Visitor put up by Bob Wentworth, Ph.D. (Utilized Physics)
I’ve been enthusiastic about some concepts that WUWT contributor Willis Eschenbach (WE) has proposed. Specifically, WE has instructed that tropical cumulus clouds and thunderstorms present a “thermostatic mechanism” that helps to stabilize the temperature of the Earth inside a slim vary. WE has additionally supplied a process for predicting floor temperatures modifications in response to elevated radiative forcing.
I discover each these concepts intriguing. But, there are assumptions implicit in WE’s thermostat speculation and predictive process—and I haven’t been in any respect sure that these assumptions are legitimate.
So, I needed to do what I might to test these assumptions.
The Earth is a fancy thermodynamic system. In relation to understanding thermodynamic techniques, my expertise is that verbal reasoning usually results in incorrect conclusions. So, I all the time wish to know what the what the mathematics and physics inform us.
I’m not about to attempt to produce a whole, reasonable mannequin of Earth’s local weather. Nonetheless, I made a decision to use math and physics to a simplified “toy mannequin” of the Earth and its ambiance.
Simplified fashions get some issues proper, and different issues incorrect. They’re not fully reliable. But, such fashions can nonetheless supply beneficial insights, past what one can get to with verbal reasoning alone.
With that in thoughts, I’ve analyzed a toy mannequin I name “Thunderstorm World,” to see what mild it might shed on WE’s process and speculation.
I’ve utilized this mannequin to analyzing these questions:
- Does WE’s process for predicting how floor temperature modifications in response to forcing appear possible to offer legitimate predictions?
- Does it appear possible that tropical cumulus clouds and thunderstorms would possibly regulate the temperature of a planet to maintain it inside a slim vary?
This can be a pretty lengthy essay. So, I’ll supply an summary, and you’ll resolve how a lot of the detailed exposition you wish to learn.
I describe the Thunderstorm World (TW) mannequin, a easy mannequin of a planet and its ambiance which incorporates convective and radiative warmth switch and cloud-induced albedo modifications.
The TW mannequin reveals robust convection and cloud formation at low latitudes. Amongst different outcomes, the mannequin yields a curve of floor temperature vs. complete floor irradiance. This curve is qualitatively much like the curve that emerges from measurements on Earth. This similarity affords a measure of validation for the mannequin.
I apply a process proposed by Willis Eschenbach (or my understanding of that process) to making an attempt to foretell the response of the TW mannequin to a “forcing” resulting from an elevated focus of greenhouse gases. Sadly, the process predicts a rise in imply world floor temperature that’s too small by 45 p.c. The process additionally fails to appropriately predict the variation of temperature with latitude. I determine two mistaken assumptions implicit within the process that result in these flawed predictions.
I look at whether or not tropical thunderstorms (or, extra exactly, low-latitude convection and cloud formation) reasonable or restrict will increase in planetary temperature. Inside the TW mannequin, it seems that convection and cloud formation do reasonable temperature will increase. However these mechanisms don’t impose any onerous restrict on such will increase. Though the onset of tropical convection would possibly seem to behave as a “thermostat” limiting floor temperature, within the TW mannequin, the setting of this “thermostat” is relative to the temperature of the higher layer of the ambiance. So, if a “forcing” warms the higher troposphere, then tropical floor temperatures also can rise.
Thus, to the extent that the TW mannequin bears a relationship to real-world local weather dynamics, the outcomes of the mannequin recommend that (a) the proposed process for predicting responses to forcing will not be reliable, and (b) tropical thunderstorms possible reasonable however don’t place any absolute cap on planetary warming.
The Thunderstorm World Mannequin
The Thunderstorm World (TW) mannequin is designed to be so simple as potential whereas nonetheless accounting for convective warmth switch, cloud formation, radiant warmth switch, and variations in floor temperature.
To this finish, the TW mannequin assumes:
- The planet has a uniform floor, excessive thermal inertia, rotates quickly, and has no inclination, in order that diurnal and seasonal temperature variations will be ignored, and there’s no variation with longitude. Temperatures and vitality flows rely solely on the latitude, 𝛳.
- The ambiance has two layers. Every layer of the ambiance is characterised by a single temperature at a given latitude.
