Re: VMRs
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
Re: VMRs
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
Re: VMRs
Hej igen, > Yes, this puts some weight on the user. Hydrostatic equilibrium (HSE) is a > similar case. Input profiles do not always fulfil HSE (this is the case for > Fascod, if not a mater of geopotential vs geometric altitudes?). Could this for Fascod also be due to the VMR definition, perhaps? > 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. /Stefan
Re: VMRs
Hi again, Great that we agree on the problem. OK, let's keep the present definition of VMR (that it refers to sum of all gases, not just "constant" ones). We should then for sure introduce a rescaling method (or maybe several). I expressed myself poorly, I rather meant that introducing such a method is not a fully complete solution, if we consider the "fine print". What I had in mind is the Jacobian, the coupling between variable and constant gases should theoretically go into the expressions for the Jacobian. But that's just a "smart" comment. I don't say that it should be implemented, which would be a pain. Then Stuart's comment is more relevant, this could have consequences for the values given to absorption models. To make the rescaling method easy to apply, I would suggest to make one specific for Earth, that automatically base the rescaling on H2O. There could be a generic one as well. Yes, this puts some weight on the user. Hydrostatic equilibrium (HSE) is a similar case. Input profiles do not always fulfil HSE (this is the case for Fascod, if not a mater of geopotential vs geometric altitudes?). 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. Bye, Patrick On 2021-09-16 15:01, Stefan Buehler wrote: Hej, With our present definition of VMRs, we agree on that having 78% N2, 21% O2 and e.g. 3% H2O is unphysical? That with a lot of H2O (or any other non-fixed gas) the standard values of the fixed gases should be scaled downwards. In the example above, with 0.97. Do you agree? Yes, I agree. It seems a bit weird to me to use this definition at the (low) level of the absorption routines. Perhaps one solutions would be to have an option for this behaviour when ingesting concentration profile data? Perhaps by passing in a list of species that should be considered as not adding to the denominator for the VMR definition. If we agree on the above, then this is the simplest (but not most theoretically correct) solution. Why not correct? /Stefan
Re: VMRs
Hej, > With our present definition of VMRs, we agree on that having 78% N2, 21% O2 > and e.g. 3% H2O is unphysical? That with a lot of H2O (or any other non-fixed > gas) the standard values of the fixed gases should be scaled downwards. In > the example above, with 0.97. Do you agree? Yes, I agree. >> It seems a bit weird to me to use this definition at the (low) level of the >> absorption routines. Perhaps one solutions would be to have an option for >> this behaviour when ingesting concentration profile data? Perhaps by passing >> in a list of species that should be considered as not adding to the >> denominator for the VMR definition. > > If we agree on the above, then this is the simplest (but not most > theoretically correct) solution. Why not correct? /Stefan
Re: VMRs
Hej, No time for writing a lot. Right now just want to make a basic check of our understanding. With our present definition of VMRs, we agree on that having 78% N2, 21% O2 and e.g. 3% H2O is unphysical? That with a lot of H2O (or any other non-fixed gas) the standard values of the fixed gases should be scaled downwards. In the example above, with 0.97. Do you agree? It seems a bit weird to me to use this definition at the (low) level of the absorption routines. Perhaps one solutions would be to have an option for this behaviour when ingesting concentration profile data? Perhaps by passing in a list of species that should be considered as not adding to the denominator for the VMR definition. If we agree on the above, then this is the simplest (but not most theoretically correct) solution. Bye, Patrick Note that for once the special thing about water is here not the fact that it’s condensible, I think, but just that there is so much of it, and at the same time very variable. Other gas species have also very variable concentrations, but it doesn’t matter for the total pressure. All the best, Stefan On 15 Sep 2021, at 20:19, Patrick Eriksson wrote: Stefan, Neither I had considered this definition of VMR. But would it not make sense to follow it? Then a statement that the atmosphere contains 20.95% oxygen makes more sense. You yourself pointed at that it would make sense to scale N2 and O2 for low humid altitudes, where