Understanding the Role of Latent Heat Energy Transport in Climate Change

[vanesch]

so even a modest increase in temperature should result in much more convection and therefore much more cooling. Earth's temperature should therefore be, in a larger sense, relatively stable.

What you are describing is actually the capacity of convection to have the temperature profile "stick to" the adiabatic lapse rate. It is also what Andre (correctly) described: an "unstable" atmosphere (one that has an actual lapse rate that is bigger than the adiabatic one) will give rise to very strong convection, which will quickly try to restore the adiabatic lapse rate. All that is correct. But this is about *deviations* from the steady-state situation we are considering. Mostly meteorological phenomena. What people are considering are the changes in the steady-state situation itself.

The point is that a greenhouse gas *shifts* the temperature profile, while keeping the same lapse rate (well, humidity itself will change this lapse rate by itself, make it softer, and this effect will also result in a relative cooling, that is to say, a less strong heating than if the lapse rate were kept constant). This shift comes about simply because the final radiating layers are now higher-up, and need to be at a similar temperature as before in order to be able to radiate as much as before.

 

[Andre]

It seems that we have made considerable progress. But the question remains if convection (especially in the hadley cell) is merely a mechanism that restores the lapse rate and dies out once established; or is it a continuous motoring engine of the Earth's air conditioner that transports surface heat (via evaporation in the form latent energy) to higher levels for easier out radiation.

Back later.

 

[vanesch]

It seems that we have made considerable progress. But the question remains if convection (especially in the hadley cell) is merely a mechanism that restores the lapse rate and dies out once established; or is it a continuous motoring engine of the Earth's air conditioner that transports surface heat (via evaporation in the form latent energy) to higher levels for easier out radiation.
.

Of course it is an "air conditioner" that works continuously and it DOES cool the Earth significantly (as I said, a LARGE part of the heat transport in the lower troposphere is on the account of convection).

I didn't say that "once the lapse rate is established, convection dies out" either, as there is a continuous drive (the heating of the surface) that tries to pull the lapse rate to higher values than the adiabatic lapse rate. As I said also to granpa, it is correct that this implies that there is a continuous small deviation from the adiabatic lapse rate to "drive" the convection. There will be a steady state situation where the deviation from the adiabatic lapse rate is just enough to give you the right amount of convection to transport all the heat it has to transport. But this deviation can be supposed to be small, so we can say that the steady state situation is "essentially" given by the adiabatic lapse rate.

What is important, however, is to see that this deviation (and hence the strength of the convection) is auto-regulated in such a way that the heat transport is OK. Indeed, if there wouldn't be "enough" heat transport, then it would get "hotter below" and the actual lapse rate would deviate more from the adiabat, hence increasing the convection rate. If there would be "too much" heat transport, it would get cooler below, and hence the deviation from the adiabat would get smaller (or would even reverse sign), which would slow down the convection rate (or even bring it to a stop).

So convection will regulate itself (together with the small deviation from the adiabat) such that heat transport is what it has to be to get "steady state". Of course, this is "on average". Locally, and temporarily, you can get strong deviations from this "average steady state", that's meteorology.

And, on average again, the lapse rate will be (close to) the adiabat.

But yes, it has an important cooling effect, and is in fact the main transportation of heat at low altitudes. However, this is already taken care off when considering the lapse rate.

And now back to "how much power do you need to get the air wet through evaporation". Well, GIVEN that convection will ADAPT to transport whatever power is to be transported, whether the air is humid or dry, this doesn't matter. ANY amount of power can be transported by convection, with humid OR with dry air. With humid air, as the transport is more efficient (and does, indeed, require evaporation as part of the heat transport process), the convection will "slow down" to adapt itself to it. With dry air, as the heat transport is somewhat less efficient (no evaporation, no latent heat), convection will need to speed up to transport the same amount of heat.

But the power, by itself, doesn't determine the humidity of the air by itself. If things are such that the air is humid, then convection will HAVE TO slow down.

This, up to the caveat that the adiabatic lapse rate is ALSO altered with humid air, so this complicates matters somewhat.

 

[vanesch]

Of course it is an "air conditioner" that works continuously and it DOES cool the Earth significantly (as I said, a LARGE part of the heat transport in the lower troposphere is on the account of convection).

