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Reply to M&Gs reaction

Last modified October 16, 2009 15:17

This is a reply to the text “The biotic pump physics is maturing to a novel theory of atmospheric circulation: Everybody’s invited!” by Makarieva and Gorshkov, published on 13-10-2009 on this site, in reaction to “A ‘Forests as biotic pump’ hypothesis discredited due to errors in basic atmospheric physics” by Stigter and Meesters, published on 8-10-2009.

Antoon Meesters

 

Introduction

This is a reply to the text “The biotic pump physics is maturing to a novel theory of atmospheric circulation: Everybody’s invited!” by Makarieva and Gorshkov, published on 13-10-2009 on this site, in reaction to “A ‘Forests as biotic pump’ hypothesis discredited due to errors in basic atmospheric physics” by Stigter and Meesters, published on 8-10-2009. The text of Makarieva and Gorshkov will be called here “Invitation” for convenience.

[The INSAM website is in my opinion not the proper place for lengthy discussion about the “Biotic Pump Theory”. I write this only because I have to reply to the remarks by Makarieva and Gorshkov. My reply has to start with a brief survey of the preceding discussion for newcomers, which survey will be restricted to the physical foundations.]

The Biotic Pump Theory (BPT) was published in its original form in MG2007, and the Comment by Meesters et al. (MDB2009) was directed to this. The same theory but with a somewhat different application focus occurs in MG2009a. A Reply to the Comment was given in MG2009b. This original theory infers from the vertical profile of water vapor (with necessarily a partial pressure that rapidly decreases with increasing height) the existence of an “Evaporative Force” which causes upward motion. It is subsequently concluded that the evaporative force is strongest for the columns with the most vigorous evaporation and condensation, so that close to the surface, those columns must suck air from the surroundings. The contribution by Stigter and Meesters refers to this old exposition of the theory.

The BPT was also included in section 4 of a submitted but rejected paper for Atmospheric Chemistry and Physics. The very extensive discussion about this paper (which also dealt with other subjects) has been published on a website (MGL2008). In the course of time, the way in which the BPT is explained has been changing substantially (“maturing” according to MG), which sometimes complicates the discussion. New points are the immediate effect of condensation (other than the obvious effect on the vapor profile), first emphasized in MGL2008, and the weights of air columns as such, first emphasized to my knowledge in the “Invitation”. On the other hand, there is one fundamental aspect in the BPT that has always been emphasized: the fixation on changes in the water vapor component of the air, neglecting all other effects. This in spite of the critique of MDB2009.

 

Replies to accusations

Let us treat first the accusations in the third paragraph of the “Invitation”: “The central point in the biotic pump theory is that condensation creates a local drop .... initiated condensation”. For newcomers: The world “local” means that this concerns the pressure in the cloud, not at the surface, and the whole point is rather technical. We fully agree that a temperature drop (which occurs as a parcel travels upward) causes a lower saturation pressure of the water vapor. But MG miss the point: our remark about pressure rise concerns the local pressure not of the water component but of the wet air as a whole (and compared to dry air at the same height), which is something very different for the following reason (already explained e.g. in section 4 of the comment MDB2009, see also the discussion about MGL2008).

As vapor condensates in ascending air, latent heat is released which mitigates the temperature drop, so that the wet parcels become in general warmer than dry parcels at the same height. The heat causes the wet parcels first to have a pressure surplus (this is what we mean) and then to expand, pushing away the dry air at the same height. This is counteracted only partly by the shrinking effect by the disappearance of water molecules from the gas phase (the only effect considered by MG thus far). The greater heat content and expansion of the wet air compared to the dry air at the same height has been confirmed by observations, and can be fully explained from the thermodynamic equations, since about 150 - 200 years. On the other hand, the “local pressure drop” of MG would necessarily cause a shrinking, or “negative explosion” (as MG emphasized themselves in MGL2008). This contradicts the observations, and is based on the neglecting of the dominating effect of latent heat release. If this part of the BPT would be right, cumulus clouds would have greater specific density than their surroundings (same mass contracted in smaller volume), and would descend instead of ascend by Archimedes’ Law.

In the fourth paragraph (“In considering the effects ….”), it is first stated that we fail to note the change of weight of an air column due to condensation, and then (at the end) that we predict pressure increase on condensation. To start with the last point: this prediction concerns the immediate condensation effect on the pressure within the cloud, not at the surface. Our answer to the first point: our alleged failure to note the change of the weight of the column, is twofold. First, this change of weight is a new topic in the discussion: e.g. the theory of MG2007 did not speak about weight differences between columns but only about the weight distribution within columns. Second, it must be obvious to any reader that condensation can by itself never cause a change of weight of an air column. We assume (neglecting MG’s phrase “nearly instantaneous”) that it is intended that the weight is reduced when the water precipitates (that is also the point of the authors cited in paragraph 4). If this is the intention, we fully agree with this. But if this is the core of the (nowadays version of the) BPT, then the BPT amounts to saying that this effect is of more basic importance than all others in driving atmospheric motion, so this point must be investigated now.

