Biotic pump questions, Happy 2021!

[Anastassia Makarieva]

Dear colleagues,

As we are now definitely into 2021, I wish from all my heart that we all navigate through this year with ease and success and in good health, inspired with good ideas, expanding illuminating knowledge and opening new green horizons.

Thank you so much for your persistent interest in biotic regulation and for your questions and comments. Let me suggest that we begin this year by considering a few questions about the biotic pump posted by our group member Pascal Cuissot, who is currently producing a film for the television channel ARTE featuring the biotic pump in Brazil and Russia  (see here for a trailer and here about some other Pascal's  award-winning works like  "Meet the Neanderthal" ), and also here (password "otaviodores") for the Brazilian part. The questions are below and I will respond to them one by one. Then I will get back to our preceding discussions and to the points advanced by Mats and Arie.

Best wishes,
Anastassia

Biotic Pump Questions

At what height does condensation happen above the forest? Around 1200 meters?

What is the temperature when condensation can start?

Do the changes of density and of pressure happen only within the air column under where the condensation takes place?

How fast does the change of pressure happen? Is it homogeneous all throughout the air column even if water vapor still comes up?

Why does the change of pressure affect only the lowest layers of the troposphere (i.e., the bottom of the air column)?

And the tough one, why does the decrease of pressure attract only the air loaded with moisture from the ocean? Why, for instance, does not it attract the air from the inner part of the forest?

[Anastassia Makarieva]

At what height does condensation happen above the forest? Around 1200 meters? 

Brief answer: It depends on the amount of water vapor in the atmosphere. At about 500 m for a typical relative humidity of 80%.

What is the temperature when condensation can start?

Brief answer: At the so-called dew point temperature. The dew point temperature also depends on the amount of water vapor in the atmosphere.

To answer the above two questions more meaningfully, we need to consider condensation in more detail.

1.     In liquid water, the water molecules are closely packed and kept together by the intramolecular forces of attraction. To become a gas molecule, the water molecule must overcome the attraction force and break away via the liquid-gas interface. Apparently, only the most energetic molecules can achieve that.

2.     The number of such energetic molecules leaving the liquid phase per unit time grows exponentially with increasing temperature. It approximately doubles per each ten degrees of temperature increase.

3.     On the other hand, those molecules that are in the gas (water vapor) can occasionally enter the liquid phase. To do so, the molecule does not need to be particularly energetic (because the molecules of gas are free to move). Therefore, the number of vapor molecules entering the liquid phase per unit time depends mostly on how many of them are around.

 4.     So, in equilibrium, the energetic molecules leave the liquid phase for gas and some gas molecules return to the liquid. The situation is like in a closed bottle with water. We do not see either evaporation or condensation (which are macroscopic processes).

 5.     Now imagine that the temperature decreases. The number of energetic molecules leaving the liquid phase abruptly diminishes. If there are enough vapor molecules around, they will continue to enter the liquid phase. We will see condensation (formation of droplets). This happens when you bring a cold bottle to a warm room. You can see mist on the bottle immediately.

 6.     If there are not enough vapor molecules around, nothing will happen. Upon the decrease of temperature, evaporation will continue at a slower rate, but no condensation will occur. You will need to decrease the temperature even further for the condensation to start.

 7.     For condensation to happen, you need to decrease the temperature to a point when the number of energetic water molecules leaving the liquid phase per unit time becomes less than the number of vapor molecules that enter the liquid phase. The first number depends on the air temperature, the second number depends on relative humidity (how “dry” is the air compared to the saturated equilibrium). Relative humidity is the ratio of the number of vapor molecules to the maximum equilibrium number of vapor molecules that can be present in a given volume at a given temperature.

 8.     So, to answer the first question above, the height at which condensation will occur will depend on the relative humidity of air at the surface. If the relative humidity is close to 100% (the air bears as much water vapor as possible at a given temperature), then as soon as the moist air rises just a little bit and cools, condensation will occur. Below are two pictures of how condensation happens just immediately over the forest, the first photo was shared by our colleague and friend Phil Shearman (hi, Phil!) and shows condensation above the virgin forests of Papua New Guinea, another is from the Prisursky Nature Reserve in Russia.

This is how PNG forest works.

 And this is his Russian brother.

For a typical 80% humidity at the surface, the height at which condensation occurs will range from about 0.5 km to about 1 km depending on how fast the temperature declines with height in the ascending air. Below is Fig. 1B from Makarieva et al. 2013 JAS, where this height is shown as dependent on surface temperature and the vertical lapse rate, see the lowest three lines, green triangles are for 5 K/km, blue circles are for 6.5 K/km and red squares are for 9.8 K/km (it is the lowest line). All heights are below 1 km.

 

9.  To answer the second question, the dew point temperature when condensation occurs also depends оn relative humidity. When relative humidity is 100%, the dew point temperature is simply equal to air temperature, condensation starts at once. The lower the relative humidity, the lower the dew point temperature in comparison to the air temperature.