[ ... ]

And, on average again, the lapse rate will be (close to) the adiabat.

But yes, it has an important cooling effect, and is in fact the main transportation of heat at low altitudes. However, this is already taken care off when considering the lapse rate.

To add to this: in fact, if there wouldn't be any convection, and only radiative heat transport, then the lapse rate would be WAY HIGHER as, in the lower troposphere, radiative transport is a much less efficient heat transport mechanism. Convection sets into REDUCE this lapse rate to the adiabatic one (up to our small deviation) because convection cannot reduce it any more (if the lapse rate is SMALLER than the adiabatic lapse rate, the atmosphere would be in buoyancy equilibrium - it would be a stratosphere).

With a "fixed" temperature at the "last black layer" which has to radiate heat into space, the smaller the lapse rate, the cooler it is at the surface. So without convection - and hence a very large lapse rate - it would be very very hot at the surface, and convection reduces this (works - as you say - as a air conditioner) to the extend that it can, by bringing the lapse rate to its minimum possible value that is still compatible with convection, namely the adiabatic lapse rate.

So it is true that humid air would "cool" the surface somewhat more than dry air, because the wet adiabat is less steep than the dry adiabat. Having a condensible vapor in the air does "cool" the surface more.
(however, in the case of water, it is ALSO a strong greenhouse gas, so this sets off the cooling by heating through increasing the average height of emission by the "last black layer").

 

[Skyhunter]

I certainly did not say that warming results in net cooling.

I said that modest warming greatly increases the amount of water vapor in the air which increases the rate at which warm moist air at ground level is lifted up to the upper troposphere where it is above most of the Earth's 'blanket' of greenhouse gases and so can cool quickly by radiating its heat to space. the world still warms up but not by much.

Thank you for the clarification.

I am not suggesting that latent heat transport is minor, just that it is not being excluded from the equation. As Vanesch has pointed out in this thread, it is an integral component of the adiabat.

 

[Andre]

Meanwhile, I don't think that things get even more complicated when looking at the evaporation map again.

A closer look would reveal that the evaporation around the equator is less than in the areas around the both tropics. (+/-20 degrees lattitude) where the evaporation tends to be at maximum. Also evaporation rates towards the polar areas seem to keep up high values despite much cooler sea surface temperatures.

Evaporation rate is defined by the Penman equation, however that doesn't help us much because we are dealing with variation in temperature, windspeed and relative humidity here as main factors governing the evaporation rate and those only...

...impact the values of m, g, cp, ρ, and δe.

However the reduced evaporation at the equator despite higher sea surface temperatures can be explained by a reduction in windspeed (doldrums) and increased relative humidity accumulated during the trade wind phase (leg #4) in the Hadley cell.

The relatively high evaporation rates in the colder lattitudes can best be explained by higher wind speed (roaring forties). And this appears to make sense as we learn from blowing a hot spoon of soup. It is especially the mechanical mixing of the lower level atmosphere under the turbulence of higher winds, while calm winds remain mostly sort of laminair in stable atmospheric conditions, preventing the moisture to mix in the higher layers.

Therefore a reduction of the convection rate in the hadley cell, would slow down the trade wind speed and this would decrease the evaporation rate, which as we have seen, deals with a lot of watts per square meters. Moreover, remember, in the increased greenhouse effect setting we needed an increase in the absolute humidity to account for the 'maintaining relative humidity' assumption to generate the positive feedback in climate sensitivity

Hence things seem certainly a bit more complex, enough perhaps to obscure the judgement about the prevailance of positive or negative feedback. This would make the different measured outcomes of negative feedback in the multiply quoted Karner and Lindzen studies maybe more acceptable.

 

[vanesch]

Therefore a reduction of the convection rate in the hadley cell, would slow down the trade wind speed and this would decrease the evaporation rate, which as we have seen, deals with a lot of watts per square meters. Moreover, remember, in the increased greenhouse effect setting we needed an increase in the absolute humidity to account for the 'maintaining relative humidity' assumption to generate the positive feedback in climate sensitivity

I think that if you really want to know the real humidity, that you need to do an entire climate modeling. The "keeping constant relative humidity" is simply a very coarse idea, that you keep things "all else equal" in a way, and that the drive for higher absolute humidity (because constant relative humidity) is the higher temperature of the water at the surface.

