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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.

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the ecmwf shortwave radiation schemes 1
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

the ecmwf shortwave radiation schemes 2
The ECMWF shortwave radiation 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 21
The ECMWF shortwave radiation schemes - 2
  • 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 3
The ECMWF shortwave radiation schemes - 3
  • Transmission functions for O3, H2O, CO2, N2O, CH4 are fitted with Pade approximants from reference calculations
sw6 vs sw4
SW6 vs. SW4
  • 6 spectral intervals from 0.185 to 4 mm
  • 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 mm, visible in 1 interval, 0.4-0.69 mm

4 spectral intervals from 0.25 to 4 mm

Based on statistical models of the transmission functions

UVBA and visible in one interval from

0.25 to 0.69 mm

the new sw radiation scheme 2
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

surface

Standard tropical atmosphere:

full line = LbL

dash line = SW6

Top of the atmosphere

the new sw radiation scheme 3
The new 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

the new sw radiation scheme 4
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 properly

distributed on the vertical.

the new sw radiation scheme 5
The new 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.

the new sw radiation scheme 6
The new 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

rrtm vs m91 g00 1
RRTM vs. M91/G00 - 1

The ECMWF LW radiation schemes:

RRTM_LW vs. M91/G00

00

m91 g00 morcrette 1991 jgr 96d 9121 9132 gregory et al 2000 qjrms 126a 1685 1710
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
m91 g00 2
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

m91 g00 3
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.
rrtm lw mlawer et al 1997 jgr 102d 16663 16682 morcrette et al 1998 ecmwf tech memo 252
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

Ro is the radiance incoming to the layer,

B(n,T) the Planck function at wavenumber n and temperature T

tn is the transmittance for the layer optical path

t’n the transmittance at a point along the layer optical path

Discretized over j

(k, k+Dk) intervals of

width Wj

rrtm lw vs m91 g00 2 morcrette et al 2001 ecmwf newsletter 91 2 9
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

rrtm vs m91 g00 3
RRTM vs. M91/G00 - 3

Differences in OLR: RRTM-M91/G00

OLR derived from AVHRR from April 99

OLR from ECMWF model with RRTM

rrtm lw vs m91 g00 4 morcrette 2002 j clim 15 1875 1892
RRTM_LW vs. M91/G00 - 4 Morcrette, 2002, J.Clim., 15, 1875-1892.

ARM-NSA

1 SURFRAD station

Comparisons over April and May 1999

ARM-TWP2

ARM-TWP1

rrtm lw vs m91 g00 5
RRTM_LW vs. M91/G00 - 5

Objective scores: RRTM vs. M91/G00

New system with RRTM

Old system with M91/G00