Here’s an idea for commenters to sink their teeth into — Stephen Wilde postulates a mechanism where convection can neutralize the effect of changes in greenhouse gases. The focus in David’s series of blog posts has been on radiation, but the troposphere is governed by convection. The tropopause is the boundary where convection runs out of oomph, and above the tropopause, in the stratosphere, radiation rules supreme. Airliners like to fly at the bottom of the stratosphere (“thirty thousand feet”), just above the convection and water vapor in the troposphere, because it’s calm. But sometimes turbulence from the troposphere punches up into the stratosphere, so think of Fig 1 below if you are asked to suddenly sit in your seat and fasten your seat belt.
All the focus on radiation imbalances tends to ignore the powerful effects of gravity and convection. To get a sense of how important gravity is, ponder that a mere 3km above the surface the pressure is 300hpa lower, but gravity stops the surface air from rushing up and equalizing that. Imagine if there were two sites on the ground where pressure was, say, 1000hpa and 700hpa, and they were only three kilometers apart, we’d have some kind of monster Cat-10 cyclone on our hands. (If gravity ever “stopped”, we get some sense of just how fast the air would leave the planet.)
I have this vision of the tropopause as being flat (as it is in so many diagrams) but Stephen Wilde points out that it’s a lumpy roiling creature. Sometimes it’s a lot further or closer to the surface than the “average” and in that variation lies a kind of loophole. Air is constantly rising and falling in a tricky “adiabatic” way — where blobs of air (officially called “parcels”) rise and fall without mixing with the surrounding air. A blob heats up over a desert, say, and rises, expanding to the the top of the tropopause (or overshooting it). Blobs of cold air can also sink and reverse that process, but Wilde points out that it isn’t symmetrical. The blobs can overshoot at the top, then slide and mix with the stratosphere in a non-adiabatic way, until they reach a lower altitude before turning into a “parcel” and sinking back to the surface. That gap where the air slides from a high spot in the bumpy tropopause to a low point is important.
The troposphere is constantly churning – low pressure systems work like vacuum cleaners drawing warm air up from the surface to the tropopause. High pressure cells do the opposite, drawing in cold air from the tropopause and pulling it back down to the surface. Generally the high’s are broad and slow moving, and the lows are smaller, faster and there are more of them.
There is constant interplay between convection and radiation, with convection attempting to follow the lapse rate but radiation distorting it.
The “lapse rate” is how quickly it cools as one ascends — on average about 6.5 C per km of vertical ascent. If the air were perfectly dry it would be nearer 10 C per km (the dry adiabatic lapse rate), and if it were as wet as could be then it would drop to around 5 C per km (the moist or saturated adiabatic lapse rate).
Stephen Wilde, who has a long and avid interest in meteorology, explains an idea he has on interplay between radiation, convection and lapse rates. Though it is not quantified, it is worth a discussion. See where that leads…
Neutralizing Radiative Imbalances Within Convecting Atmospheres
Stephen Wilde, 3 October 2015
This article sets out a simple mechanism whereby planetary atmospheres can be rendered thermally stable over time despite huge variations in the atmospheric content of radiatively active molecules such as greenhouse gases, material released by volcanic outbreaks of a vast size, and material vaporized in large asteroid or meteor strikes.
It is established science that convective adjustments can stabilise or neutralise radiative imbalances:
“Radiative equilibrium profile could be unstable; convection restores it to stability (or neutrality)”
Note that the hydrostatic equation depicts the vertical balance of force for a piece of fluid at rest. The balance is between the upward pressure gradient force and downward gravitational force The hydrostatic equation is the vertical component of the momentum equation (Newton’s equation of motion) for the fluid parcel when the forces are in prefect balance and the net acceleration = 0.”
Readers could study that lecture since it explains the concept of hydrostatic balance within atmospheres.
It appears that those climate scientists who apply the radiative gases theory of climate change have overlooked the means by which convection neutralizes radiative imbalances.
Let’s start by looking closely at the tropopause, which is known to undulate upwards above rising air masses and downwards above falling air masses. The height of the tropopause is set by ozone in the stratosphere reacting directly with incoming solar energy so as to create a temperature inversion that blocks further upward convection.
