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The ECMWF radiation schemes

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

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The ECMWF radiation schemes

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  1. The ECMWF radiation schemes • A number of radiation schemes are in use at ECMWF. SinceJanuary 2011, have been active • McRad including RRTM_LW and RRTM_SW is used in the forward model for operational 10-day forecasts at TL1279 L91, EPS 15-day forecasts at TL639 L62, and seasonal forecasts at TL159 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)

  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

  3. A quick run through the past fi fi fi

  4. The ECMWF shortwave radiation schemes - 1 • Photon path distribution method originally developed by Fouquart and Bonnel (1980). [see on-line documentation for details] • Vertical integration: • with reflectance at the top and transmittance at the bottom of a layer

  5. 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

  6. 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

  7. The ECMWF shortwave radiation schemes - 3 • Transmission functions for O3, H2O, CO2, N2O, CH4 are fitted with Pade approximants from reference calculations

  8. 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

  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 surface Standard tropical atmosphere: full line = LbL dash line = SW6 Top of the atmosphere

  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

  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 O3 is more properly distributed on the vertical.

  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.

  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

  14. RRTM vs. M91/G00 - 1 The ECMWF LW radiation schemes: RRTM_LW vs. M91/G00 00

  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

  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

  17. 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. NB: This code, with its six spectral intervals is now only used in its adjoint and tangent-linear form in the 4D-Var assimilation

  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 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

  19. RRTM_LW vs. M91/G00: Impact when operationally introduced in 2000 MLS profile

  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

  21. RRTM_LW vs. M91/G00 - 3 Morcrette, 2002, J.Clim., 15, 1875-1892. ARM-NSA 1 SURFRAD station Comparisons over April and May 1999 ARM-TWP2 ARM-TWP1

  22. RRTM_LW vs. M91/G00 - 4 Objective scores: RRTM vs. M91/G00 New system with RRTM Old system with M91/G00

  23. RRTM vs. M91/G00 - 5 RRTM M91/G00

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