Again, don't confuse "evaporation rate" with "humidity". If you don't blow over your hot soup, then the evaporation rate is probably lower, but the humidity above the soup is higher. If you blow over the soup, the evaporation rate is higher, but (or rather BECAUSE) the humidity in the air flow is lower.

So if the trade winds slow down, humidity might even rise stronger than "keeping relative humidity constant", all together with less evaporation. Or not. If you really want to know, and go beyond the educated guess of a simple model like this, you need to do all the modeling, in a detailed way, including winds, ocean currents, and all that.

 

[Andre]

.
So if the trade winds slow down, humidity might even rise stronger than "keeping relative humidity constant", all together with less evaporation..

That would be really hard to quantify because as said, you loose the mixing with higher levels when the wind decreases. Sure at sea level under no wind condition the relative humidity can rise easy to 100% but at -say- 10 meters altitude the humidity will be a lot lower, and that's where the greenhouse effect should have been enhanced.

Light winds can be laminair, with little mixing with higher layers under the stable conditions in the trade wind area, again, still limiting evaporation as well as limiting humidity where it counts for the greenhouse effect. Only when the wind strenghen and more pronounced waves are formed, the mechanical turbulence can cause water vapor mixing to 'higher'-lower layers, say some 100s of meters. Pilots of low flying gadgets like hang gliders balloons, sail plane can tell about mechanical turbulence for hours.

So yes reduced wind can increase humidity, but only at levels directly at the ground and under stable atmospheric conditions. But not at higher levels where it counts more for greenhouse effect.

Yes modelling. It would seem logical that the relationship between the pace of the hadley cell structure, evaporation and radiative effects are incorporated in the Global Climate Models, but I don't see evidence of that so far.

 

[Skyhunter]

So yes reduced wind can increase humidity, but only at levels directly at the ground and under stable atmospheric conditions. But not at higher levels where it counts more for greenhouse effect.

Do you have a citation to support that hypothesis?

If there is no wind, or light wind, Then it would seem to me that convection would carry the water vapor to higher altitudes as the surface warmed. The surface flux would have to be high to maintain 100% relative humidity, which would lead to strong surface warming, especially considering the strong LW absorption by the saturated air.

 

[Andre]

Do you have a citation to support that hypothesis?

If there is no wind, or light wind, Then it would seem to me that convection would carry the water vapor to higher altitudes as the surface warmed. The surface flux would have to be high to maintain 100% relative humidity, which would lead to strong surface warming, especially considering the strong LW absorption by the saturated air.

You missed the second condition: stability:

Light winds can be laminair, with little mixing with higher layers under the stable conditions in the trade wind area

meaning no convection, meaning not a lot of mixing.

 

[Skyhunter]

You missed the second condition: stability:



meaning no convection, meaning not a lot of mixing.

But that is not representative of the conditions under which WV saturation would occur. If there is enough warming at the ocean surface to maintain 100% relative humidity there must be convection to maintain the adiabat.

 

[Andre]

That's irrelevant

We're talking about the transition of leg #3 to leg #4. In the subsidence phase of leg #3 the descending (very dry) air heats up dry adiabatically, such a process with colder air below is usually suppressing convection.

The warm (desert) air that get's in contact with the ocean in leg #4 cools due to evaporation, therefore the lower levels are cooler than the environment lapse rate making the vertical temperature conditions stable. Hence no convection, only mechanical turbulence.

 

[granpa]

Do you have a citation to support that hypothesis?

If there is no wind, or light wind, Then it would seem to me that convection would carry the water vapor to higher altitudes as the surface warmed. The surface flux would have to be high to maintain 100% relative humidity, which would lead to strong surface warming, especially considering the strong LW absorption by the saturated air.

his point seems pretty simple and clear to me. higher winds = more turbulence = more mixing with higher levels, meaning the lower 1000-2000 meters of the troposphere (marine layer?)

 

[Skyhunter]

his point seems pretty simple and clear to me. higher winds = more turbulence = more mixing with higher levels, meaning the lower 1000-2000 meters of the troposphere (marine layer?)