The situation at the tropopause can become very complex:
The above picture focuses on frontal zone distortions between low pressure (ascent)) and high pressure (descent) across the latitudes between equator and poles but our interest here is on the interaction between the high and low pressure centers themselves at any latitude or at the same latitude so we can simplify the graphics thus:
The first diagram shows, by way of a broken green, line an undulation in tropopause height between rising and falling columns of air. If one then looks at the green arrows in the cloud to the left of the picture, the lateral flow corresponds with the high level winds in the simplified second diagram and in the absence of the frontal zone distortion one should extend the flow downwards beneath the distorted tropopause to the top of the adjoining subsidence column, which is actually situated to the right of the first diagram. That scenario applies between all high and low pressure cells even if they adjoin one another at the same latitude.
Taking an atmosphere in hydrostatic balance (as they all must be), the raised areas are, on average, over time, equal in volume to the lowered areas. If that were not the case then the atmosphere would constantly expand or constantly contract.
Cold air at the top of ascending columns is forced to one side by warmer air continuing to flow up from beneath but it is blocked by the warmer stratospheric air above and so flows laterally and downward to the top of descending columns, following the undulating slope of the tropopause.
Whilst flowing across the area between the two columns that cold air does not warm up by compression despite falling in height because it remains in contact with stratospheric air with which it can freely exchange energy by conduction and mixing.
Falling air in the descending column will not start to warm by compression until parcels of falling air become detached from the tropopause.
At that point, energy exchange with stratospheric air ceases and a process known as adiabatic descent begins. In meteorology an adiabatic process is one whereby no heat passes between the vertically moving parcel and the surroundings.
For those who wish to go deeper into that aspect this is a useful source: PSU Courseware
“As an air parcel rises, it moves into an environment of increasingly lower pressure (remember that pressure decreases with increasing altitude). In order to equalize the pressure difference between the the rising parcel and its new environment, air molecules inside the higher-pressure air parcel push out the sides of the parcel. That requires molecules to do work, which results in a loss of kinetic energy. With kinetic energy expended by air molecules to push out the sides of the expanding parcel, the temperature of the air inside the parcel decreases (recall that temperature corresponds to the average kinetic energy of molecules). The energy spent by molecules to push out the sides of the parcel amounts to a flat rate of 5.5 degrees Fahrenheit per 1000 feet of ascent (10 degrees Celsius per kilometer).”
The process is reversed in the descent phase.
The important point to note is that adiabatic descent involves a falling parcel of air contracting and heating despite there being no heat drawn by it from the surroundings. A rising or falling parcel in a pure adiabatic process will retain its temperature differential with its surroundings throughout the ascent or descent. No process is perfectly adiabatic so there will always be some diabatic energy exchange in or out of the moving parcel but here we are only concerned with the adiabatic portion. Any work done against surrounding molecules would be diabatic and not adiabatic.
Once the convective loop is established the air molecules in the direction of travel move away without providing any resistance
The parcel of air changes its own temperature and pressure at the same rate as the temperature and pressure of the surroundings changes. It follows that air rising and expanding into a region of lower pressure need do no work on the surrounding molecules because it simply expands into the additional space made available by the reducing density gradient..
Similarly, a parcel falling into a region of higher pressure simply contracts to fit into the reduced space made available by the increasing density gradient.
Now, bear in mind that convective uplift in the rising column had previously pushed tropopause height down above the descending column by the same volume as it pushed tropopause height up above the ascending column.
Recall that the air flowing across laterally and downward to the top of the descending column did not warm up adiabatically due to its contact with the stratosphere above.
What we then see is air at the top of the descending column colder than it ‘should’ be for its position along the dry adiabatic lapse rate slope AND a reduced distance from the surface so that it cannot warm up as much as it otherwise would have warmed up during its descent to the surface.
It follows that as a direct result of the tropopause having been raised above the rising column there is less warming by compression than there would otherwise have been at the base of the descending column.
The next point to consider is that the rising column only developed in the first place due to uneven surface heating which causes density variations in the horizontal plane. Lighter, warmer air will always rise above heavier, colder air within an atmosphere suspended off a surface against gravity in hydrostatic equilibrium. The declining density gradient with height (caused by gravity) permits lighter, warmer air to rise further away from the surface than can colder, heavier air at the same upward pressure gradient force.
Greenhouse gases are not required in order to cause uneven surface heating. Such unevenness at the surface is the inevitable consequence of the physical characteristics of a rough surfaced, rotating sphere illuminated by a point source of light such as a sun.
Convective overturning is therefore inevitable even with no radiatively active material in an atmosphere at all. Uneven conduction to the atmosphere from the illuminated surface is all that is needed combined with the declining density gradient with height.
