the ecmwf shortwave radiation schemes - 1
DESCRIPTION
The ECMWF shortwave radiation schemes - 1. Photon path distribution method originally developed by Fouquart and Bonnel (1980). [see lecture notes for full details] Vertical integration: with. reflectance at the top and transmittance at the bottom of a layer. - PowerPoint PPT PresentationTRANSCRIPT
ECMWFThe ECMWF Radiation Transfer schemes 1
Photon path distribution method originally developed by Fouquart and Bonnel
(1980). [see lecture notes for full details]
Vertical integration:
with
The ECMWF shortwave radiation schemes - 1
)1()()(
)()( 0
jjFjF
kFjF
top
N
jk
bot
downup
down
clearcldcldcldbot
clearcldcldcldtop
CC
CC
)1(
)1(
, reflectance at the top and transmittance at the bottom of a layer
ECMWFThe ECMWF Radiation Transfer schemes 2
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
,
gac gac
ac
ac
aacc ggg
ECMWFThe ECMWF Radiation Transfer schemes 3
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 ke is an absorption coefficient
approximating the spectrally averaged transmission of the clear-sky
atmosphere
The ECMWF shortwave radiation schemes - 2
0
0
etopctopeetopctope
ebotcboteebotcbote
kuku
kuku
/)/ln( /)/ln(
/)/ln( /)/ln(
00
00
botctopc ,
)]/(ln[ /
10
0
tot
tote utu
k
ECMWFThe ECMWF Radiation Transfer schemes 4
The ECMWF shortwave radiation schemes - 3
Transmission functions for O3, H2O, CO2, N2O, CH4 are fitted with Pade
approximants from reference calculations
ECMWFThe ECMWF Radiation Transfer schemes 5
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
H2O, CO2, O3, O2, CH4, CO, N2O
resolution 0.01 cm-1 from 2000 to 20000 cm-1, then resolution of the O3 continuum, i.e. 5 to 10 cm-1
UVCBA 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
UVBA and visible in one interval from 0.25 to 0.69 m
ECMWFThe ECMWF Radiation Transfer schemes 6
The new 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 = LbLdash line = SW6
surface
Top of the atmosphere
ECMWFThe ECMWF Radiation Transfer schemes 7
The new SW radiation scheme - 3The new SW schemeSW6 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
ECMWFThe ECMWF Radiation Transfer schemes 8
The new 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 O3 is more properlydistributed on the vertical.
ECMWFThe ECMWF Radiation Transfer schemes 9
The new SW radiation scheme - 5
Station Total daytime clsky tot clsky day ovcst tot ovcst day
Bondville SW4 196.8 357.9 291.2 619.5 114.9 177.6SW6 192.4 349.8 285.0 606.5 112.8 174.4
Desert Rock SW4 266.4 499.3 304.4 608.8 159.5 310.3SW6 262.0 484.9 299.3 598.6 157.3 306.0
Fort Peck SW4 223.6 395.6 318.2 495.5 179.3 294.1SW6 219.0 387.5 311.5 485.0 176.6 289.8
Penn State U SW4 208.0 379.1 313.4 520.2 94.5 128.7SW6 203.9 371.6 307.5 510.4 93.2 126.9
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.
ECMWFThe ECMWF Radiation Transfer schemes 10
The new SW radiation scheme - 6
Within the ECMWF forecast model, the effect of the new SW scheme isfelt 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
ECMWFThe ECMWF Radiation Transfer schemes 11
RRTM vs. M91/G00 - 1The ECMWF LW radiation schemes: RRTM_LW vs. M91/G00
00
ECMWFThe ECMWF Radiation Transfer schemes 12
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 intervals0-350 + 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
p
pp
kdBksurfBkoksurfk
surf
k dBrpptpTBpptTBTBpF'
),',())(( ),()]()([)(
0
'
),',())(( )0,()]()([)(pp
kdBkBkktopk dBrpptpTBptTBTBpF k
ECMWFThe ECMWF Radiation Transfer schemes 13
M91/G00 - 2
Transmission functions represented by Pade approximants from transmission functions computed with Malkmus and Goody statistical models
with the effective absorber amount
2
0
2
2
0
2
2
1
2
1
)(
),()(
),(
j
i
j
i
i
iu
u
eff
eff
ud
uc
dTB
dTputTB
Tput
),( puTpuru ueff
])250( )( )250( )(exp[),( 2 uuu TpubTpuapuT
Diffusivity factor 2
1
p
p
dpuppuPressure-weighted amount of absorber
ECMWFThe ECMWF Radiation Transfer schemes 14
M91/G00 - 3
Effective cloudiness
kabs,liq from Smith and Shi (1992), kabs,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.
)] exp(1[** , , iceiceabsliqliqabscldeff ukukCFCFCF
ECMWFThe ECMWF Radiation Transfer schemes 15
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
2
1
1
12')())'(,()(
121
t
oo dtRtTBRdvv
R
2
1
21 )),,(exp()],()([),(1
012
aTpvktBRtBdvv
R effeff
Ro is the radiance incoming to the layer, B(,T) the Planck function at wavenumber and temperature Tt is the transmittance for the layer optical patht’ the transmittance at a point along the layer optical path
j
jjeffojjeffj aBRBWR )exp( ,,21 Discretized over j (k, k+k) intervals of width Wj
ECMWFThe ECMWF Radiation Transfer schemes 16
RRTM_LW vs. M91/G00 - 1
MLS profile
ECMWFThe ECMWF Radiation Transfer schemes 17
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
TRO MLS MLW SAS SAWOLR RRTM 286.2 276.2 226.6 257.7 196.1
M91/G00 291.0 281.8 230 264.3 198.5
SDLW RRTM 399.3 350.5 224.3 301.1 173.9M91/G00 390.9 344.4 220.4 296.1 170.9
TRO MLS MLW SAS SAW90% low cloudOLR RRTM 275 267.8 220.5 249.7 197.7
M91/G00 277.1 269.2 221.5 250.6 197.9SDLW RRTM 440.2 406.3 293.5 363.4 240.4
M91/G00 438.6 405.4 290 362.9 223.6
90% high cloudOLR RRTM 279.8 265.9 218.4 240.4 190.7
M91/G00 286.3 277.2 226.5 256.6 196.1SDLW RRTM 399.6 351.2 226.3 304.1 176.2
M91/G00 391.2 344.8 221.2 297.4 171.9
For clear-sky situations
For overcast low- and high-level cloudiness
ECMWFThe ECMWF Radiation Transfer schemes 18
RRTM vs. M91/G00 - 3
OLR derived from AVHRR from April 99
OLR from ECMWF model with RRTM
Differences in OLR: RRTM-M91/G00
ECMWFThe ECMWF Radiation Transfer schemes 19
RRTM_LW vs. M91/G00 - 4 Morcrette, 2002, J.Clim., 15, 1875-1892.
Comparisons over April and May 1999
ARM-NSA 1 SURFRAD station
ARM-TWP1 ARM-TWP2
ECMWFThe ECMWF Radiation Transfer schemes 20
RRTM_LW vs. M91/G00 - 5 Objective scores: RRTM vs. M91/G00
New system with RRTM
Old system with M91/G00
ECMWFThe ECMWF Radiation Transfer schemes 21
RRTM vs. M91/G00 - 6M91/G00 RRTM