Guest Post by Willis Eschenbach
I’ve been messing about with the “Modtran” online calculator for atmospheric absorption. It’s called “Modtran” because it is a MODerate resolution program to calculate atmospheric infrared absorption written in ForTRAN, which calculates the result for each 1 cm-1 wide band of the wavenumber across the spectrum. Not quite a “line-by-line” calculation, but close. Here’s a sample of the input page:
Figure 1. User input page for the Modtran online calculation for infrared absorption. Left side is user input. Upper right graph shows absorption as a function of frequency. The lower right graph shows the GHG concentrations, pressure, and temperature, as a function of altitude. See here for an overview of the model. Click to enlarge
This shows the situation during the subarctic summer, with no clouds or rain.
Along the way, I ran into a curious mystery, one for which I have no answer.
Here’s the peculiarity I found. I decided to see what Modtran had to say about the “instantaneous forcing”. This is the forcing immediately after a change in e.g. CO2 or other greenhouse gas. In Table 1 of “Efficacy of Climate Forcings” , James Hansen et al. say that the instantaneous forcing from a doubling of CO2 is 4.52 W/m2.
So I tested that claim with Modtran using a variety of different locations, with different combinations of clear skies, cloud, and rain. I started by testing every few hundred PPMV increase, to see if the results were linear with the log (to the base 2) of the change in CO2. Finding that they were perfectly linear, I then tested each situation using 375 ppmv, doubled CO2 (750 ppmv) and two doublings of CO2 (1500 ppmv). I noted the absorption at each level, and compared that to the logarithm (base 2) of CO2. That let me calculate the forcing, which is typically given as the change in forcing for a doubling of CO2. Using Modtran, I get the following results:
Figure 2. Instantaneous forcing calculated by Modtran for different scenarios.
Now, this has the expected form, in that the forcing is highest at the equator and is lowest at the poles. The addition of either rain or clouds reduces the forcing, again as we’d expect, except during subarctic winter when some kinds of clouds increase the forcing slightly.
So the mystery is, according to Modtran, the absolute maximum instantaneous forcing from a doubling of CO2 is 3.2 W/m2 in the clear-sky tropics. I can’t find any combination of locations and weather that gives a larger value for the instantaneous forcing than that. And the minimum value I can find is subarctic winter plus cirrus, at 1.57 W/m2. I can’t find any combination giving less than that, although there may be one.
As a result, according to Modtran the planetary average instantaneous forcing from CO2 doubling cannot be any more than 3.2 W/m2, and is likely on the order of 2.4 W/m2 or so … but according to Hansen et al., the real answer is nearly double that, 4.5 W/m2.
So the mystery is, why is the accepted value for instantaneous forcing nearly twice what Modtran says?
Note that the answer to the mystery is not “feedbacks”, because we’re looking at instantaneous forcing, before any response by the system or any possible feedbacks.
All suggestions welcome, except those that are anatomically improbable …
w.
DATA: Excel spreadsheet here. You don’t need it, though. For any situation, simply use Modtran successively for two CO2 values where one CO2 value is double the other, and note the difference in the calculated upwelling radiation. This is the instantaneous climate sensitivity for that situation.
THE USUAL: If you disagree with me or someone else, in your comment please quote the exact words that you disagree with. This lets everyone know your exact subject of disagreement.
NOTE: I see as I finish this that they have an upgraded user interface to Modtran here … the results are the same. I prefer the older version, the graphics are more informative, but that’s just me.
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“MODTRAN is very good at simulating the planet IR balance. This is because it was derived from the correct physics; 160 W/m^2 mean surface thermalisation. Because it’s proprietary, it can’t be ‘altered’. ”
MODTRAN can be “altered” in fact during the 1980s our group submitted many upgrades to LOWTRAN that made it into MODTRAN. But your changes have to be validated.
Willis Eschenbach says:
April 12, 2014 at 6:37 pm
“Yeah, those dumb scientist chumps …”
————————————————-
Willis, that paper was written by empiricists using the variances demonstrated by one flight to call for much more of the same type of empirical experiment –
“A major problem with estimates of the short-wave and long-wave DRE of dust aerosol using satellite data and radiative transfer modelling is the lack of concurrent detailed information of the aerosol optical properties, atmospheric vertical structure and surface reflection properties. This can only be overcome by multi-disciplinary experiments”
The argument of the authors of that paper are congruent mine. (only my experiment design is better 😉 )
“If the atmosphere were 10 meters deep. I might consider modtran predictions meaningfull. Modtran is just another simulation. No one it seems is intelligent enough to define, predict or measure molar absorptivity for gasses.”