- Though the floor temperature, T₁(𝛳), and the temperature of the decrease layer of the ambiance, T₂(𝛳), differ with latitude, the temperature of the higher layer of the ambiance, T₃, is identical in any respect latitudes. (On Earth, the typical temperature within the higher troposphere at a stress of 190 mbar is just weakly depending on latitude, so this assumption of fixed temperature isn’t unreasonable.)
- Convection occurs every time the temperature distinction between the floor and the decrease ambiance layer, or between the decrease and higher ambiance layers, exceeds a threshold worth 𝚪H, the place 𝚪 is the adiabatic lapse price and H is the elevation change between layers.
- The warmth switch price related to convection is assumed to be proportional to how a lot the temperature distinction exceeds 𝚪H.
- When convection happens on the floor, that is assumed to result in cloud formation which results in reflection of shortwave radiation from the Solar. This improve in albedo is assumed to be proportional to how a lot the temperature distinction exceeds 𝚪H.
- The layers of the ambiance have radiative properties much like these assumed in my prior essay, Atmospheric Power Recycling. Every layer of the ambiance absorbs absolutely a fraction f of thermal radiation wavelengths, and is clear to a fraction (1– f) of thermal radiation wavelengths. The parameter f is taken to narrate to the focus of greenhouse gases current within the ambiance.
- At a given latitude, the floor and the decrease layer of the ambiance are assumed to regulate their temperatures to make sure vitality steadiness, in order that the speed of vitality getting into and leaving match. For the higher layer of the ambiance, vitality steadiness can be assumed, however this requires integrating vitality gained and misplaced over all latitudes, since air circulation is taken to take care of a uniform temperature for the higher layer of the ambiance.
- Warmth switch between latitudes through atmospheric circulation isn’t absolutely modeled, however is addressed partially through the idea that the temperature of the higher ambiance is unbiased of latitude.
These assumptions vastly oversimplify the way in which Earth’s local weather works. But, they embody sufficient components, and sufficient thermal physics, that maybe some dynamics of the actual system shall be reproduced by the mannequin.
The mannequin is depicted beneath.
The floor receives vitality from the Solar, extra on the equator and fewer on the poles, and exchanges vitality with the decrease layer of the ambiance, in addition to radiating some vitality on to house. The decrease and higher layers of the ambiance additionally trade vitality, and the higher layer radiates vitality to house.
At low latitudes (close to the equator), floor heating results in the adiabatic lapse price being exceeded in a manner that triggers convection.
Usually, the zone the place convection occurs between the layers of the ambiance could also be completely different than the zone the place convection occurs between the floor and the decrease layer of the ambiance. (In a extra advanced variant of the TW mannequin, these zones are extra related.)
That’s the TW mannequin.
In what follows, I supply outcomes for the dynamics of the mannequin, based mostly on mannequin parameters as specified within the Appendix. Given these parameter values, I’ve numerically solved for the temperatures T₁(𝛳), T₂(𝛳), and T₃ and the vitality flows that yield vitality steadiness in steady-state.
Primary TW Mannequin Predictions
For the mannequin parameters I’ve thought of, throughout the TW mannequin temperatures differ with latitude as proven within the following determine.
Please take into account that I’m not anticipating the TW mannequin to precisely mannequin Earth in any quantitative manner. I’m simply hoping to see some normal qualitative similarities between dynamics of the mannequin and a few of the dynamics on Earth.
The determine exhibits how floor temperature (pink curve) and the temperatures of the 2 layers of the ambiance (inexperienced and blue curves) differ with latitude.
The floor temperature (pink curve) rises as one strikes from the polar area in direction of decrease latitudes, till a latitude of 42º the place a threshold temperature of 25.four℃ is achieved. After that threshold level, the floor temperature rises solely very slowly, reaching 26.9℃ on the equator.
The floor temperature could be very chilly (-80℃) on the poles. It’s because the TW mannequin doesn’t account for the air and ocean currents which heat Earth’s polar areas.
Averaging the floor temperature over the globe, the typical floor temperature is 19.three℃, a bit of hotter than Earth. (In computing the typical, latitudes nearer the equator are weighted extra closely than latitudes nearer the poles, as a result of the floor has extra space at decrease latitudes.)
To grasp why the temperature plot seems to be because it does, it helps to have a look at convective cooling results, as proven beneath.