[Anastassia Makarieva]

And the tough one, why does the decrease of pressure attract only the air loaded with moisture from the ocean? Why, for instance, does not it attract the air from the inner part of the forest?

The air flows from a higher to a lower pressure just as the water flows from a higher to a lower elevation. We can make use of this similarity to visualize what happens during condensation.

It is a very simplified scheme but hopefully it will be helpful.

Suppose that without condensation air pressure is the same over the forest and the two adjacent oceans. They are all at the same “pressure elevation”.

Upon condensation, all forest trees reduce the amount of water vapor above them by the same amount. Pressure falls equally for all of them. Now on the “pressure landscape” the forest is lower than the ocean.

Moist air begins to flow from over the ocean to the forest, to fill up the “pressure pit”. Due to this restoring flow, those forest parts that are closer to the ocean will have a slightly higher pressure than the interior parts of the forest. (Indeed, air pressure cannot be discontinuous.) That is why the central forest (with its lowest pressure) will receive air from the adjacent forest parts (with their slightly higher pressure).

Approximately in this manner the boreal forest belt in summer receives moisture from its three adjacent oceans, the Atlantic, the Pacific and the Arctic.

https://bioticregulation.ru/common/pdf/taac/taac-fig6b.gif

We can modify this conceptual picture to shed light on a more complex situation.

Suppose there are two oceans, A and D, at different latitudes, with and without intense condensation over them due to the different seasons. The continent between them also has two parts, B and C. The “pressure landscape” for the lower atmosphere would look like this:

Ocean A and Land B have higher pressure than Land C and Ocean D because of the seasonal differences (e.g., there is more sunlight, evaporation and condensation in region CD). At the same time, Land B and Land C have higher pressures than, respectively, Ocean A and Ocean D, because there is less condensation over land and more over the ocean.

In this case, Land BC will be the zone of descending air motion, from which dry air will be flowing to the adjacent ocean. Nothing will grow on such a land.

Let us restore natural forest on Land B. Now air pressure is lower over Forest B than over Ocean A (because of condensation dominating over the forest). The positions of C and D did not change.

Governed by the resulting modified "pressure landscape", the moist air flows from Ocean A to Forest B to Land C to Ocean D.

In the result, Land C now receives moisture from Ocean A! This happens due to Forest B.

Think of Forest B as the Amazon forest and of Land C as the agricultural regions of South-Eastern Brazil.

Best wishes,

Anastassia

[Pokorný Jan]

Dear Anastassia, many thanks for clear explanation. I like it.

I have a comment  to the point 7:  condensation is catalysed  by volatile organic matter ( VOC - isoprene, monoterpene etc.)  produced by the forest. Can we imagine the VOC like “tiny hooks” or continuation of forest stand up to atmosphere which keep the humidity produced by transpiration trees?

Best regards

Jan

--
You received this message because you are subscribed to the Google Groups "Biotic Regulation of the Environment" group.
To unsubscribe from this group and stop receiving emails from it, send an email to biotic-regulat...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/biotic-regulation/CAKz3-98byPt3dD26zYXHEWNh%2BKGF%2B-W1H9dAewOTNMotsoOibw%40mail.gmail.com.

[Anastassia Makarieva]

How fast does the change of pressure happen? Is it homogeneous all throughout the air column even if water vapor still comes up?

Condensation rate in the ascending air depends on the vertical air velocity and on the local concentration of water vapor. These two magnitudes change in an opposing manner with altitude: water vapor concentration declines, but vertical velocity often grows. So condensation rate can be uniform throughout the air column.

 Do the changes of density and of pressure happen only within the air column under where the condensation takes place?

Condensation initiates a change of pressure. Then the surrounding air begins to react – to readjust according to the instantaneous pressure gradient. In the result, pressure changes may redistribute in space, see below.

 Why does the change of pressure affect only the lowest layers of the troposphere (i.e., the bottom of the air column)?

The change of pressure occurs at every point where condensation occurs. Then there is gravity. Under the action of gravity, our atmosphere tends to adjust to a state when the air pressure at each point is balanced by the weight of air above that point (so-called hydrostatic equilibrium).

As upon condensation and precipitation there is less moisture in the column, this means that the weight of the column has reduced, and so must the surface air pressure. In this way condensation aloft causes the air to readjust vertically such that the pressure shortage is relocated to the surface.

The precise configuration of the pressure gradient will depend on the geometry of the condensation area.

If the condensation area is vertically and horizontally small (a localized shallow cloud), then the drop of pressure will be compensated by air inflow from all sides (see Fig. (a) below, red arrows). These opposing air flows will extinguish each other, and the condensation-induced air circulation will stop.

(a)    A shallow non-precipitating cloud and (b) a large area with deep convection. Red arrows indicate the direction of air motion during the pressure adjustment initiated by condensation.