MODTRAN is a ENGINEERING TOOL. we use it to build working Machinery that protects our nation.
Simple example. In the design of the F-22 and F-35 one critical aspect of survivability is the probability of detection and engagement by a ground based system.
Testing would consist of the following. Since the aircraft wasnt built yet you had to predict what its broadband signature would be.. either in RF ( Radar cross section) or in IR..
The IR signature would be a 3D volume in three different bands. One for Hot gas, one for aero heating, and one for hot metal parts.
These signatures would later be measured when you had real aircraft. but you had to use validated codes to predict your signature.
Next comes the ground site. Here we would simulate a ground based IR sensor. Sometime we had captured soviet systems. Sometimes we had to estimate the sensitivities of the systems based
on our projections of soviet technology. This was known as Threat Analysis ( I did that for about 5 years ). It basicaly comes down to S/N ratio and a probability of detection.
Now that you have a ground site and an aircraft you run a bazillion different profiles using lowtran or modtran ( depending on the requirements) Those profiles give you a probability of being detected and engaged by the ground site. then comes launch, guidance and kill estimation.
These profiles are basically footprint diagrams around sites.. and so you get Pd as a function of speed, altitude and signature. That is, given a signature and an altitude and a speed you get a detection footprint around the site.. you can also use this in mission planning to avoid detection.
So the same technology would be used in providing mission planning for a UAV that needs to penetrate an airspace undetected. Does it work? is the physics sound? ask the fricken bad guys.
Opps you cant, they are dead.
In any case. Look at the F-22. That aircraft was designed using these codes. These codes work well enough to protect our country. only a libtard would doubt the Airforce
Silly Willis. You just forgot to check the box for the “applyn 10.76 mw-ovens for 30 minutes” option under the Trenberth tab .
I am forced, from my experience and firsthand knowledge of relevant DoD systems, to confirm the substance of what Mosher is saying above. MODTRAN is indispensable in these systems and has proven its real world accuracy time and time again across many applications.
However, I cannot say I agree with the manner in which he said it. Can’t we treat each other well, even those who annoy us?
It would appear the Hansen et al worked backwards from their temperature estimate of 3C.
http://www.giss.nasa.gov/research/briefs/hansen_05/
There was an interesting post a while back from a chap who had somehow managed to engage Gavin Schmidt in a discussion (possibly by playing really dumb and impressionable). He managed to drop a really tough question as to what plans were in the works to change the models to more accurately reflect reality. Gavin’s reported reply was that the CO2 response in the models was an emergent property of the equations and that these equations could not be changed without affecting a lot of other stuff.
Still thinking about that but all it would take is a little LOTRAN code getting into HITRAN fed into a dynamic equation…
What the Air force was interested in was radiative signal from the air to the ground. This direction is akin to incoming solar radiation, a significant portion of which is near IR, and CO2 is a very small player in these spectra. I don’t know what temperature a plane radiates at but I bet it’s a lot hotter than the ocean.
Willis,.
What temperature profile did you use?
Over the “tropics” (actually over the half of the planet centred on the latitude at which the sun is overhead), the temperature profile is not “American Standard” (tropopause 11-20km) but a profile where the Tropopause is a single point of inflection at an altitude of between 14 and 18km.
The strong emissions from the centre of the CO2 absorption band, roughly 630-710 cm^-1, are from an altitude of 15km at least, if they are to get to Space, that is they are mostly from the Stratosphere, not the Troposphere, in the tropics.
The implication is that emissions to Space from this band will INCREASE in the “Tropics” when CO2 is doubled, whereas over the Temperate zones there will be little change in emissions because of the extensive tropopause.
So the “radiative forcing” increase obver the tropics should reduce due to the increased stratospheric emissions.
Willis, Nick and Mosher,
Your last comments are what make this site special. Real science gets sorted out and understood without a lot of bluster and BS. Well done!
The answer to you question is not a secret: positive feedback from water vapor.
That’s why Svensmark’s theory is so important to confirm or falsify.
If Svensmark’s theory is correct, then under certain circumstances, water vapor in the atmosphere causes cooling by negative feedback.
ckb says:
April 12, 2014 at 8:39 pm
Dang … and here I was thinking that Mosher was being quite mellow …
in any case, what Mosher said.
w.
Frederick Colbourne says:
April 12, 2014 at 9:58 pm
Frederick, perhaps you missed this part of the head post:
w.
Willis,.
