The ECMWF radiation schemes

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ECMWF

The ECMWF Radiation Transfer schemes
1



A number of radiation schemes are in use at ECMWF.
Since January 2011, have been
active


McRad

including RRTM_LW and RRTM_SW
is
used in the forward model for
operational 10
-
day forecasts at
T
L
1279
L91, EPS 15
-
day forecasts at
T
L
639
L62, and
seasonal forecasts at T
L
159 L62.


The tangent linear and
adjoint

of the “old” SW radiation scheme in a 2
-
spectral interval
version


The tangent linear and
adjoint

of the “old” LW radiation scheme with 6 spectral intervals,
replacing a
neural network version of the
same “old
” LW radiation
scheme (
Morcrette
,
1991;
Janiskova

and
Morcrette
, 2005)


These last two schemes are used in the assimilation (cf.
P.Lopez’s

presentation in TC
PA module)


… and all the dedicated RT scheme used to simulate radiances (RTTOV
-
based) in the
analysis of satellite data (cf. TC DA module)

The ECMWF radiation schemes

ECMWF

The ECMWF Radiation Transfer schemes
2


The ECMWF radiation schemes


For more details than the following quick “run through”, look at


http://www.ecmwf.int/research/ifsdocs/CY38r1/index.html


Part IV. Physical Processes


Chapter 2: Radiation




ECMWF

The ECMWF Radiation Transfer schemes
3


fi

fi

fi

A quick run through the past

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The ECMWF Radiation Transfer schemes
4



Photon path distribution method originally developed by Fouquart and Bonnel
(1980).
[see on
-
line documentation for details]



Vertical integration:





with

The ECMWF shortwave radiation schemes
-

1

reflectance at the top and transmittance at the bottom of a layer

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The ECMWF Radiation Transfer schemes
5



Delta
-
Eddington method (Shettle and Weinman, 1970; Joseph et al., 1976) to
compute from the total optical thickness , single scattering
albedo , and asymmetry factor g, which account for the combined effect
of cloud condensed water, aerosol, and molecular absorption






The ECMWF shortwave radiation schemes
-

2

ECMWF

The ECMWF Radiation Transfer schemes
6



Laplace transform method to get the photon path equivalent gaseous absorber
amounts from 2 sets of layer reflectances and transmittances, assuming
successively a non
-
reflecting underlying medium ( ) then a reflecting
one ( )





where are the layer reflectance and transmittance corresponding
to a conservative scattering medium and k
e

is an absorption coefficient
approximating the spectrally averaged transmission of the clear
-
sky
atmosphere

The ECMWF shortwave radiation schemes
-

2

ECMWF

The ECMWF Radiation Transfer schemes
7


The ECMWF shortwave radiation schemes
-

3


Transmission functions for O
3
, H
2
O, CO
2
, N
2
O, CH
4

are fitted with Pade
approximants from reference calculations


ECMWF

The ECMWF Radiation Transfer schemes
8


SW6 vs. SW4


6 spectral intervals from 0.185 to 4

m


Based on a line
-
by
-
line model of the
transmission functions


LbL based on STRANSAC
(Scott, 1974, Dubuisson et al., 1996)


modified to account for HITRAN
2000


H
2
O, CO
2
, O
3
, O
2
, CH
4
, CO, N
2
O


resolution 0.01 cm
-
1

from 2000 to
20000 cm
-
1
, then resolution of
the O3 continuum, i.e. 5 to 10
cm
-
1


UV
CBA

in 2 intervals, 0.185
-
0.25
-
0.4

m, visible in 1 interval, 0.4
-
0.69

m



4 spectral intervals from 0.25 to 4

m

Based on statistical models of the
transmission functions

UV
BA

and visible in one interval from

0.25 to 0.69

m

ECMWF

The ECMWF Radiation Transfer schemes
9


The 6
-
interval SW radiation scheme
-

2



Comparison with a line
-
by
-
line
model of the SW radiation transfer
on standard cases shows an
excellent agreement on the flux
profiles

Standard tropical
atmosphere:


full line = LbL

dash line = SW6

surface

Top of the atmosphere

ECMWF

The ECMWF Radiation Transfer schemes
10


The 6
-
interval SW radiation scheme
-

3


The new SW scheme

SW6 is compared to

the old SW4, and to

results obtained from

a different scheme

linked to a different

line
-
by
-
line model,

RRTM


Differences in

tropospheric SW

heating rates:


A small impact is seen
in the troposphere,
related to a water

vapour absorption
including both


a p
-

and e
-
type
absorption



ECMWF

The ECMWF Radiation Transfer schemes
11


The 6
-
interval SW radiation scheme
-

4


Differences in

stratospheric SW

heating rates



The main impact of a
better representation
of the gaseous
absorption is found in
the stratosphere,
where the heating by
O
3

is more properly

distributed on the
vertical.