the amount of water can be several %. In code preparing data for ARTS I normally do this adjustment. Should be more correct!? A problem is to define what is the wet species when we go to other planets. Or maybe there are even planets with several wet species? That is, I would be in favour to define VMR with respect to dry air, if we can find a manner to handle other planets. Bye, Patrick On 2021-09-15 18:27, Stefan Buehler wrote: Dear all, Eli Mlawer brought up an interesting point in some other context: we recently had a LBLRTM user get confused on our vmr, which is amount_of_gas / amount_of_dry_air. They weren’t sure that dry air was the denominator instead of total air. I’m too lazy to look at the link above that @Robert Pincus provided, but I hope it is has dry air in the denominator. So much easier to simply specify evenly mixed gases, such as 400 ppm CO2 (and, 20 years from now, 500 ppm CO2). I’ve never considered that one could define it this way. Perhaps this convention explains, why VMRs in climatologies like FASCOD add up so poorly to 1. I’m not suggesting that we change our behaviour, but want to make you aware that this convention is in use. (Or perhaps you already were, and just I missed it.) All the best, Stefan
Re: VMRs
Patrick, I think it doesn't make sense to switch to this in ARTS. In fact, I don't think it makes sense to use it at at all unless you want a fixed low-altitude model. It does make a lot of sense to provide some level of automation to take care of the adjustments for fixed low-altitude models, but it's not that easy elsewhere. First issue. Even on Earth, a constant VMR is not OK at higher altitudes. You have only 20% O2 at 80 km, and less above. So the dry values has to be provided as a profile to make any sense. This is a minor inconvenience. Second issue, is the pressure grid/field going to be the dry pressure? It has to be, otherwise x * P / k T is no longer the number density required for absorption strength. I don't see how you can work around this. Third issue, related strongly to the second issue. This means that pressure broadening calculations can no longer rescale to 1-VMR, and that the VMR profile must be 1 or higher at all altitudes for each of the absorption lines pressure broadening species. This is a deep change, as we can no longer have the meaning of air be "everything required to add up to 1-VMR". It would be a lot of effort to fix this. Lastly, and perhaps a bit too philosophical. Given the same temperature profile, if you add water to the atmosphere, the actual mixing ratio of all the other species goes down. Their number densities or relative pressure are the ones that are supposed to remain somewhat constant. It seems to me therefore that if anything should be changed in ARTS, it's that we shouldn't be using VMR but instead number densities. This would certainly simplify absorption strength calculations, but take quite a bit of effort to fix in broadening calculations. With hope, //Richard Den ons 15 sep. 2021 kl 20:19 skrev Patrick Eriksson < patrick.eriks...@chalmers.se>: > Stefan, > > Neither I had considered this definition of VMR. But would it not make > sense to follow it? Then a statement that the atmosphere contains 20.95% > oxygen makes more sense. You yourself pointed at that it would make > sense to scale N2 and O2 for low humid altitudes, where the amount of > water can be several %. In code preparing data for ARTS I normally do > this adjustment. Should be more correct!? > > A problem is to define what is the wet species when we go to other > planets. Or maybe there are even planets with several wet species? > > That is, I would be in favour to define VMR with respect to dry air, if > we can find a manner to handle other planets. > > Bye, > > Patrick > > > > On 2021-09-15 18:27, Stefan Buehler wrote: > > Dear all, > > > > Eli Mlawer brought up an interesting point in some other context: > > > >> we recently had a LBLRTM user get confused on our vmr, which is > amount_of_gas / amount_of_dry_air. They weren’t sure that dry air was the > denominator instead of total air. I’m too lazy to look at the link above > that @Robert Pincus provided, but I hope it is has dry air in the > denominator. So much easier to simply specify evenly mixed gases, such as > 400 ppm CO2 (and, 20 years from now, 500 ppm CO2). > > > > I’ve never considered that one could define it this way. Perhaps this > convention explains, why VMRs in climatologies like FASCOD add up so poorly > to 1. > > > > I’m not suggesting that we change our behaviour, but want to make you > aware that this convention is in use. (Or perhaps you already were, and > just I missed it.) > > > > All the best, > > > > Stefan > > >