So, we are now ready to pull all that together in order to show how a planetary atmosphere uses convective overturning to neutralise the effect of radiatively active materials of any type so that hydrostatic balance can be maintained whatever fate throws at it.
We will start with a theoretical radiatively inert atmosphere which will still have undulations at the tropopause due to uneven surface heating below but they will be minimal and so for all intents and purposes the rising and falling columns will both follow the dry adiabatic lapse rate as they move up and down:
Now we will turn to a non-condensing radiatively active material such as CO2 but the same principle applies to all other radiatively active materials such as various other gases and particulates.
Figure 3: DALR = dry adiabatic lapse rate, about 10 deg C per km
The most important feature is the vertical displacement without a change in temperature (shown in green) because that is what changes the distance along the lapse rate slope which is available to allow adiabatic compression at the dry adiabatic lapse rate in descent.
The existence of such vertical displacements around convective columns is shown here:
Although the above picture shows the most dramatic scenario between the cloud tops at the equator and those at higher latitudes the truth is that the pattern is exactly the same but to a lesser degree around all convective columns at whatever latitude and whether clouds are present or not.
Taking the process step by step:
i) In an ascending column the CO2 at lower levels is a net absorber of radiation from the ground so that the slope of the lapse rate is reduced in an ascending column which slows down the rate of convection from the surface. The steeper the lapse rate slope the faster is convection so reducing the slope reduces the speed of convection. Reducing upward convection allows the surface to warm beneath the rising column to a higher temperature than it otherwise would have done.
Note that the position is exactly reversed in the descending column. Whereas the CO2 has reduced the rate of uplift in the ascending column (which warms the surface) CO2 at lower levels reduces the rate of descent in the descending column which reduces surface temperature because less warmth is then being generated via compression of descending air.
At higher levels CO2 becomes a net radiator of energy to space so that the slope of the lapse rate increases again. The cooling higher up reinvigorates convection from below and since the entire ascending column contains more energy than it otherwise would have done the tropopause is raised more above the rising column than would otherwise have been the case.
Kinetic energy at the surface acting via conduction and convection supplies the upward pressure gradient force which offsets the downward force of gravity in order to constantly hold the mass of an atmosphere off the surface in hydrostatic balance.
In order to maintain hydrostatic balance the necessary kinetic energy cannot be radiated away to space hence that ‘additional’ energy must be held at the surface over and above that which is required (by the surface and atmosphere combined) solely to radiate enough energy to space to match energy being received from space. That additional kinetic energy at the surface is the true greenhouse effect and it is mass induced rather than GHG induced.
ii) We see an increased tropopause height above the rising column but, as described above, that induces a decreased height above the descending column so that the surface below the descending column warms up less during adiabatic descent than would otherwise have been the case. Thus the net thermal effect at the surface is zero.
iii) Temperature differentials at the surface between the base of rising columns and the base of falling columns will be enhanced but the average surface temperature remains unchanged. Surface winds increase to deal with the enhanced differentials across discrete regions at the surface.
iv) An AGW proponent might ignore the lack of any net surface temperature rise and instead seize on the idea that CO2 might make a large enough difference to increase storminess at the surface. To resolve that aspect one need only consider that convective overturning involves the entire mass of the atmosphere reacting to huge variations induced by solar and oceanic processes. Any contribution to average surface wind speed from CO2’s radiative capability relative to the overturning of atmospheric mass induced by conduction and convection would be magnitudes less and impossible to measure.
We should now look at water vapor as an example of a condensing radiatively active component of the atmosphere.
The scenario is similar:
The difference lies in the absence of water vapor in the descending column which then warms at a different rate to the cooling in ascent. Additionally, the distortions of the lapse rate in ascent are greater than for CO2 because water vapor is lighter than air and contains more energy in latent form which heats the air around it when condensation occurs during uplift.
The above application of known meteorological principles appears not to have been considered by the climate establishment.
To summarize, lapse rate distortions caused by GHGs always slow down the rate of convection at lower levels. In the ascent column that slows surface cooling via decompression for a warming effect and in the descent column that slows surface warming via compression for a cooling effect. To maintain hydrostatic balance the two effects must be equal and opposite.
My thanks to David Evans for creating the above diagrams from my hand drawn originals, and to Joanne Nova for tracing some source material that helped to substantiate the above narrative.