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Improvements for the computation of Cp, viscosity and dust mixing ratio in MCD v4.3.EXM.5

Improvements for the computation of Cp, viscosity and dust mixing ratio in MCD v4.3.EXM.5. E. Millour, F. Forget April 6, 2011 LMD/TAS-I Progress Meeting. Specific heat Cp.

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Improvements for the computation of Cp, viscosity and dust mixing ratio in MCD v4.3.EXM.5

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  1. Improvements for the computation of Cp, viscosity and dust mixing ratio in MCD v4.3.EXM.5 E. Millour, F. Forget April 6, 2011 LMD/TAS-I Progress Meeting

  2. Specific heat Cp • Until now, Cp was computed from the contribution (weighed by molar fraction yi ) of main constituents (CO2,CO,O,N2 and Ar; assuming yAr=0.6yN2): Cp = i yi Cpi • We now, in addition, take into account the fact that Cp of CO2 depends on temperature (Lebonnois et al. 2010): CpCO2(T) = 1000 MCO2 (T/460)0.35 with MCO2 = 44.01 10-3 kg/mol ; T in Kelvins

  3. Illustrative example of impact of improvement Large temperature variations from surface to ~200km Significant compositional variations from ~120km to the top of the atmosphere

  4. viscosity • The evaluation of the viscosity of the atmosphere has been rewritten to better account for the combined effect of temperature and composition dependencies: • At a given temperature, the viscosity of the mixture  is computed from the viscosities of its components i (Herning and Zipperer method): • Where the temperature dependence of the i are taken into account: yi : Molar fraction Mi : molar mass CO2: μCO2 = 1.572.10-6 T3/2 / (T + 240) “Standard” Sutherland Formula CO: μCO = 1.428.10-6 T3/2 / (T + 118) “Standard” Sutherland Formula N2: μN2 = 1.407.10-6 T3/2 / (T + 111) “Standard” Sutherland Formula Ar: μAr = 7.592.10-6 (T/90)0.883Van Itterbeek and Van Paemel, 1938 O: μO = 3.34.10-7 T0.71 Dalgarno and Smith, 1962

  5. Illustrative example of impact of improvement

  6. Improved dust mixing ratio Reference equation to compute Mass Mixing Ratio in a well mixed atmospheric layer of thickness Δp and opacity τ : • MMR = (4/3) (g ρD reff τ) / (Qext Δp ) (kg/kg) • With the following assumption in the MCD - g=3.72 m/s2 - D~2500 kg/m3 (Dust mass density. The possible range is 1550 - 3300 kg/m3) • reff : Dust Effective radius (see below) • Qext : Extinction coefficient at reference wavelength =0.67µm (see below) 2 estimation of MMR are now available (assuming constant effective radius (overestimation:extvar(37) ) or a decreasing radius with altitude (underestimation extvar(51) )

  7. Estimation of dust effective radius • Previous versions of the MCD used reff = 1.8 µm • In MCD v4.3.EXM.5 we now use • reff = 1.65 µm (as shown by Wolff et al. [2006,2009]) if we assume a constant dust effective radius (MMR= extvar(37) in call_mcd.F) • reffvarying with pressure if we assume a varying dust effective radius

  8. Qext Extinction coefficient at reference wavelength ref • Former MCD use Qext = 3.0 at ref = 0.67 µm • In MCD v4.3.EXM.5 we now use • Qext = 2.4 at ref = 0.67 µm (as shown by Wolff et al. [2006,2009]) if we assume a constant dust effective radius • If we assume a variable dust effective radius, we take Qext = 2.4 for reff > 0.235 µm, and Qext = (0.07 +5 reff )4 for reff < 0.235µm to account for the decrease of extinction for small particles in the Rayleigh scattering regime.

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