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 Simple Radiative Models of the AtmosphereModerators: Forum Managers, ldavidcooke Jump to page : < ... 7 8 9 10 11 12 13 > Now viewing page 13 [20 messages per page] | View previous thread :: View next thread |
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ldavidcooke |
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 Moderator Location: Graham, North Carolina | Hey Jan; Just a quickie OT, as to Bessel Functions, in FM signaling the amplitude is changed to slope or frequency around a central frequency. However, the dynamic range cannot be translated to the amplitude of the signal, hence it has to be encoded into the frequency variation. The extraction of dynamic range in ann FM signal, to some degree, is similar to the AM Vestigial Sideband used for Color in NTSC. We did a number of experiments at AT&T in the late 50's and early 60's prior to the digitization of analog with a 4 bit sampling of each waveform until we found we could achieve a reasonable simulation with only a two bit sampling. However, before we got to this point the idea had been to simply pump the analog into a varactor circuit in which the variation in amplitude would simply drive the change in frequency. Initially, we would overdrive the signal to develop varying pulse widths, place the pulse widths in a digital frame and forward the frame. The idea had been all we needed to do on the far end was to put the signal in to the driving circuit of a Phase Lock Loop circuit and we could recreate the original frequency. (However, we found we would have needed a secondary circuit to also digitize the amplitude of the frequency.) We found that to combine the signals we needed to multiplex them and it was much easier to assign a code to the various pulse widths and simply forward the codes, which resulted in the ability to multiplex multiplex multiple signals on a single circuit. This lead to the adding codes for the amplitude as well. Where in the analog world you would encode the data in a similar method in the relation of the frequency to the slope angle. We had moved to a encoding of the frequency at a given amplitude and forwarding these codes in frames and combining multiple frames into a Super Frame. Dave Cooke | ||
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| Jan Pompe |
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Location: Sydney Australia | Hi David, Just a quickie OT, as to Bessel Functions, in FM signaling the amplitude is changed to slope or frequency around a central frequency. Do you mean phase here? Phase and frequency are related in that the differential of the phase is frequency so in a sense frequency is the differential of phase in an angle modulated signal. | ||
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| ldavidcooke |
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Moderator Location: Graham, North Carolina | Hey Jan; Actually phase is not necessarily part of the modulation, as the change in frequency provides the change in slope of apparent phase. The change in frequency (FM)is in essence similar to pulse width modulation However, the frequency alone does not represent the character of the signal. As most analog data is a combination of amplitude and frequency we have to embed both characteristics into the FM signal. To shift the phase of a given frequency signal is in essence a form of pulse width modulation. Normally, slope angle is associated with the frequency or the changes in the time period between peak sinusoidal voltages, Where phase change may be more similar to pulse width modulation or the distance between the rising voltage and falling voltage in a wave form. I have seen the use of both methods in a analog signal in which the voltage peaks occurred in a specific rage for a given input voltage, however, the signal at between 2/3 and -2/3 of the peak voltage may have a period which is non-linear with regard to the sinusoidal signal. I do not know that this was the technique of modulation used in FM Radio Broadcast. Phase Modulation did not seem to become part of any analog signaling until we get to the Color Burst on the NTSC Video Signal "Back Porch". Could you help me to see your point? It is very difficult to instantaneously change the phase angle for at given frequency unless you use a device something like a butter worth filter to maintain specific sinusoidal frequency while you modify the pulse width. However, it has been several years since I was in the middle of this technique... Dave Cooke | ||
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| Jan Pompe |
