cz
T
zz
G
t
TC
soil, shomogeneou anFor
ydiffusivit Thermal C
tyconductivi Thermal
capacityheat volumetric Soil
In the absence of phase changes, heat conduction in the soil obeys a Fourier law
Boundary conditions:• Top Net surface heat flux• Bottom No heat flux OR prescribed climate
Slide 8
PA Surface III of IV - training course 2013 Slide 8
Diurnal cycle of soil temperature
summer
winter
bare sod
Rosenberg et al 1983
50 cm depth
surface
summer
Slide 9
PA Surface III of IV - training course 2013 Slide 9
TESSEL
Solution of heat transfer equation with the soil discretized in 4 layers, depths 7, 21, 72, and 189 cm.
No-flux bottom boundary condition
Heat conductivity dependent on soil water
Thermal effects of soil water phase change
↓ 10.6 ~ 55.8 d
↓0.1~0.6 d
↓ 1.1~5.8 d
Time-scale for downward heat transfers in wet/dry soil
Slide 10
PA Surface III of IV - training course 2013 Slide 10
TESSEL soil energy equations
0G
TTG
DD5.0
TTG
D
GGTT
t
C
2/41
i1i,ski,sk2/1
1jj
1nj
1n1j
2/1j,T1n2/1j
j
1n2/1j
1n2/1jn
j1n
jj
conditions Boundary
1,...,4j
DjTj
j-1
j+1
Gj+1/2
Gj-1/2
Slide 11
PA Surface III of IV - training course 2013 Slide 11
Case study: winter (1)
Model vs observations, Cabauw, The Netherlands, 2nd half of November 1994
Soil
140 m
2 m
Slide 12
PA Surface III of IV - training course 2013 Slide 12
Case study: winter (2)Soil Temperature, North Germany, Feb 1996: Model (28-100 cm) vs OBS 50 cm
Observations: Numbers
Model: Contour
Slide 13
PA Surface III of IV - training course 2013 Slide 13
Model bias:- Net radiation (Rnet) too large
- Sensible heat (H) too small
But (Tair-Tsk) too large (too large diurnal cycle)
Therefore f(Ri) problem
- Soil does not freeze (soil temperature drops too quickly seasonally)
Case study: winter (3)
T air
T skin
Rnet H LE
G
))(( skairHnairp TTRifCUCH
Stability functions
Soil water freezing
Viterbo, Beljaars, Mahfouf, and Teixeira, 1999: Q.J. Roy. Met. Soc., 125,2401-2426.
Slide 14
PA Surface III of IV - training course 2013 Slide 14
Winter: Soil water freezing
water frozen Soil
T
I
Iwfs t
Lz
T
zt
T)C(
Soil heat transfer equation in soil freezing condition
)T(f)T(II
z
T
zt
T
T
fL)C( Twfs
Apparent heat capacity
Slide 15
PA Surface III of IV - training course 2013 Slide 15
Case study: winter (4)
1 Oct 1 Jan1 Dec
1 Nov 1 Feb
Germany soil temperature: Observations vs Long model relaxation integrations
Observations
Control
Stability
Stab+Freezing
Slide 16
PA Surface III of IV - training course 2013 Slide 16
Northern Hemisphere
Case study: winter (5)
Soil water freezing acts as a thermal regulator in winter, creating a large thermal inertia around 0 C.
Simulations with soil water freezing have a near-surface air temperature 5 to 8 K larger than control.
In winter, stable, situations the atmosphere is decoupled from the surface: large variations in surface temperature affect only the lowest hundred metres and do NOT have a significant impact on the atmosphere.