What he said was:

Sure at sea level under no wind condition the relative humidity can rise easy to 100% but at -say- 10 meters altitude the humidity will be a lot lower,

10 meters is not 1000-2000, so I fail to see how you can interpret it as such.

I agree that higher winds = more turbulence = more mixing. But his point about no convection above 10 meters is erroneous because it is not a stable condition. With 100% humidity the surface flux will cause the air to warm and rise.

 

[Skyhunter]

The warm (desert) air that get's in contact with the ocean in leg #4 cools due to evaporation, therefore the lower levels are cooler than the environment lapse rate making the vertical temperature conditions stable. Hence no convection, only mechanical turbulence.

So where does the desert air make contact with the ocean?

Most of the air in contact with the ocean does not come from a desert.

It seems to me that you are describing an extremely rare condition that would have little effect on global climate.

 

[granpa]

he said under 'no wind[/color]' conditions. he was going to extremes to to make his point more obvious.

also I think 'desert air' means any air in the descending leg of the cycle. (where all the worlds deserts are located)

 

[Skyhunter]

he said under 'no wind[/color]' conditions. he was going to extremes to to make his point more obvious.

also I think 'desert air' means any air in the descending leg of the cycle. (where all the worlds deserts are located)

Dry air, not desert air would be the accurate term. And he was not exaggerating to make a point. He specifically used 10 meters because all LW radiation from the surface that can be absorbed by GHG is absorbed in the first 10 meters, provided there is water vapor present. That is why he said there would be no added WV to the layers where it would have a greater effect.

He is attempting to redefine the adiabat using exceptions that may or may not exist in nature.

 

[granpa]

http://en.wikipedia.org/wiki/Planetary_boundary_layer

 

[Andre]

This is what I mean with humidity of the lower 10 meters under stable no wind conditions:

http://www.bbc.co.uk/bristol/content/images/2008/02/11/fog_shallow_john_rawlings_470x258_2.jpg

 

[Skyhunter]

http://en.wikipedia.org/wiki/Planetary_boundary_layer

The stably stratified boundary layer is a night time phenomenon except in the higher latitudes where the surface is colder than the air, even during the day. Andre is proposing this mechanism for the tropics using the penman equation which includes solar forcing, the penman equation is not applicable for this phenomenon in the tropics since this condition will not exist during the day over the tropical oceans.

The reason that there is less evaporation over the ocean at the equator is evident in the Hadley cell diagram. The circulation is from high latitudes toward the equator at the surface, in the tropics, where most of the evaporation takes place. (4), by the time it gets there it is already near saturation. This nearly saturated air rises once again (1). The WV either condenses around CCN releasing it latent heat, remains as a vapor, or precipitate out. Both clouds and vapor are carried to higher latitudes (2a), (2b). Finally the air cools and falls back to the surface (3), to begin the cycle again..

During the night time there is less convection as the surface cools relative to the air, but come morning, solar energy will once more drive convection regardless of the relative humidity.

Low clouds (fog), which are liquid not vapor will inhibit evaporation from the surface as they absorb and reflect SW radiation during the day.Day and night they will trap LW in significant frequencies from surface emission. Net forcing for clouds can be positive or negative depending on numerous factors.

What Andre is proposing is a well understood and phenomenon, and well accounted for in the wet lapse rate. There will be plenty of water vapor in the layers above the first ten meters to once again with CO2 trap most of the outward LW flux from the first 10 meters of atmosphere, and without convection and 100% humidity, you would almost have an actual greenhouse with a glass ceiling. The Hadley cell diagram shows that this is not the case.

 

[Andre]

The Hadley cell is indeed accounted for but I cannot see that the changes in the Hadley cell are accounted for with increase of greenhouse effects, especially what the required changes in evaporation rate are for maintaining relative humidity to explain positive water vapor feedback. Also, how the required energy is provided to attain those evaporation rates.

All I see is the assumption that relative humidity is assumed to be more or less constant in climate sensitivity calculation. Period. I attempted to demonstrate here that it would be pretty hard to maintain that assumption, given the substantial energy required to increase evaporation rates. If there is more evaporation required, which Vanesch challenged, then this process would act as a negative feedback, which I have not seen to be accounted for.

