There is a provocative new paper available at an online discussion journal:
Makarieva, Gorshkov, Sheil, Nobre, Li: Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics. link
See discussion at the Air Vent (here and here)
I first became aware of Makarieva’s research about a year ago, I encountered her at a blog (probably climateaudit) and suggested that she send me copies of her papers. My curiosity was struck initially by her hurricane papers. We began an extensive e-dialogue about her work, and I started to dig deeply into her hurricane papers. And then climategate struck, and my attention unfortunately became diverted. I am delighted to take this opportunity to revisit her work and discuss her new paper at Climate Etc. Anastassia Makarieva is one of the people that I have invited to host a thread at Climate Etc.
In my opinion, the most significant characteristic of an important paper is that it changes the way you think about a problem. This qualifies as an important paper, by my standards. While the paper is controversial, it has potentially far reaching implications for our understanding of how the climate system works.
Summary: This paper presents a theory as to how condensation influences atmospheric pressure through the mass removal of water from the gas phase with a simultaneous account of the latent heat release. The mechanism described by Makarieva et al. is correct, and it is not currently included in climate models. It is not clear to what extent this mechanism “matters;” their thermodynamic analysis is insufficient to demonstrate the relative magnitude of this effect. Nevertheless, this mechanism raises important issues regarding the structural adequacy of the atmospheric dynamical core used in climate models. Given the overwhelming importance of water vapor and cloud feedbacks in climate model simulations, re-examination of the atmospheric dynamical core used in climate models is called for.
Note: the summary paragraph will be revised based upon the discussion, as per TomFP’s suggestion.
“Abstract. Phase transitions of atmospheric water play a ubiquitous role in the Earth’s climate system, but their direct impact on atmospheric dynamics has escaped wide attention. Here we examine and advance a theory as to how condensation influences atmospheric pressure through the mass removal of water from the gas phase with a simultaneous account of the latent heat release. Building from fundamental physical principles we show that condensation is associated with a decline in air pressure in the lower atmosphere. This decline occurs up to a certain height, which ranges from 3 to 4 km for surface temperatures from 10 to 30 °C. We then estimate the horizontal pressure differences associated with water vapor condensation and find that these are comparable in magnitude with the pressure differences driving observed circulation patterns. The water vapor delivered to the atmosphere via evaporation represents a store of potential energy available to accelerate air and thus drive winds. Our estimates suggest that the global mean power at which this potential energy is released by condensation is around one per cent of the global solar power – this is similar to the known stationary dissipative power of general atmospheric circulation. We conclude that condensation and evaporation merit attention as major, if previously overlooked, factors in driving atmospheric dynamics.”
Review
The paper makes its arguments using the first and second laws of thermodynamics, conservation of mass (in the form of the mass continuity and scalar continuity equations), and the hydrostatic equations (my text Thermodynamics of Atmospheres and Oceans explains the basics behind these equations as applied to the atmosphere). Their analysis does not include the Navier-Stokes equations or a dynamical (time dependent model). Rather they calculate mechanical work derived from the atmospheric pressure gradient induced by condensation processes.
The basic points made in this paper are valid, and the mechanism they have identified is a real one. There is one point where I disagree with the authors, and that is on evaporation vs. condensation. They identify “salient differences” between them which in fact do not exist. Evaporation is not a surface specific process. When a cloud forms in the atmosphere, the condensed water has one of two fates: fallout in the form of precipitation or evaporation. The precipitation efficiency of clouds is rather low, much less than 10%. So most of the condensed water in the atmosphere eventually evaporates in the atmosphere. But I don’t see that this has much impact on their overall argument.
It is not clear to what extent their mechanism “matters;” their thermodynamic analysis is insufficient to demonstrate the relative magnitude of this effect. They provide some back-of-the-envelope estimates for the Hadley cell and hurricanes. Investigation of whether this matters requires a more comprehensive scale analysis that includes the Navier-Stokes equations and model simulations to test these ideas. The time and space scales of the adjustment to the small mass disturbances engendered by condensation, and how the adjustments occur in the vertical or horizontal direction, can only be determined in the context of a simulation.
I think that this mechanism might have some effect on storms (e.g. hurricanes and extratropical cyclones) where evaporation and precipitation are large. In terms of climate and larger scale circulations, I think the biggest effects would be in areas where there are large gradients of evaporation minus precipitation (see here). In Dec-Feb, large gradients are seen near the Atlantic coasts of South America and southern Africa, and between tropical and subtropical ocean regions. In June-Aug, large gradients are seen in the Australasian monsoon region, north-south gradients on the South America and African continents, and between the tropical and subtropical ocean regions. Climate models do not do a good job with monsoon circulations. Weather models do not do a good job with hurricane intensity. It is worth testing to see if their mechanism could possibly provide at least a partial explanation for these deficiencies.