Europe
850 hPa T RMS forecast errors
Stab+freezing
Control
Slide 17
PA Surface III of IV - training course 2013 Slide 17
Introduction
General remarks
Model development and validation
The surface energy budget
The surface water budget
The surface CO2 budget
Soil heat transfer
Soil water transfer
Snow
Initial conditions
Conclusions and a look ahead
Layout
Slide 18
PA Surface III of IV - training course 2013 Slide 18
More soil science miscellany
3 numbers defining soil water properties
- Saturation (soil porosity) Maximum amount of water that the soil can hold when all pores are filled 0.472 m3m-3
- Field capacity “Maximum amount of water an entire column of soil can hold against gravity” 0.323 m3m-3
- Permanent wilting point Limiting value below which the plant system cannot extract any water 0.171 m3m-3
Hillel 1982Jacquemin and Noilhan 1990
Slide 19
PA Surface III of IV - training course 2013 Slide 19
Schematics
extractionroot ie k,source/sin water Soil
flux water Soil
water soil 12
33
S
skgmF
mm
Sz
F
t ww
Hillel 1982
Root extraction The amount of water transportedfrom the root system up to the stomata(due to the difference in the osmoticpressure) and then available fortranspiration
Boundary conditions:Top See laterBottom Free drainage or bed rock
Slide 20
PA Surface III of IV - training course 2013 Slide 20
Soil water flux
1
12
ty conductivi hydraulic
y diffusivit hydraulic
)(
ms
sm
zF w
Darcy’s law
> 3 orders of
magnitude> 6 orders of
magnitude
Mahrt and Pan 1984
Slide 21
PA Surface III of IV - training course 2013 Slide 21
TESSEL: soil water budget
Solution of Richards equation on the same grid as for energy
Clapp and Hornberger (1978) diffusivity and conductivity dependent on soil liquid water
Free drainage bottom boundary condition
Surface runoff, but no subgrid-scale variability; It is based on infiltration limit at the top
One soil type for the whole globe: “loam”
Slide 22
PA Surface III of IV - training course 2013 Slide 22
A new hydrology scheme
A spatially variable hydrology scheme is being tested following Van den Hurk and Viterbo 2003
Use of a the Digital Soil Map of World (DSMW) 2003
Infiltration based on Van Genuchten 1980 and Surface runoff generation based on Dümenil and Todini 1992
Van den Hurk and Viterbo 2003
Slide 23
PA Surface III of IV - training course 2013 Slide 23
A new hydrology scheme(2)
Dominant soil type from FAO2003 (at native resolution of ~ 10 km)
█coarse █medium █med-fine █fine █very-fine █organic Soil Diffusivity
Soil Conductivity
control
control
Slide 24
PA Surface III of IV - training course 2013 Slide 24
TESSEL soil water equations (1)
2/412/412/12/1
2/11
111
2/11
2/1
,
12/1
12/1
1
conditionsBoundary
5.0
ws
jjj
nj
nj
jwnj
jwj
nj
nj
nj
nj
w
FEYTF
DDF
SD
FF
t
Dj
j-1
j+1
Fj+1/2
Fj-1/2
j ↓ 11.7 ~ >> d
↓0.1~19.7 d
↓ 1.2~195.9 d
Time-scale for downward water transfers in wet/dry soil
Slide 25
PA Surface III of IV - training course 2013 Slide 25
TESSEL soil water equations (2)
2
32
,/,
,/,//,
tscoefficien Hydraulic
1,...,4j
n vegetatio(H/L) high/lowfor separate j,layer at extractionRoot
b
satsat
satsat
b
satsat
jjliqjLHj
jliqjLHjLHLHjw
b
DR
DRCS
Slide 26
PA Surface III of IV - training course 2013 Slide 26
Introduction
General remarks
Model development and validation
The surface energy budget
The surface water budget
The surface CO2 budget
Soil heat transfer
Soil water transfer
Snow
Initial conditions
Conclusions and a look ahead
Layout
Slide 27
PA Surface III of IV - training course 2013 Slide 27
Snow
Snow insulates the ground (30% to 90% of the snow mantle is air)
A snow covered surface has a higher albedo than any other natural surface (0.2-0.3 in the presence of forests, 0.5-0.8 for bare ground/low vegetation)
Snow melting keeps the surface temperature at 0 C for a long period in spring
Slide 28
PA Surface III of IV - training course 2013 Slide 28
Snow energy budget
waterliquidoffractionTf
capacitywaterliquidSnowS
G
Q
L
QGHELRt
T
T
TfSLDC
sn
Cl
m
s
msnsn
sn
snClfsn
fluxheat Basal
melting ofHeat
nsublimatio ofheat Latent
fraction Snow
Meltwater
fusion ofheat Latent
sn
f
msn
wffm
C
M
L
t
S
CLMLQ
Apparent heat capacity
Slide 29
PA Surface III of IV - training course 2013 Slide 29
Snow mass budget
)(
mass) (snow equivalent water Snow
Snowfall
Snowfall
snowpackthelevingwaterliquidRunoffR
S
F
F
RFcEcFt
S
sn
l
snlsnsn
Snow mass (S) and snow depth (D)
mD
C
SD
sn
snsn
w
area, covered-snow for thedepth Snow
density Snow
Slide 30
PA Surface III of IV - training course 2013 Slide 30
Metamorphism, density, albedo, Density
- Weighted average between current density and the density of fresh snow, in case of snowfall
- Overburden, thermal metamorphism (new formulation)
Albedo
- Exponential relaxation with different time scales for melting and non-melting snow
- Surface albedo for high vegetation regions with snow underneath from MODIS
Snow cover fraction
- Function of snow depth (10 cm deep – fully cover)
1.0,1min sn
sn
S
c
Slide 31
PA Surface III of IV - training course 2013 Slide 31
Case study: Boreal forest albedo (1)
Slide 32
PA Surface III of IV - training course 2013 Slide 32
0
-1
Forecast day0 10
Northern Hemisphere
FAL
CON
Case study: Boreal forest albedo (2)
Forecast day0 10
-1
0
-2
-3
Eastern Asia
CON
FAL
850 hPa temperature bias20 forecasts every 3 days, March-April 1996
No data assimilation
Considering a lower value of snow Forest ALbedo was beneficial.