 

Important issues to consider

In doing so, we should not overlook the fact that precipitation continues a cycle which starts with evaporation (the more as evaporation and ”evaporative force” were originally the focus of the BPT). The weight loss by precipitation equals the weight gain in the previous evaporation. Moreover, treating all other effects as secondary (again: this is the critical point), the air columns which contain most water must be the heaviest of all, and must have the highest surface pressure. Very high surface pressures should then be expected for the tropical rainforests and the tropical waters, and much lower surface pressures for the subtropical deserts. This is just the converse of what we see on climate maps. Moreover, as a consequence the surface flow would be directed mostly from the wet to the dry areas, which is just the opposite of what is seen in the pictures which use to illustrate the Biotic Pump Theory. Note that this drawback was not present in the old (less mature) exposition, which focused not on horizontal but on vertical pressure gradients.

It must be obvious from this that the weight change of whole columns by evaporation/ condensation/precipitation (as explained above, we cannot avoid to include the evaporation to the list) cannot by itself be the main driver of the atmospheric circulation (though it is of local importance, especially in cases with vigorous precipitation when other effects are acting relatively slow). Readers may wonder how the low surface pressures of the wet areas are actually generated. Above we spoke already about the effects of condensation on air (according to traditional understanding): it causes higher temperature and expansion than when no condensation occurs, other circumstances being equal. In regions where condensation has a dominating effect, such as the tropics, air columns are on average more expanded horizontally than elsewhere (we mean “air columns” containing some given amount of dry air component). So their weight is distributed over a larger horizontal surface, decreasing the surface pressure. This expansion-effect is so strong that it overwhelms the weight gain which is caused by the higher water content. This is essentially the “traditional” explanation of the well-known relation between low-pressure reading at the surface, and “bad weather”. Precipitation, if it occurs, also contributes to the lowering of the pressure, but this effect is embedded in a larger system as shown above, and as such it is a part of the conventional theory (though there are sometimes problems in getting it accurately represented in models, see the references in paragraph 4 of the “Invitation”).

 

Final remarks

Concerning the tornados and hurricanes (paragraph 5 of the “Invitation”), what I have seen of the new explanations is still based on the “old” BPT, so the critique in the Comment of Meesters et al. (2009) on the deduction of the “evaporative force” (which will not be repeated here) also applies to this. See also the discussion about MGL2008.

Again, the INSAM website is in my opinion not the proper place for lengthy discussion about the “Biotic Pump Theory”, and I write this only because I have to reply to the remarks by Makarieva and Gorshkov. I think that if a further discussion would be worthwhile, it should be carried out elsewhere.

 

What was said earlier and referred to above

  • [Invitation] Makarieva AM, Gorshkov VG (2009): The biotic pump physics is maturing to a novel theory of atmospheric circulation: Everybody’s invited! INSAM website.
  • Stigter K, Meesters A. (2009) A “forests as biotic pump” hypothesis discredited due to errors in basic atmospheric physics. INSAM website.
  • [MG2007] Makarieva AM, Gorshkov VG (2007) Biotic pump of atmospheric moisture as driver of the hydrological cycle on land. Hydrol. Earth Syst. Sci. 11:1013-1033.
  • [MGL2008] Makarieva AM, Gorshkov VG, Li BL (2008) On the validity of representing hurricanes as Carnot heat engine. Atmos. Chem. Phys. Discuss. [http://www.cosis.net/members/journals/df/index.php?a_id=8972]
  • [MDB2009] Meesters AGCA, Dolman AJ, Bruijnzeel LA (2009) Comment on “Biotic pump of atmospheric moisture as driver of the hydrological cycle on land” by Makarieva AM, Gorshkov VG (2007). Hydrol. Earth Syst. Sci. 13: 1299-1305.
  • [MG2009a] Makarieva AM, Gorshkov VG (2009) Condensation-induced dynamic gas fluxes in a mixture of condensable and non-condensable gases. Physics Letters A 373, 2801-2804.
  • [MG2009b] Makarieva AM, Gorshkov VG (2009) Reply to A.G.C.A. Meesters et al.’s Comment on “Biotic pump of atmospheric moisture as driver of the hydrological cycle on land”. Hydrol. Earth Syst. Sci. 13, 1307-1311.
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