For latitudes beneath 52º, convection transports warmth between the 2 layers of the ambiance. For latitudes beneath 42º, floor convection transports warmth into the ambiance and kinds clouds that replicate a few of the incident shortwave radiation.
The onset of convection explains why the curve for floor temperature (in Determine 1) modifications slope at these two threshold latitudes.
(On Earth, the edge latitudes for the onset of main convection and ocean thunderstorms are nearer to the equator than they’re within the TW mannequin for the parameters I’ve chosen. The oversimplifications within the TW mannequin imply that one can select just a few of Earth’s parameters to suit correctly. I selected to roughly match the insolation and imply floor temperature values for Earth, on the expense of permitting the edge latitude to be considerably completely different than what’s noticed on Earth. I believe that is okay, as a result of I’m within the qualitative conduct of the mannequin, not absolutely the worth of any quantitative outcomes.)
These cooling results will also be plotted as a operate of floor temperature, as proven beneath.
One can see that floor cooling will increase quickly for floor temperatures above 25.four℃. This appears qualitatively much like what one sees in WE’s Determine three. This affords reassurance that the TW mannequin is reproducing a few of the local weather options that WE’s evaluation depends upon.
Let’s take a look at one other kind of graph that WE makes use of.
This chart exhibits floor temperature as a operate of complete downwelling irradiance on the floor throughout the TW mannequin. It’s notable that the slope of the curve drastically flattens for irradiance values above about 450 W/m². This seems to be qualitatively fairly much like WE’s Determine 2, although the particular irradiance threshold worth is completely different for the TW mannequin and for the Earth.
In Determine 5, temperature will increase monotonically with irradiance. This matches WE’s Determine three for land-based knowledge, however differs from WE’s Determine four for ocean-based knowledge. Within the latter determine, temperature above the edge declines considerably with growing floor irradiance.
Can the TW mannequin account for such non-monotonic conduct?
It seems variant of the TW mannequin reveals such conduct.
The straightforward type of the TW mannequin makes use of the identical adiabatic lapse price, 𝚪, in all places. In actuality, the adiabatic lapse price is dependent upon the extent to which water vapor is current. For moist air, the lapse price is smaller, and for dry air it’s bigger.
I’d anticipate that the ambiance is more likely to be extra humid the place floor convection (presumed to be above an ocean) is going on, and fewer humid the place there is no such thing as a floor convection. So, the lapse price for convection between the layers of the ambiance must be bigger when there is no such thing as a floor convection, and smaller when there’s floor convection. That’s the idea used within the variant of the TW mannequin that yields the temperature vs. irradiation curve in Determine 6.
In Determine 6, the temperature for a given irradiance drops as floor convection begins. That is qualitatively related to what’s noticed in WE’s knowledge for ocean areas.
As soon as once more, I really feel reassured that the predictions of the TW mannequin qualitatively reproduce what WE has seen in knowledge for Earth.
For the rest of this essay, I’ll follow the model of the TW mannequin that yielded Determine 5, since that mannequin is less complicated to know.
Response to Greenhouse Fuel Forcing
What does the TW mannequin predict will occur if the focus of greenhouse gases is elevated?
Let’s take into account a top-of-atmosphere (TOA) radiative forcing ∆F = 7.four W/m², which I perceive to be roughly the radiative forcing predicted to happen on Earth if the focus of CO₂ was quadrupled.
As I perceive climatologists’ use of the time period, radiative forcing is a measure of the radiative imbalance that may happen at TOA if greenhouse gasoline concentrations had been elevated, however the ambiance and floor had been in any other case unchanged thermodynamically. I assume which means all temperatures stay the identical, as do convection and cloud protection.
Primarily based on this understanding, a TOA imbalance of seven.four W/m² happens within the TW mannequin if the longwave absorption fraction, f, is elevated from f = zero.600 to f = zero.631. So, to compute the impact of a TOA forcing ∆F = 7.four W/m², I re-ran the TW mannequin for f = zero.631, fixing for the brand new temperatures, convective warmth flows and cloud-induced albedo will increase.
The previous and new temperatures are proven beneath.
The imply world floor temperature within the TW mannequin will increase by 1.84℃.