Slide 33
PA Surface III of IV - training course 2013 Slide 33
Case study: Boreal forest albedo (3)
The surface albedo is the direct regulator of the energy available to the surface. The albedo of natural surfaces has a limited range (0.1-0.3), but in non-forested snow covered areas can reach values up to 0.8.
Snow covered boreal forests have a much lower albedo than grassland to their south and tundra to their north; the presence of boreal forests has a direct control on the climate of high-latitudes.
1997Operational
Bias(FAL)
1996Operational
Bias(CON)
Slide 34
PA Surface III of IV - training
course 2013
30R1
32R1
Dutra et al. 2008
Slide 34
Snow sublimation, role of roughness
Slide 35
PA Surface III of IV - training course 2013 Slide 35
Snow insulation, role of snow density (1)New snow formulation improves
snow mass and snow density.
Lower snow densities -> increased snow thermal insulation
Dutra et al. 2010
Slide 36
PA Surface III of IV - training course 2013 Slide 36
Snow insulation, role of snow density (2)
Increased snow thermal insulation
Stronger decoupling of land –atmosphere
Warm T2M bias are reduced (soil was warming the atmosphere)
Clear impact on long climate runs
Small impact on short range forecasts
Hazeleger et al 2010
Slide 37
Liquid Water in the snow-pack
A diagnostic formulation can take into account liquid water in the snow pack without new prognostic variable (similar to soil ice)
Slide 37PA Surface III of IV - training course 2013
Slide 38
PA Surface III of IV - training
course 2013
Liquid Water in the snow-pack (II)
The liquid reservoir is function of temperature, snow mass and snow density
Slide 38
Slide 39
PA Surface III of IV - training
course 2013
Snow density: I. New snow
Slide 39
Slide 40
PA Surface III of IV - training
course 2013
Snow density: II. Snow on the ground
In the old scheme rainfall infiltrates directly into the soil even in presence of snow on the ground. In the new snow scheme it is intercepted by the snow-pack
Slide 40
Slide 41
PA Surface III of IV - training
course 2013
Forest-Snow Albedo
Albedo of Forest areas with snow underneath was fixed to 0.15 (quite dark). A MODIS-derived vegetation dependent albedo is used.
Slide 41
Slide 42
PA Surface III of IV - training
course 2013
Open-Area Snow Albedo
Open area albedo is aging from 0.85 in case of fresh snow to 0.5 for old snow, however in case of new snowfall the albedo was istantaneously re-whitened to 0.85, regardless of the snowfall amount.
Slide 42
Slide 43
PA Surface III of IV - training
course 2013
Snow fractionSnow fraction is made dependent of snow
depth rather than snow mass.
A 10cm snow depth is required for full coverage.
Slide 43
Slide 44
PA Surface III of IV - training
course 2013
Verification
Slide 44
Slide 45
PA Surface III of IV - training
course 2013
SNOW-MIP2
Slide 45
Slide 46
PA Surface III of IV - training
course 2013
GSWP-2 and Runoff verification
Slide 46
Slide 47
MODIS albedo as snow verification product
Slide 47PA Surface III of IV - training course 2013
Slide 48
PA Surface III of IV - training
course 2013
Summary and conclusions for snow processes impactThe new snow scheme first validated in offline simulations (SNOW-MIP2
and GSWP2) and showed improvements on the water cycle and on the land albedo
Further tests in forecasts mode both at ECMWF and in a climate model have demonstrated that snow can improve near-surface temperatures in cold climate.
The introduction of multi-layer snow scheme is recognized as high priority in the next years to improve the representation of the diurnal cycle and introduce the right level of decoupling between snow and atmosphere.
Perspectives for snow modelling
Slide 48