The profiles are all set by the model. There are six profiles: tropic, midlatitude summer and winter, subarctic summer and winter, and US standard atmosphere. If you set the “Save” dropdown in Modtran to save the results, you can then run a scenario, take a look at the results, and see the entire thermal profile.
w.
“””””…..dbstealey says:
April 12, 2014 at 7:27 pm
Another very interesting article from Willis. This is habit forming.
Also, george e smith is smarter than I am. He says:
I think the whole idea is rubbish (of CO2 induced warming)……”””””
I hasten to point out; nobody should think that I believe CO2 and other GHG don’t do anything in the atmosphere. That is NOT my point. The physics of molecular vibrational and rotational absorption spectra, which are typically manifested in the LWIR (and longer) region is well established, and that is NOT a hill “skeptics” should choose to die on.
But there are a lot of things, which do stuff in the atmosphere physically, that are well researched.
But do you guys, and gals ever read and understand some of the stuff Willis puts up here.
Did you not read his account of the MEASURED drop in surface solar brightness, in Hawaii, due to attenuation by major volcanoes, that persisted for some months. This is short wave solar energy that did not reach the ground.
And Willis showed that the Temperature signal from that MEASURED drop in surface solar irradiance, DID NOT even show up in the MEASURED Temperature record (even locally).
Roy Spencer nailed it; there’s a very powerful negative feedback loop operating via the clouds that simply wipes out all of these gnat level pestilences like CO2 and aerosols, and TSI variations, and a whole host of other things.
So long as Earth has its oceans, we couldn’t change the Temperature of this planet, if we wanted to.
And if we could, what would YOU set the thermostat to ?? And WHY ??
Well said that guy, George. Conclusions are so much more interesting than graphs etc.
I wrote, “Something has changed in the online MODTRAN calculator. It produces somewhat different results than it used to. Ref. note #2, here.”
Ferdinand Engelbeen replied, “Indeed, in the current online version refered by Willis, you still can choose between water vapour influence as pressure or relative humidity, but the results are exactly the same”
And Willis Eschenbach replied, “Thanks, Dave. According to your data at the link, the 2xCO2 forcing is 3.2 W/m2 … which is identical to my results in Figure 1. So I’m not sure what you mean. I calculated the CO2 forcing from your data as the I_out for 285 ppmv minus the I_out for 570 W/m2.”
It’s the “constant relative humidity” calculations which have changed. I’ve made an updated version my spreadsheet with columns calculated using the new and old MODTRAN pages, so that you can see the differences, here:
http://www.burtonsys.com/climate/MODTRAN_etc4c.html
First I asked MODTRAN to calculate Iout for various CO2 levels at ground level temperature of 299.7 K, and compiled them in column C. Those numbers are identical for the two MODTRAN versions.
(Note: the “current” CO2 number was chosen as 392 ppmv, because I first did these calculations in January, 2012.)
Then, using constant water vapor pressure (i.e., no water vapor feedback) I found the temperature offsets needed to maintain the “current” (Dec. 2011) Iout at each of the other CO2 levels. (Some trial & error was required.) Those results are compiled in column D, and the numbers were again identical for the two MODTRAN versions.
Then I did the same thing using constant “relative humidity” instead of constant water vapor pressure (i.e., including water vapor feedback). The results from the old MODTRAN page are in column E, and the results from the new MODTRAN page are in column F. They are very different!
The results from the old MODTRAN page showed water vapor amplification for clear sky and tropical atmosphere to be about +65%, but the new MODTRAN page calculates it to be only about +8%, i.e., almost, as Ferdinand said, exactly the same as with constant water vapor pressure.
I also did those calculations for one of the supported cloud scenarios; those results are in columns G & H.
Back in January I started to compose an email to the folks at U. Chicago, asking them about the discrepancy, but I never finished it and sent it. Thanks to this thread, I finally got a round tuit tonight. Perhaps they can figure out what’s wrong.
The answer is in the source code – Fortran reads easy, unless the coder has NO style and desire to document with clear and meaningful comments.
Otherwise, someone is lying, guess who ? ( should be easy )
Berényi Péter says: “And indeed, according to RSS, during the past 35 years temperature of the lower stratosphere has decreased at an average rate of -0.28 K/decade. The only trouble is that almost all the decline happened close to the beginning of observations, during the last 2 decades this rate is only -0.03 K/decade, almost an order of magnitude smaller.”
Actually the change in stratospheric temps has everything to do with volcanoes and nothing to do with CO2: Note how the ever increasing CO2 since 1998 has no visible impact:
http://climategrog.wordpress.com/?attachment_id=902
Drawing straight line “trends” through anything in climate is nothing but deceiving. The practice should be banned and those who insist should be put before some kind of Nuremberg tribunal for crimes against humanity 😉
There is another way to look at this issue.