ECMWF

The ECMWF Radiation Transfer schemes
12


The 6
-
interval SW radiation scheme
-

5

In these 1
-
D calculations, whatever the state of the atmosphere, clear
-
sky,

overcast, or mixed, the surface downward flux from SW6 is always smaller

than the one from SW4.

ECMWF

The ECMWF Radiation Transfer schemes
13


The 6
-
interval SW radiation scheme
-

6

Within the ECMWF forecast model,

the effect of the new SW scheme is

felt at the surface where it decreases the
SW radiation available at the surface.


In terms of temperature field, the effect is
almost exclusively in the

stratosphere, where it improves the
agreement with
climatologies
:


270 K and more at the
stratopause

around
1
hPa

NB: This code, with only 2 spectral
intervals (UV
-
Vis + NIR) is now only
used in its
adjoint

and tangent
-
linear
version in the 4D
-
Var assimilation

ECMWF

The ECMWF Radiation Transfer schemes
14


RRTM vs. M91/G00
-

1

The ECMWF LW radiation schemes:

RRTM_LW vs. M91/G00

00

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The ECMWF Radiation Transfer schemes
15


M91/G00
Morcrette, 1991, JGR, 96D, 9121
-
9132


Gregory et al., 2000, QJRMS, 126A, 1685
-
1710.


Band
-
emissivity

type of scheme, i.e., solves for a (N+1)
2

matrix of
transmission functions







Six spectral intervals


0
-
350 + 1450
-
1680 cm
-
1

970
-
1110 cm
-
1


500
-
800 cm
-
1

350
-
500 cm
-
1


800
-
970 cm
-
1

1250
-
1450 + 1880
-
2820 cm
-
1


mixed vertical quadrature:


2
-
point Gaussian for layers adjacent to level of computation


trapezoidal rule for distant layers




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The ECMWF Radiation Transfer schemes
16


M91/G00
-

2


Transmission functions

represented by Pade approximants from
transmission functions computed with Malkmus and Goody statistical
models







with the effective absorber amount







Diffusivity factor

Pressure
-
weighted

amount of absorber

ECMWF

The ECMWF Radiation Transfer schemes
17


M91/G00
-

3


Effective cloudiness




k
abs,liq

from Smith and Shi (1992), k
abs,ice

from Ebert and Curry
(1992)




Effect of clouds on LW fluxes following Washington and Williamson
(1977). Formulation allows for maximum,
maximum
-
random
, or random
cloud overlap.

NB: This code, with its six spectral intervals is now only used in its
adjoint

and
tangent
-
linear form in the 4D
-
Var assimilation


ECMWF

The ECMWF Radiation Transfer schemes
18


RRTM_LW
Mlawer et al., 1997: JGR, 102D, 16663
-
16682


Morcrette et al., 1998: ECMWF Tech.Memo., 252


The use of the correlated
-
k method (mapping k
-
> g) allows radiative transfer
to be performed as a monochromatic process

R
o

is the radiance incoming to the layer,

B(

,T) the Planck function at wavenumber


and temperature T

t


is the transmittance for the layer optical path

t’


the transmittance at a point along the layer optical path

Discretized over j

(k, k+

k) intervals of

width W
j


ECMWF

The ECMWF Radiation Transfer schemes
19


RRTM_LW vs.
M91/G00:

Impact when operationally introduced in 2000


MLS profile

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The ECMWF Radiation Transfer schemes
20


RRTM_LW vs. M91/G00
-

2

Morcrette et al., 2001, ECMWF Newsletter, 91, 2
-
9.


Due to the increased LW absorption, RRTM provides smaller OLR and
larger surface downward LW radiation

For clear
-
sky situations

For overcast low
-


and high
-
level

cloudiness

ECMWF

The ECMWF Radiation Transfer schemes
21


RRTM_LW vs. M91/G00
-

3
Morcrette
, 2002,
J.Clim
.,
15, 1875
-
1892.

Comparisons over April and May 1999

ARM
-
NSA

1 SURFRAD station

ARM
-
TWP1

ARM
-
TWP2

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The ECMWF Radiation Transfer schemes
22


RRTM_LW vs. M91/G00
-

4

Objective scores:
RRTM

vs.
M91/G00

New system with RRTM

Old system with M91/G00

ECMWF

The ECMWF Radiation Transfer schemes
23


RRTM vs. M91/G00
-

5


M91/G00

RRTM