You are right, the cross dependence would still be there, and come through the dry pressure, which gets smaller when there is more water vapor.
Overall, I also still like the option to rescale the VMRs better. /Stefan On 16 Sep 2021, at 21:34, Patrick Eriksson wrote: > Stefan, > >>> For HSE it is up to the user to apply this "fine tuning" or not. This >>> including to include adding call of the HSE method in OEM iterations, to >>> make sure that HSE is maintained after an iteration. The VMR rescaling >>> should also be included in the iteration agenda, if the retrieval can >>> change H2O close to the ground. That is, a VMR rescaling would not be >>> something completely new, as I see it. >> >> It seems to me that this leads into a logical loop: If you retrieve H2O and >> O3, and the retrieved H2O value directly affects the O3 value due to the >> rescaling. As you write, in principle, this should even be in the Jacobian, >> as a cross-term. With more water, the lines of all other gases get weaker. >> >> It is true that if there is more of the one there has to be less of the >> other, but argh, this is so ugly. >> >> Perhaps the deeper reason why AER went for the other definition? If VMRs >> refer to the dry pressure, and the dry gases are all either quite constant >> or very rare, then retrievals are more independent. > > To switch to the other definition, than the VMR of e.g. N2 would stay the > same in a retrieval of H2O. This is why I initially found this option nice. > But it would not change the physics and the cross-dependences between species > would not disappear. You have to remember that VMR is a relative measure. To > get the absolute amount of the species, you still need to calculate the > partial pressures. That is you need to "distribute" the total pressure among > the gases, and as I understand it a general expression for this would be: > > p_i = VMR_i * p / VMR_sum > > where p_i is partial pressure of species i, VMR_i its VMR, p pressure and > VMR_sum the sum of all VMRs. > > Our present definition is based on that VMR_sum=1, while in the alternative > version it will deviate, and with more H2O VMR_sum will increase which will > affect p_i even if VMR_i is unchanged. > > Or do I miss something? > > Bye, > > Patrick