Radiative forcing is based on the assumption of an ‘equlibrium atmosphere’ in which perturbation theory is used to ‘predict’ the effects of an increase in atmospheric CO2 concentration on surface temperature. It is a mathematical ritual that has no basis in physical reality. There is no equilibrium, and it goes downhill from there.
MODTRAN is a medium resolution radiative transfer program that was developed initially by the Air Force Geophysics Lab. It is a lower resolution adaptation of the HITRAN database that is much less computer intensive. Rather than follow the radiative forcing ritual, MODTRAN gives a ‘snapshot’ of the atmospheric flux based on the spectral data and the given atmospheric temperature and species profile.
The Earth’s surface temperature is determined at the surface by the net heat stored in the surface layer. MODTRAN can provide the downward LWIR flux reaching the surface by setting the altitude to zero and looking upwards. When the CO2 concentration is increased from 280 to 380 ppm, the increase in downward LWIR flux reaching the surface is approximately 1 W m^-2. The values depend on the atmospheric model chosen and the defaults used in the U. Chicago version of MODRAN.
The diurnal solar flux is zero at night and may reach 1000 W m-2 in the middle of the day for full summer sun/tropical conditions. There is no such thing as an average solar flux. Over land, the solar heat is dissipated by a combination of moist convection and net LWIR flux. Some of the heat is also conducted below the surface and stored in the ground where it is released later. Daytime surface temperatures can easily reach 50 C. In order to determine the change in surface temperature it is necessary to perform the full engineering calculation of the short term surface energy transfer in 0.5 to 2 hr steps.
The correct physics question is then ‘How does the increase in LWIR flux at the surface change the surface temperature?’ Using the 2008 data from the Ameriflux UC Irvine ‘grasslands’ site and a value of 1.7 W m-2 for the increase in LWIR flux from CO2, I obtained an approximate upper limit of 0.07 C.
Now, this is for the surface temperature. The weather station temperature is the meteorological surface air temperature (MSAT) measured at eye level in a ventilated enclosure, 1.5 to 2 m above the ground. There can be no CO2 signature in the MSAT record. The ‘hockey stick’ is based on changes in ocean surface temperatures. The weaterh station temperature climate record is dominated by the AMO, because of the area averaging etc.
The oceans behave rather differently. For ‘pristine’ ocean, 50% of the solar flux is absorbed in the first 1 m depth and 90% is absorbed in the first 10 m depth. The LWIR flux is absorbed and emitted within the first 100 micron. The dominant ocean cooling term is wind driven evaporation, which also coupled to the surface. A change of 1 W m-2 in the LWIR flux has no effect on the measured ocean temperature.
If you dig a little deeper into the radiative transfer calculations, about 75% of the downward LWIR flux reaching the surface comes from the first 100 m layer of the atmosphere. (This value also depends on the humidity). Almost all of the downward LWIR flux comes from the first 2 km layer. The moist convection transports the heat to higher altitudes where it is radiated to space. The upward radiation to space by the atmosphere is decoupled from the surface temperature. Some of the LWIR flux however, also escapes to space through the LWIR transmission window (~800 to ~1200 cm-1).
Rather than argue over meaningless forcings, feedbacks and climate sensitivities, perform the real engineering calculations of the climate heat transfer. CO2 induced global warming then disappears into the noise of the surface fluctuations. There is no such thing as an average climate, average temperature or average flux.
References
Akasofu, S-I, Natural Science 2(11) 1211-1224 (2010), ‘On the recovery from the Little Ice Age’
http://www.scirp.org/Journal/PaperInformation.aspx?paperID=3217&JournalID=69
Clark, R. Energy and Environment 24(3, 4) 319-340 (2013) ‘A dynamic coupled thermal reservoir approach to atmospheric energy transfer Part I: Concepts’
Clark, R. Energy and Environment 24(3, 4) 341-359 (2013) ‘A dynamic coupled thermal reservoir approach to atmospheric energy transfer Part II: Applications’
(These 2 papers are paywalled. Most of the info is available at http://venturaphotonics.com/RootCause.html)
Goulden, M. L., 2012, Ameriflux Data, Grasslands Site Data
http://ameriflux.ornl.gov/fullsiteinfo.php?sid=193
Manabe, S. and Wetherald, R. T., J. Atmos. Sci., 24 241-249 (1967), ‘Thermal equilibrium of the atmosphere with a given distribution of relative humidity’ http://www.gfdl.noaa.gov/bibliography/related_files/sm6701.pdf
Yu, L., J. Climate, 20(21) 5376-5390 (2007), ‘Global variations in oceanic evaporation (1958-2005): The role of the changing wind speed’
Yu, L., Jin, X. and Weller R. A., OAFlux Project Technical Report (OA-2008-01) Jan 2008, ‘Multidecade Global Flux Datasets from the Objectively Analyzed Air-sea Fluxes (OAFlux) Project: Latent and Sensible Heat Fluxes, Ocean Evaporation, and Related Surface Meteorological Variables’ (Available at: http://oaflux.whoi.edu/publications.html)
What’s the definition of parameter Water Vapor Scale in MODTRAN?