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Location: Sydney Australia | Hi David, Actually phase is not necessarily part of the modulation, as the change in frequency provides the change in slope of apparent phase. It isn't part of pulse density because there is no carrier to compare. Phase and frequency modulation are very much related, in that you are not going to get one without the other in the same signal and the instantaneous phase is the time integral of the frequency variations. We can this from the general formula for frequency modulation x(t) = Accos[ωct + φc + Amsin(ωm + φm)] that while the carrier frequency term ?ct remains constant the phase changes by the modulating term (Amsin(ωm + φm) of course the sine is the integral of the cosine. There are a number of means of frequency modulating a signal the most common and simplest is to use a voltage controlled reactance in an oscillator or to phase shift a sinusoid from a crystal locked oscillator. We can pass a pulse density signal through a low pass filter and recover the modulating signal you can't do that with FM or PM all you'll get there is a reduction in signal strength. The variation in solar intensity seems to have a floor and the sunspot cycles are increasing excursions in intensity above this so they can be viewed as pulses and the variation in density over the 87 year Gleissberg cycle as the modulating signal and the ocean a large storage device acts as a low pass filter so we should I suspect see that finger print in the Sea Surface Temperature though it may have too long a time constant. Edited by Jan Pompe 16/12/2007 08:38 | ||
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| Jan Pompe |
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Location: Sydney Australia | Hi David, I forgot to mention Phase Modulation did not seem to become part of any analog signaling until we get to the Color Burst on the NTSC Video Signal "Back Porch". The colour signal is a Quadrature Amplitude Modulation which is a species of AM where two carriers 90o out of phase carry a different amplitude modulated signal which can be separately detected. One phase carries R-Y (red - luminance) the other B-Y the green can be recovered by adding and subtraction the colour burst is simply a reference for a phase locked local oscillator needed to detect the phase so the right colour signal is detected be the right circuit so the right additions and subtractions can be made or we get strange colours. I'm surprised I have remembered this much, more detail is bound to give me a headache and is OT so please let's drop it. Perhaps the right thing to do is to start a new thread if we want to discuss insolation signatures, if we can find them etc, in more detail. | ||
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| gmoran |
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| I’ve noticed that the atmospheric greenhouse effect explanations at Wikipedia, RC, IPCC 4AR all seem to involve a violation of 2nd law TD (sun heats earth, earth heats atmosphere, atmosphere heats earth a bit more). Obviously this has been raised before and I’ve seen rebuttals from Eli, Atmoz and at RC; unfortunately I didn't find them convincing. These appear to be the possibilities: 1. IPCC and mainstream climate science have missed something! Seems somewhat unlikely. 2. It’s semantics. GHGs warm the atmosphere, reduce heat loss etc., raise average temperature. 3. Complexity: appears to violate 2nd law, but doesn’t for technical reasons not obvious to laity. I've raised this at ClimateAudit, however Steve doesn't like these discussions, because they confuse what he is trying to do; no explanations so far, and I don't expect one. I've done some very direct googling at RC; and it's clear that they really do believe that back radiation heats the surface, see this: http://www.realclimate.org/?comments_popup=483 465. [[ We both know that neither a greenhouse interior nor the earth’s surface is warmed by back radiation]] No matter how many times you repeat this, it still won’t be true. The Earth receives 324 watts per square meter, on the average, of back radiation from the atmosphere. We’ve measured it with instruments. It’s there. Deal with it. Comment by Barton Paul Levenson — 9 November 2007 @ 12:23 PM The basic idea appears to be that the earth is struggling to shed its excess radiation? GHGs raise the height of the radiation window in the atmosphere, and that the surface warms in response.This sounds like a complex but clear violation of 2nd law? Also there seems no attempt to deal with this paradox? Is this a case of 3 above (complexity)? I know I must be missing something, but I can't see it, and frankly it's making me feel like I'm losing it. An informed and succinct explanation would be appreciated. | |||
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| Jan Pompe |
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Location: Sydney Australia | Hi Gary, No matter how many times you repeat this, it still won’t be true. The Earth receives 324 watts per square meter, on the average, of back radiation from the atmosphere. We’ve measured it with instruments. It’s there. Deal with it. He gets that number for back radiation from the J. T. Kiehl and Kevin E. Trenberth for cloudy skies. The number is bigger than they have for radiation from the sun which they have at 168 watt/m2. A rather odd state of affairs IMHO. Aspects of this paper were discussed here you might like to take a look and comment. | ||
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| cba |