Please don’t connect significance to this explicit worth. I don’t consider absolutely the magnitude of this quantity to be significant, given the restrictions of the TW mannequin. What’s more likely to be significant, nevertheless, is how this worth compares to different predictions of temperature change related to the identical mannequin.
Checking WE’s Process for Predicting Response to Forcing
As I perceive it, WE’s process for computing the Floor Response to Elevated Forcing goes like this:
- For a given TOA radiative forcing worth ∆Fₜ, compute an equal improve in downwelling floor irradiance, ∆Fₛ. In WE’s instance, on Earth, a TOA forcing of ∆Fₜ=three.7 W/m² was thought to result in a downwelling forcing 1.three instances as massive (presumably resulting in a ∆Fₛ=four.eight W/m² improve in downwelling radiation).
- Given a graph of floor temperature T₁ versus floor irradiance 𝚽, compute the spinoff dT₁/d𝚽. (That graph may be WE’s Determine three or four or my Determine 5.)
- At every level on the planetary floor, compute the temperature change ∆T₁ as ∆T₁ = ∆Fₛ × (dT₁/d𝚽).
Let’s name this process Temperature-Irradiance Curve Following, or TICF. TICF would possibly or may not be an correct illustration of the process that WE is advocating. He can tell us. Regardless, we are able to consider how properly TICF works with respect to the TW mannequin.
With regard to step #1 above, evaluating the imply complete (SW+LW) floor irradiance earlier than and after making use of the ∆Fₜ=7.four W/m² TOA forcing (i.e., earlier than and after growing f from zero.600 to zero.631), the imply complete floor irradiance will increase by ∆Fₛ=12.15 W/m². (So, on this case ∆Fₛ=1.6 × ∆Fₜ.)
After I apply step #2 to my Determine 5, then apply step #three utilizing ∆Fₛ=12.15 W/m², and common over the floor of the planet, the TICF process predicts a imply world floor temperature change of 1.02℃. That’s 45 p.c lower than the “precise” imply temperature change worth of 1.84℃ produced by the TW mannequin.
So, the TICF process didn’t do an excellent job of predicting temperature modifications within the TW mannequin.
Why Doesn’t TICF Predict Temperature Appropriately?
The TICF process is interesting intuitively. So, why doesn’t it appropriately predict temperature modifications?
So far as I can inform, there are two methods by which the TICF process as I outlined it goes incorrect.
One drawback with TICF, as I’ve outlined it, is that the floor irradiance forcing ∆Fₛ is assumed to be a relentless that’s unbiased of latitude.
Let’s take a look at how the floor irradiance modifications when the forcing is utilized (i.e., when f =zero.600 modifications to f =zero.631).
The pink curve (∆ SW+LW) signifies the change in complete irradiance absorbed by the floor, ∆𝚽. As one can see, that is nowhere close to being a relentless. It relies upon strongly on latitude, 𝛳.
Let’s assume we all know ∆𝚽(𝛳), and check out utilizing this to foretell temperature modifications through the method ∆T₁ = ∆𝚽(𝛳) × (dT₁/d𝚽). We might name this process Spatially-Various-Forcing TICF, or SVF-TICF. (This process isn’t more likely to very helpful in follow, even when it really works, as a result of anybody who is aware of ∆𝚽(𝛳) in all probability additionally already is aware of the temperature change.)
How properly does SVF-TICF predict temperature modifications?
The chart above exhibits the change in floor temperature, as a operate of latitude, as predicted by TICF, as predicted by SVF-TICF, and as within the precise answer of the TW mannequin.
It’s obvious that the TICF process which assumes a relentless forcing ∆Fₛ (inexperienced curve) matches the precise reply (pink curve) nearly nowhere. It’s no marvel that its prediction of the change in imply floor temperature is manner off.
What about SVF-TICF? For latitudes between 90º and 44º, the SVF-TICF predicted temperature (blue curve) change carefully tracks the “Precise” temperature change throughout the TW mannequin (pink curve). So, that’s an enchancment.
Nonetheless, whereas it may be a bit of tough to see within the chart, for latitudes between 44º and 0º, the TICF (inexperienced curve) and SVF-TICF (blue curve) predictions be a part of collectively, and each predict tropical floor temperature will increase a lot smaller than the “Precise” outcome (pink curve).