If, on the other hand, condensation occurs throughout most of the atmospheric column (height hc of condensation is comparable to atmospheric height h ~ 10 km) and if the condensation area is horizontally large (horizontal size lc >> hc), then there will be a consistent air flow with air ascending within the condensation area and flowing in horizontally at the surface.

In this case, the pressure shortage due to condensation is restored in the vertical dimension more quickly than it does in the horizontal direction. The pressure shortage is located at the surface and drives in moisture-laden air.

This simple picture illustrates the importance of the forest (and condensation area) being horizontally large.

 ****

I think I have now replied to all Pascal’s questions. The topic is complex. Creating an informative animation about the physical and ecological basis of the biotic pump is an ambitious task which could well demand a separate project. So I believe in the framework of the current Flying Rivers project one will need to find a compromise between simplicity and informativeness.

I agree with Jan’s comment that we should pay more attention to the role of biogenic aerosols acting as cloud condensation nuclei and initiating condensation.

On another point, I personally find this laboratory experiment quite illuminating as to how condensation makes fluids move.

Best wishes,

Anastassia


ср, 6 янв. 2021 г. в 12:22, Pokorný Jan <pok...@enki.cz>:

[Ugo Bardi]

Happy Orthodox Christmas to all the members of the list!

--
You received this message because you are subscribed to the Google Groups "Biotic Regulation of the Environment" group.
To unsubscribe from this group and stop receiving emails from it, send an email to biotic-regulat...@googlegroups.com.

To view this discussion on the web visit https://groups.google.com/d/msgid/biotic-regulation/CAKz3-98YnxwjA%3DfyaXyCHH893bTB3u-UrXoefBob_deWMeCOWw%40mail.gmail.com.

-- 
***********************************
Prof. Ugo Bardi
Dipartimento di Chimica, Università di Firenze, Italy
Full member of The Club of Rome
Registered Expert of the United Nations "Harmony With Nature" Programme
Chief Editor of "Biophysical Economics and Sustainabilty," a Springer Journal
Delegato del Rettore per la gestione della Rete Università Sostenibili (RUS)
ugo....@unifi.it
www.cassandralegacy.blogspot.com
www.theproudholobionts.blogspot.com

[Mats Almgren]

Anastassia, there is a point in the biotic pump mechanism that I am ashamed to admit that I have failed to understand so far: the difference between the pressure as in the ideal gas law, and the hydrostatic, or barometric pressure as given by the weight of the column above. Suppose that there is only some condensation, or aggregation, so that the number of water molecules are reduced, and a much smaller number of aggregates are formed (but no precipitation). The pressure according to the gas law will be reduced, but the hydrostatic pressure would not be affected. This question was up to discusssion in some of the long discussions threads about the Biotic pump papers, but never answered in a way that I understood. Could you give a solution to this paradox?

Best regards

Mats

--
You received this message because you are subscribed to the Google Groups "Biotic Regulation of the Environment" group.
To unsubscribe from this group and stop receiving emails from it, send an email to biotic-regulat...@googlegroups.com.

To view this discussion on the web visit https://groups.google.com/d/msgid/biotic-regulation/CAKz3-98YnxwjA%3DfyaXyCHH893bTB3u-UrXoefBob_deWMeCOWw%40mail.gmail.com.

[Anastassia Makarieva]

Hello, Mats.

There are nuances, but it would not be much error to say that the hydrostatic pressure won't reduce until the condensate falls out. If this does not happen sufficiently soon (e.g. if the condensate particles are very small and have small terminal velocity), then the reduction of air pressure upon condensation will bring about a horizontal air inflow at the level of condensation (and not at the surface). For example, if condensation begins at a height of about 500 m, the air inflow will be there. This air inflow can either feed the ascending air motion and enhance condensation or it can cause a pressure surplus and extinguish the condensation.

There is some confusion in the literature about these dynamics.

Best wishes,

Anastassia

чт, 7 янв. 2021 г. в 12:53, Mats Almgren <mats.a...@gmail.com>:

[Anastassia Makarieva]

Thank you, Ugo!

Here we are having quite a nice, cheerful winter.

чт, 7 янв. 2021 г. в 12:50, Ugo Bardi <ugo....@unifi.it>:

To view this discussion on the web visit https://groups.google.com/d/msgid/biotic-regulation/b5a15a56-09ff-3dea-a931-0e1bfdda9e7f%40unifi.it.

[Mats Almgren]

Anastassia,

Thank you for the clarification. Your paper (tough reading!) was helpful also in this respect.

Mats

To view this discussion on the web visit https://groups.google.com/d/msgid/biotic-regulation/CAKz3-99_c63xk5oYmfM4uxeuvv488T3wBjJVpSoBxcudRikVXA%40mail.gmail.com.

[Anastassia Makarieva]

Link shared by our colleague and friend Prof. German Poveda:

https://www.ttbook.org/show/plants-persons