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| Gary, Ultimately, it's simply about how much energy comes to the earth from the sun and how much radiates off into space. When these numbers balance out, there is equilibrium of a sort and the system will continue to stay as is. If there is more arriving than leaving, it will warm up and if there is more leaving than arriving, it will cool off. Everything with a temperature will try to radiate energy off. In a closed room at uniform temperature, a ball hanging from a string in the center will radiate off energy based upon its temperature. In this case the walls being the same temperature will radiate energy back and the ball will remain at the same temperature as the rest of the room. If you hang the ball outside, the ground will radiate energy based upon its temperature and the sky will radiate energy back towards the ball. Assuming the temperature of the ball and ground are at 300K (room temperature) the ball will radiate about 400W/m^2 (assuming it's a good radiator) and so will the ground. The bottom half of the ball will basically get back the same amount of radiation that it emits while the top half will emit about 400W/m^2 and get back 324W/m^2 so it is losing 76 Watt/m^2 and is cooling off unless there is a heat source inside it. Note too that the 324 is an average and isn't present everywhere at all times. Note, if this weren't the case, you'd be staring at 2.7K deep space during the night with approximately 0 W/m^2 coming back and so at room temperature, you'd have 400 W/m^2 radiating out to space. Ignoring the drop in T and the lower radiation rates due to less T, if you had a pan of h2o at 300K that was 1cm thickness of h2o and assuming you set it on an insulator so that the surface radiation wouldn't affect it, you'd have 10,000 g or 10kg of h2o radiating at 400W/m^2. Note it takes around 4 joules of energy added or removed to change the temperature of a gram of h2o 1 deg and 1 watt is 1 joule / second. Over an hour's time 400W is 1440 KJ of energy. For 10 kgs of h2o, this is 144 joules per gram. Dividing by 4 j/deg K we'd get a 36 degree drop - except that it can only go down about 26 degrees before hitting the freezing point and it will take an additional 2 hours of radiating away energy for there to be the 333 joules necessary to freeze the gram of h2o. Note too, in reality the power being radiated would decrease by 40% by the time it reached 273K so it would take even a bit longer since only about 274 W/m^2 would be radiated by then. As for 2nd law, this is not a closed system. Also, you must recognize that the ht flow is from hot to cold net and not no flow from cold to hot. Cold will radiate if above absolute 0, just not as much as hotter objects will radiate so there is still net flow from hot to cold. Finally, AGW assumptions when discussed look pretty much at just radiative factors (although their fancy models are supposed to have convection - but GIGO). Convection is important and is already in play, mixing the atmosphere and transfering heat. My guess is when they are able to precisely predict the weather for several months in advance, right down to the minute a cloud will form over lower slobovia and precisely rain out on the left bank of the upper mubatto, then, they might have a chance of getting their detailed modeling to more or less function for climate prediction after ignoring all unforeseen transients. But then, that's probably a big stretch on my part. Other than that, the most that can be hoped for is to ascertain what effects combine in what fashion to affect the climate. | |||
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| gmoran |
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| Jan Thank you. I'd already read the debate with Hans Erren thread (in fact it's what piqued my interest in the physical aspects of GHGs in the first place). I've been re-reading it again, and there is a lot on 2nd law. Yes I find the numbers odd; however my maths is nowhere near good enough to comment further :( . Cba If I'm reading you right, you (and I think De Witt Payne in the other thread) are saying that net heat flow is from the surface to the atmosphere, so it doesn't matter that so much is being back radiated from the atmosphere, and I can accept that. That also appears to me to be saying that the atmosphere isn't warming the surface, however over time the surface is warmer than it ordinarily would have been because heat loss is reduced? Again, that's fine. In which case it's a case of Semantics. Also you say in relation to 2nd law, this isn't a closed system, something I've heard before, and which didn't make sense. I thought a closed system was one in which mass cannot be exchanged. Also that there is a further case: an isolated system in which neither mass nor energy can be exchanged. I would have thought the earth was a closed system, not being able to meaningfully exchange mass? Thanks for anything you can help with here. Edited by gmoran 21/12/2007 10:53 | |||
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| Jan Pompe |
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Location: Sydney Australia | Hi Gary, Also you say in relation to 2nd law, this isn't a closed system, something I've heard before, and which didn't make sense. The 2nd Law is a rather tricky one, but it doesn't only apply to closed systems it also applies to heat pumps which are anything but closed. Of course the direction of heat flow also refers to open systems as well as closed. The only statement of the law that applies to closed systems is the entropy statement. The problem with the K&T paper and the comment you posted that the 324 W/m^2 downward flux is measured, is that the paper doesn't ever mention where, how or when it was measured. Those guys are pretty sharp they are not going to make trivial mistakes like not have the system add up. They do seem to get this extra energy doing the rounds up and down coming out of nowhere and none of it ever seems to get lost. Might be a violation of the first law but then it's not being created or destroyed, and there really are no conservation laws for radiation, it's just there. Like I said odd. | ||