As a result of a world common weights low latitudes strongly, the imply world floor temperature improve predicted by SVF-TICF is 1.05℃, simply barely bigger than the 1.02℃ predicted by TICF, and nonetheless a lot lower than the precise improve of 1.84℃.
So, even with correct details about spatial variations within the downwelling irradiance forcing, the TICF process fails to precisely predict temperature modifications.
What’s the core drawback with TICF?
TICF is dependent upon the idea that the curve of floor temperature vs. floor irradiance is fastened, and “forcing” will merely trigger completely different areas on the floor to vary the place they seem on this fastened curve.
So, the process is critically depending on the temperature vs. irradiance curve not altering.
Sadly, the curve does change.
As seen within the chart above, the curve of floor temperature vs. complete floor irradiance superficially seems to be principally the identical earlier than and after the forcing is utilized. However what’s going on to the higher proper? Let’s take a look at that half extra carefully.
As soon as complete floor irradiance exceeds about 450 W/m², the “preliminary” and “ultimate” curves are completely different. Sadly, this area of the graph applies to a majority of the floor space of the planet.
If a forcing raises the temperature of the higher ambiance layer (as will be seen to occur in Determine 7), this will increase the temperature at which tropical thunderstorms “cap” the floor temperature. That is what shifts the temperature vs. irradiance curve.
To generalize this outcome a bit, the temperature vs. irradiance curve within the TW mannequin is unchanged by forcing in areas the place vitality switch is fully radiative, however the curve modifications in areas the place convection is essential.
Since convection and atmospheric circulation are essential, albeit to various levels, nearly in all places on Earth, it appears possible that the temperature vs. irradiance curve on Earth would possibly shift in response to forcing.
Thus, the TICF process appears unlikely to be efficient in precisely predicting floor temperature modifications in response to forcing.
Do Tropical Clouds and Convection Average Warming?
WE has instructed that tropical cumulus cloud formation and thunderstorms (supporting robust convective warmth flows) assist to reasonable Earth’s temperature.
Let’s see what the TW mannequin has to say about this speculation.
I redid the temperature change calculation, holding some components fastened. As soon as once more, I assumed a 7.four W/m² TOA forcing, modeled by growing the longwave absorption fraction from f=zero.600 to f=zero.631. The outcomes for the rise in imply world floor temperature had been:
- 2.61℃: cloud albedo and convective warmth switch held fastened.
- 2.05℃: cloud albedo held fastened and convective warmth switch allowed to regulate.
- 1.84℃: cloud albedo and convective warmth switch each allowed to regulate.
So, if a researcher didn’t account for elevated cloud albedo, they might predict a temperature change 11 p.c bigger than what truly occurs within the TW mannequin. If a researcher didn’t account for each elevated cloud albedo and elevated convection, they might predict a temperature change 42 p.c bigger than what truly occurs.
(I’ve the impression that the GCM pc codes utilized by climatologists all mannequin convection modifications. Some studying means that GCM’s sometimes additionally mannequin cloud. However, I’m not an skilled on GCM’s and would moderately not get right into a debate about them. Let’s follow speaking about what the TW mannequin tells us.)
(The TW mannequin possible overestimates the cooling impact of clouds as a result of the mannequin accounts for elevated reflection of daylight from clouds, however doesn’t account for elevated longwave absorption and emission from clouds, which are likely to have a warming impact. On Earth, on common, longwave warming by clouds compensates for about 60 p.c of the shortwave cooling by clouds, though cooling results predominate extra strongly within the tropics, cf., WE Determine 2.)
The underside line is that the speculation that “will increase in tropical cloud formation and convection reasonable planetary warming” is legitimate throughout the TW mannequin.
Do Tropical Clouds and Convection Cap Warming?
The outcomes of the TW mannequin do not assist the speculation that “tropical cloud formation and thunderstorms place a onerous restrict on planetary temperature will increase.”
Within the TW mannequin, there’s not an absolute “thermostat” impact that forestalls tropical floor temperatures from growing in response to a forcing. On the contrary, the temperature on the equator elevated by 1.46℃ in response to the forcing.
What could also be complicated is that there’s a relative “thermostat” impact that forestalls tropical floor temperatures from growing an excessive amount of, throughout the context of a given higher ambiance layer temperature.