Implications
The reason this research resonates with me is that I have long been concerned about the fitness for the climate task of the atmospheric dynamical cores designed for weather models. Yes, the use of the weather model cores provides confidence that they climate models can simulate weather systems. But what about processes involved in water vapor and cloud feedback, the big gorillas in driving the sensitivity to greenhouse warming?
So are climate models wrong? Well, there are a number of simplifications made in the dynamical core of the atmospheric models. These simplifications derive from the history of numerical weather prediction, and have arisen variously from considerations of numerical stability and computational efficiency to simplifications introduced over 50 years ago into moist thermodynamic equations for ease in deriving analytical solutions. While continuing the use the same atmospheric dynamical core used in weather models, climate models have evolved to have far more sophisticated radiative transfer and cloud parameterizations and most climate models now have prognostic equations for cloud water content.
While the current atmospheric dynamical cores provide mostly sensible simulations of the broad features of the atmospheric circulation, there are some nagging concerns. Is it possible that the impacts of all these approximations accumulate in the tropical upper troposphere, where there is some large discrepancies between the models and observations?
In What can we learn from climate models?, I quoted this statement from Thuburn (2008), which directly targets the issue raised by Makarieva et al.:
“Moist processes are strongly nonlinear and are likely to be particularly sensitive to imperfections in conservation of water. Thus there is a very strong argument for requiring a dynamical core to conserve mass of air, water, and long-lived tracers, particularly for climate simulation. Currently most if not all atmospheric models fail to make proper allowance for the change in mass of an air parcel when water vapour condenses and precipitates out. . . However, the approximation will not lead to a systematic long term drift in the atmospheric mass in climate simulations provided there is no long term drift in the mean water content of the atmosphere.”
In thinking about how the atmospheric dynamical core could be improved to address processes associated with water vapor and clouds, I am wondering whether climate models need to be nonhydrostatic and also might need to account for multicomponent fluids and multiphase flows, where moist air is treated as a binary fluid. Staniforth et al. describe the new nonhydrostatic formulation for the weather and climate models at the UK Met Office. The only application of multiphase fluid dynamics I have seen for the atmosphere is a paper by Bannon (note there is controversy over eq 5.16 in this paper). This subsequent paper by Bannon has ideas whereby the mass conservation can be incorporated without a full multiphase treatment. Note, I am not an expert on the nuances of the atmospheric dynamical cores, I am throwing these issues out for consideration.
The model that comes closest to addressing these issues is the WRF model (which is not a climate model). A relatively efficient way to proceed would be to work with WRF model to assess the impact of neglecting these various terms. Simulations of the impact of precipitation mass sink on hurricane intensity have been investigated using WRF, showing a modest but non-negligible impact (this is not published to my knowledge).
Now on the topic of thermodynamics, there are a whole host of mostly unnecessary simplifications that have been introduced that are probably pretty irrelevant on weather time scales but probably relevant on climate time scales. A topic for another post, but as an example, the heat capacity of condensed water is commonly ignored; this is ok in an analytical/theoretical analysis such as Makarieva et al., but not for a climate model.
Makarieva et al. also raise the issue of the nature of interconversions among kinetic energy, potential energy, and internal energy. To anyone that has a copy of my textbook, the relevant discussion is in section 12.2. I will do a post on this at some point, there are some issues that have been nagging me about how all this is treated, and I want to dig into these a bit more.
Addendum: the “community” response
If you go to Makarieva’s web site, you will see that she has published several papers on this general topic in journals that include the prestigious Proceedings of the Royal Society and Physics Letters. However, she has not succeeded in getting her work published in atmospheric science journals.
Her previous paper submitted to ACPD received a huge number of comments (by the standards of the journal), and the paper was eventually rejected. The discussion is well worth reading.
The previous paper received some discussion on the tropical listserv. Some people dismissed it since Emanuel’s MPI theory seems to work just fine. Others had misconceptions about what the paper said. A few people seemed quite interested.
Bottom line: it is challenging for an “outsider” to get a paper published that poses a major challenge to the status quo. Insiders are less likely to challenge the status quo, so outside challenges should be welcomed and considered carefully. Part of the challenge is for the outsider to spin up in the “culture” of the field and cite the relevant literature and use terminology and notation that is familiar to the target audience. And not to overstate the case. I think the present paper will have an easier time in the review process than its predecessor at ACPD, we shall see.
And it is important for “insiders” to engage with the “outsiders.” I understand that Makarieva has contacted a number of climate scientists for feedback on her papers; a few have helpfully replied. I guess my interaction with Makarieva “counts” as interacting with a skeptic, since I encountered her on a skeptical blog and she is challenging the status quo. If this is the kind of thing that I shouldn’t be doing according to the IPCC “in crowd” (see here), then the climate field is in a great deal of trouble. I personally think that we are very fortunate to have Anastassia Makarieva (an esteemed Russian biophysicist) and her colleagues taking a look at some of the fundamental assumptions made in our field. Whether all this turns out to be important or not, I don’t know. But I do know that it is important for us to try to figure this out.