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| gmoran |
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| thanks Jan after googling around that was my conclusion regards 2nd law and closed systems, good to have it confirmed. | |||
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| abmcdonald |
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gmoran - 21/12/2007 10:32 I’ve noticed that the atmospheric greenhouse effect explanations at Wikipedia, RC, IPCC 4AR all seem to involve a violation of 2nd law TD (sun heats earth, earth heats atmosphere, atmosphere heats earth a bit more). Obviously this has been raised before and I’ve seen rebuttals from Eli, Atmoz and at RC; unfortunately I didn't find them convincing. These appear to be the possibilities: 1. IPCC and mainstream climate science have missed something! Seems somewhat unlikely. 2. It’s semantics. GHGs warm the atmosphere, reduce heat loss etc., raise average temperature. 3. Complexity: appears to violate 2nd law, but doesn’t for technical reasons not obvious to laity. As Sherlock Holmes says in the story The Sign of Four, "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?". http://en.wikipedia.org/wiki/Sherlock_Holmes So, if you remove the impossible - that is semantics or some quirk of complexity, then it must be the improbable. "The IPCC and mainstream climate science have missed something!" You are not the first to suggest this. In a recent NRC report [Wigley, et al. 2006] states: “Although the majority of observational data sets show more warming at the surface than in the troposphere, some observational data sets show the opposite behaviour. Almost all model simulations show more warming in the troposphere than at the surface. This difference between models and observations may arise from errors that are common to all models, from errors in the observational data sets, or from a combination of these factors. The second explanation is favored, but the issue is still open.” One problem with the current models is that they are based on the greenhouse gases re-emitting all the radiation which they absorb (Kirchhoff's Law.) Thus it it is argued that the system is in local themodynamic equilibrium (LTE.) But near the surface the absorbed radiation is converted to the sensible heat of the air and so it is not available to energise the back radiation. In fact the energy for the back radiation comes mainly from the latent heat of condensation of water vapour produced by evapotranspiration. But the correct solution is more complicated than the accepted ideas. See http://www.realclimate.org/index.php?p=507#comment-78370 where I have tried to explain it.
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| cba |
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| What will happen is the ghgs will absorb the energy at various wavelengths based on the absorption curve of the molecule. These same molecule types will also be radiating at these wavelengths as a function of the absorption rate times the temperature function (planck curve). If the temperature is the same as what originates the emission, the absorption curve will be invisible. If the gas is cooler, it will absorb and if the gas is warmer, it will emit. Also, in the symmetry for an atmosphere where there is the added problem that what is absorbed coming out must be emitted going out and emitted back down again. Factors for this are both decreased line width due to less pressure and slightly less temperature causing slightly less emission. Since there must be a radiative equilibrium for this to go on, a slight increase in ghgs results in a slight increase in the absorption but if originally in equilibrium for energy in / energy out(or power in/power out), the added absorption capability translates out to a linear increase in absorption and a linear increase in emission outward - AND - a linear increase in in emission inbound - which screws up the previously existing equilibrium. Note that this balance undoubtedly includes the latent and sensible heating to achieve the temperature curve that exists. Also, added absorption implies a possible increase in convection amounts and added absorption means immediate temperature rise that makes the parcel lower density causing it to rise. In addition, more h2o vapor will make parcels lighter and aide in increasing convection. Sorry but I didn't have time to go through your RC explaination to the point of translating to more familar terms such as B planck distributions and b einstein coefficients to try to check the basic structure but it is obviously quite similar even if you are not using the terminology. happy new year, charles | |||
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