So, within the context of the baseline TW mannequin, floor convection is triggered at a floor temperature of 25.four℃ at a latitude of 42º, and as soon as convection is energetic temperature will increase slowly to a most of 26.9℃ on the equator.
This would possibly make it seem that there’s a “thermostat” set at 25.four℃.
Nonetheless, after the forcing is utilized, floor convection is triggered at a floor temperature of 26.eight℃ at a latitude of 43º, and as soon as convection is energetic temperature will increase slowly to a most of 28.three℃ on the equator.
So, there nonetheless seems to be a “thermostat”, however after the forcing, the thermostat is ready 1.four℃ greater.
The explanation it really works this fashion is that the onset of floor convection is ruled by the lapse price and the temperature of the higher layer of the ambiance. The forcing prompted the temperature of the higher layer of the ambiance to extend by 1.four℃. Within the TW mannequin, this results in a corresponding improve within the “most floor temperature” in low latitudes.
The lesson to be realized from that is that, throughout the TW mannequin, tropical thunderstorms cap the utmost floor temperature, however solely relative to the temperature of the higher ambiance layer. If the temperature of the higher ambiance layer (i.e., the higher troposphere) will increase because of forcing, then the temperature restrict enforced by tropical thunderstorms will improve as properly.
Relating the TW Mannequin to Earth
How does the TW mannequin relate to Earth?
The TW mannequin sure leaves out many processes that are essential in Earth’s local weather. Earth’s ambiance contains many layers, and is affected by world circulation patterns within the ambiance and oceans, circulation patterns that are largely unaccounted for within the TW mannequin. The radiative dynamics in Earth’s ambiance are additionally extra advanced than these assumed within the TW mannequin.
The parameters used within the TW mannequin instance I’ve offered result in conduct that matches Earth in some methods (e.g., insolation and world temperature are no less than vaguely comparable within the TW instance and on Earth) however not in others (e.g., robust convection happens over a broader vary of latitudes within the TW instance than on Earth). The simplifications within the TW mannequin imply that its conduct can’t be quantitatively matched to that of Earth besides in a number of respects.
But, the TW mannequin contains the dynamics of the onset of convection in a manner that appears more likely to be no less than considerably related to the way in which issues work on Earth. Each within the TW mannequin and on Earth, convection is stimulated when floor warming causes the adiabatic lapse price to be exceeded. This creates a threshold impact that’s relative to the temperature of the higher troposphere.
I believe the TW mannequin is correct in portraying this facet of local weather physics.
Primarily based on working with the Thunderstorm World mannequin, plainly:
- It is sensible to be skeptical concerning the capacity of the TICF process to precisely predict how floor temperatures would reply to forcing.
- Tropical cumulus clouds and convection related to thunderstorms possible reasonable planetary temperature modifications, however aren’t possible to offer any fastened restrict on planetary warming.
APPENDIX: Mannequin Particulars
At a given latitude, 𝜽, the web vitality flows throughout the Thunderstorm World (TW) mannequin are as depicted within the following illustration.
For the calculations offered on this essay, the next parameter values had been used:
- Imply insolation, Sₐ = 292.6 W/m². (That is roughly the insolation Earth experiences, if non-cloud albedo is taken under consideration.)
- Convection adiabatic lapse threshold, 𝚪H = 30 Kelvin.
- Convection energy, Z = zero.2 /Kelvin.
- Convection reference energy, Sᵣ = Sₐ.
- Convection heat-transfer vs. cloud-formation fraction, 𝝌 = zero.7.
- Lengthy-wave radiation absorption fraction, f = zero.6 (previous to added forcing).
The world-weighted world common of a amount g(𝜽) is given by the integral from −𝜋/2 to 𝜋/2 of ½⋅cos(𝜽)⋅g(𝜽).
For the variable-water-vapor mannequin variant used to generate Determine 6, the adiabatic lapse threshold 𝚪₂H for convection between the decrease and higher ambiance layers is made to transition from 33 Ok to 30 Ok as floor convection begins. (Specifically, this transition is made as the worth of T₁’ − T₂’ − 𝚪₁H transitions from -1 Ok to 1 Ok, the place T₁’ and T₂’ are the values that T₁ and T₂ would have if solely radiative warmth flows had been current, and the place 𝚪₁H = 30 Ok. This barely odd recipe was chosen as a result of it was discovered to assist numerical convergence.)