Re: VMRs
Hej Patrick! It seems a bit weird to me to use this definition at the (low) level of the absorption routines. Perhaps one solutions would be to have an option for this behaviour when ingesting concentration profile data? Perhaps by passing in a list of species that should be considered as not adding to the denominator for the VMR definition. Note that for once the special thing about water is here not the fact that it’s condensible, I think, but just that there is so much of it, and at the same time very variable. Other gas species have also very variable concentrations, but it doesn’t matter for the total pressure. All the best, Stefan On 15 Sep 2021, at 20:19, Patrick Eriksson wrote: > Stefan, > > Neither I had considered this definition of VMR. But would it not make sense > to follow it? Then a statement that the atmosphere contains 20.95% oxygen > makes more sense. You yourself pointed at that it would make sense to scale > N2 and O2 for low humid altitudes, where the amount of water can be several > %. In code preparing data for ARTS I normally do this adjustment. Should be > more correct!? > > A problem is to define what is the wet species when we go to other planets. > Or maybe there are even planets with several wet species? > > That is, I would be in favour to define VMR with respect to dry air, if we > can find a manner to handle other planets. > > Bye, > > Patrick > > > > On 2021-09-15 18:27, Stefan Buehler wrote: >> Dear all, >> >> Eli Mlawer brought up an interesting point in some other context: >> >>> we recently had a LBLRTM user get confused on our vmr, which is >>> amount_of_gas / amount_of_dry_air. They weren’t sure that dry air was the >>> denominator instead of total air. I’m too lazy to look at the link above >>> that @Robert Pincus provided, but I hope it is has dry air in the >>> denominator. So much easier to simply specify evenly mixed gases, such as >>> 400 ppm CO2 (and, 20 years from now, 500 ppm CO2). >> >> I’ve never considered that one could define it this way. Perhaps this >> convention explains, why VMRs in climatologies like FASCOD add up so poorly >> to 1. >> >> I’m not suggesting that we change our behaviour, but want to make you aware >> that this convention is in use. (Or perhaps you already were, and just I >> missed it.) >> >> All the best, >> >> Stefan >>
Re: VMRs
Stefan, Neither I had considered this definition of VMR. But would it not make sense to follow it? Then a statement that the atmosphere contains 20.95% oxygen makes more sense. You yourself pointed at that it would make sense to scale N2 and O2 for low humid altitudes, where the amount of water can be several %. In code preparing data for ARTS I normally do this adjustment. Should be more correct!? A problem is to define what is the wet species when we go to other planets. Or maybe there are even planets with several wet species? That is, I would be in favour to define VMR with respect to dry air, if we can find a manner to handle other planets. Bye, Patrick On 2021-09-15 18:27, Stefan Buehler wrote: Dear all, Eli Mlawer brought up an interesting point in some other context: we recently had a LBLRTM user get confused on our vmr, which is amount_of_gas / amount_of_dry_air. They weren’t sure that dry air was the denominator instead of total air. I’m too lazy to look at the link above that @Robert Pincus provided, but I hope it is has dry air in the denominator. So much easier to simply specify evenly mixed gases, such as 400 ppm CO2 (and, 20 years from now, 500 ppm CO2). I’ve never considered that one could define it this way. Perhaps this convention explains, why VMRs in climatologies like FASCOD add up so poorly to 1. I’m not suggesting that we change our behaviour, but want to make you aware that this convention is in use. (Or perhaps you already were, and just I missed it.) All the best, Stefan
VMRs
Dear all, Eli Mlawer brought up an interesting point in some other context: > we recently had a LBLRTM user get confused on our vmr, which is amount_of_gas > / amount_of_dry_air. They weren’t sure that dry air was the denominator > instead of total air. I’m too lazy to look at the link above that @Robert > Pincus provided, but I hope it is has dry air in the denominator. So much > easier to simply specify evenly mixed gases, such as 400 ppm CO2 (and, 20 > years from now, 500 ppm CO2). I’ve never considered that one could define it this way. Perhaps this convention explains, why VMRs in climatologies like FASCOD add up so poorly to 1. I’m not suggesting that we change our behaviour, but want to make you aware that this convention is in use. (Or perhaps you already were, and just I missed it.) All the best, Stefan