 

[granpa]

Ah. now I understand. you're not from San Francisco are you? over here we call the 2000 meter (or is it feet?) thick marine layer 'fog'.

 

[granpa]

http://en.wikipedia.org/wiki/Planetary_boundary_layer

The PBL depth varies broadly. At a given wind speed, e.g. 8 m/s, and so at a given rate of the turbulence production, a PBL in wintertime Arctic could be as shallow as 50 m, a nocturnal PBL in mid-latitudes could be typically 300 m in thickness, and a tropical PBL in the trade-wind zone could grow to its full theoretical depth of 2000 m.

 

[Skyhunter]

The Hadley cell is indeed accounted for but I cannot see that the changes in the Hadley cell are accounted for with increase of greenhouse effects, especially what the required changes in evaporation rate are for maintaining relative humidity to explain positive water vapor feedback. Also, how the required energy is provided to attain those evaporation rates.

All I see is the assumption that relative humidity is assumed to be more or less constant in climate sensitivity calculation. Period. I attempted to demonstrate here that it would be pretty hard to maintain that assumption, given the substantial energy required to increase evaporation rates. If there is more evaporation required, which Vanesch challenged, then this process would act as a negative feedback, which I have not seen to be accounted for.

When you raise the temperature you also reduce the heat of evaporation. So your calculation for energy required to vaporize water needs to be constantly adjusted downward as SST increases.

 

[Andre]

Very true,

roughly in the order of magnitude of one promille per degrees, I'd say, judging from that slope at the left hand. But the point indeed is with countering effects, quantititive judgement is difficult.

 

[granpa]

http://en.wikipedia.org/wiki/Sunlight
Hence the average incoming solar radiation, taking into account the angle at which the rays strike and that at anyone moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 342 W/m²)

from:
http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

 

[granpa]

what effect might increased temp and increased water vapor have on the frequency of severe storms? if they become much more common then that could have a drastic effect on the average pressure at the equator (air pressure under a severe storm being much less than under a regular thunderstorm) and therefore greatly increase the rate of convection

 

[mheslep]

Maybe this could explain what Lindzen and Choi 2009 observed:
Lindzen, R. S., and Y.-S. Choi (2009), On the determination of climate feedbacks from ERBE data, Geophys. Res. Lett., doi:10.1029/2009GL039628, in press.

The paper doesn't appear in that list of links. Is GRL the actual publisher?

 

[vanesch]

When you raise the temperature you also reduce the heat of evaporation. So your calculation for energy required to vaporize water needs to be constantly adjusted downward as SST increases.

That's in principle true, but we are talking about small temperature variations over which we can take the latent heat of evaporation rather constant in a first approximation I would think.

The real point is that there is no *required* amount of power to establish a given *humidity* (which was, if I understood correctly, the initial claim of the OP). You can, in principle, have any humidity in relationship with any power, as convection will transport exactly the amount of heat it has to, with the given air mixture and will hence be able to transport a low amount of power or a high amount of power, and this by adjusting the "looping speed" of the convection cell.

What will determine the amount of humidity has hence not much to do with the forcing just by itself, but rather with the temperature, the winds, the mixing of air masses and all that. Humidity and mixing and so on will also determine the final amount of evaporation.

If you want to find out how much all this changes, I don't think there's any other option but a very detailed modeling of all these processes. If you're too lazy to do so, then a reasonable assumption (which might turn out not to be correct, but only after thorough modeling) is to keep relative humidity constant (which would imply similar mixings as today between 'wet' air and 'dry air' in similar proportions, with 'wet air' completely saturated).

In a way, Andre is right of course that as convection will adapt, this will change winds, convection patterns, and so on, and this will have an influence on humidity and evaporation - only, it is not clear how much and in what sense without a deeper investigation. There could be more or less humidity. But it will never be because 'power is lacking'.

 

[Andre]

But it is an assumption, right? maintaining relative humidity constant with increased greenhouse effect, without any visible justification in the decription of all the processes as we attempted here. So one could argue about it being a reasonable assumption seeing the considerable energy required for excess evaporation and because the observations don't appear to confirm the assumptions.


For mheslep, the former link to Lindzen and Choi was to the article in press. It is now:

http://www.agu.org/pubs/crossref/2009/2009GL039628.shtml