torsdag 20 maj 2010
Computation of Atmospheric Lapse Rate
To compute the lapse rate (temperature drop with height z) of an atmosphere, we start from the 2nd Law of Thermodynamics in the form presented in Computational Thermodynamics:
c_v dT + P dV non-negative
where c_v is specific heat under constant volume, which combined with the differentiated form PdV + V dP = R dT of the gas law PV =RT gives
(c_v+R) dT - V dP non-negative.
Vertical hydrostatic balance is expressed by dP = - g Rho dz with Rho = 1/V density, from which follows that
dT/dz + g/C_p non-negative,
where C_p = (c_v+R)/V Rho = (c_v+R)/mass. The lapse rate is thus bounded below by - g/C_p, which for a dry Earth atmosphere equals - 10 C/km.
The observed lapse rate is dT/dz = - 6 C/km, which is partly (because there is also
evaporation/condensation) an effect of turbulent dissipation with c_v dT + P dV strictly positive: Rising hot air expands under temperature drop which is counterbalanced by turbulent dissipation reducing the drop.
The Joule experiment discussed in the previous post, concerns precisely this effect. We have shown that we can simulate this effect by computing turbulent soutions of the Navier-Stokes equations, and thus it should be possible to similarly compute the effective lapse rate in the
In the ideal case of isentropic expansion the 2nd Law is satisfied with equality, from which follows
c_v dT + R T dV/V = 0.
T V^a = constant
where a = R/c_v, which determines P and V (or Rho) as functions of z through T(z).