Once a number is settled on, it has to be integrated (averaged) over geographic conditions, diurnal and seasonal cycles before we get a true comparison with the 1365/4 number for inbound flux. Does anyone have a reference to where this has been done?
Pekka gave the first correct answer:
Pekka Pirilä says: April 12, 2014 at 1:20 pm
“The main reason for that is in the influence of stratosphere. … Results calculated at 17 km looking down are closer to the correct ones, but fully correct values require a model that makes the stratospheric corrections.”
Later, Nick Stokes gave the same answer, but actually provided numerical results, which was more useful.
Nick Stokes says: April 12, 2014 at 6:30 pm
“So I calculated Modtran at 17km, tropics.
375 ppmv CO2 – 289.0 W/m2 upwelling
750 ppmv 284.5 W/m2.
Difference 4.5 W/m2.”
Willis then noted we have to calculate the net flux at the tropopause, up – down flux, which increases the forcing to CO2 doubling in the tropics to 5.6 W/m2.
The IPCC says the global CO2 forcing is 3.7 W/m2 per doubling at the tropopause. I used Modtran to calculate the total earth CO2 forcing and got 4.62 W/m2, significantly higher than the IPCC value. Modtran calculates clear sky values in three regions; tropics, mid-latitudes and polar. I assumed the year average is the average of the summer and winter forcings for mid-latitudes and polar region.
The tropopause height and the net radiative flux for the three regions are:
Region Tropopause Net Flux
Tropics 17 km 5.60 W/m2
Mid-Lat 14 km 4.93 W/m2
SubArctic 10 km 4.27 W/m2
Assuming for the northern hemisphere, Tropics is 0 – 30 N, Mid-Lat is 30 – 60 N, SubArctic is 60 – 90 N, the clear sky Earth is 5.14 W/m2, weighted by the surface area of each region. Assuming a 65% cloud fraction, and Willis’ calculated cloud forcings for the tropics, the ratio of all-sky to clear sky forcing flux is about 90%. This gives the all-sky Earth forcing of double CO2 at 4.62 W/m2. These calculations are shown here:
http://www.friendsofscience.org/assets/files/Modtran.xls
So the real mystery is why Modtran gives such high CO2 forcing numbers. I suspect the answer is that the water vapor concentrations assumed in Modtran are too low. In my calculations I am not using the 1976 US Standard Atmosphere, which is known to have too low water vapor. But I suspect that the three regions also use too low water vapor, which would make the calculated CO2 forcings too high.
Maybe it would be smart the remove CO2 in total, because the effect is zero. Every time CO2 comes in it is a theoretical deal. CO2 is the effect, not the cause. CO2 comes after water vapor, not before.
That’s why you get strange answers all the time because you do the science wrong just like AGWers do.
Lapse Rate. The higher radiation goes before it encounters a CO2 molecule the less energy it will radiate back.
Willis Eschenbach says:April 12, 2014 at 7:30 pm
“This is one reason that I find this whole approach crazy. The problem is that it’s not possible to measure something “at the tropopause” in any but the most general way. In addition, I fail to see the diagnostic value in measuring the radiation in the middle of the atmosphere. If you do that, then if the stratosphere warms slightly and the troposphere cools slightly, the “TOA” radiation changes with no change in the global energy balance. How does that not make it useless as a tool to diagnose changes in the overall energy state of the system?”
At the end of the day I find we are in agreement. Thanks again for spawning a great thread. I wasn’t disparaging good work done by smart people,simply trying to stay humble about what we are able to do. The discussion gets lost in radiative physics and I just can’t divorce it from all of the kinetic stuff that dominates the world I live in down here in the troposphere, giving rise to the need for barf bags, dramamine and weathermen. (Hat tip to Anthony). The world is full of curve balls, wind shear and rogue waves. Stay safe.