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Venus Atmosphere Puzzle, one man's struggle with atmospheric physics
qraal
post Jul 5 2006, 11:18 AM
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Hi ngunn

For the first 200 km of altitude Venus's gravity is 8.87 - 0.0028z (m/s^2) with z in km.

Not bad really, but it adds a bit of mass to the air column overall.

Thing is I am not yet convinced by the hydrostatic argument, but it seems sound on numerical integration.

Adam

and it's 'Qraal' - a bit like 'kraal' and 'kroll'.


QUOTE (ngunn @ Jul 5 2006, 09:19 PM) *
Which is what I still don't understand (or believe). In this limiting 'skinny atmosphere' aproximation every air molecule is assumed to be near the surface experiencing the maximum possible g. The atmosphere cannot weigh more in any other configuration than it does in this one and the thermal and other properties of the gas become irrelevant in this approximation. There is one, and only one, way that the air molecules could exert a reaction force on the surface of the planet greater than their own weight and that is if they were being fired off ballistically faster than the escape velocity and leaving the planet altogether.
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ngunn
post Jul 5 2006, 01:08 PM
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Ah! qraal with a Q - my apologies. It's the underlining that does it. I keep getting Messenqer wronq as well.
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qraal
post Jul 5 2006, 10:32 PM
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Hi ngunn & MichaelT

As you might've guessed I have performed a numerical integration using a variable 'g' and the results are similar. Atmospheric mass only differs slightly between the two (<1%).

My real puzzle is why integration of the density equation, r = ro(T/To)^(Cp/(Cp-Cv)), didn't give the mass as Po/g. Constant lapse rate, as used in the International Earth Atmosphere, still gives Po/g after numerical integration - but I must've made a stupid assumption somewhere when I derived an analytical solution that differed so strongly. Over the temperature range in question, for Earth, Cp/Cv is practically constant, and only begins to differ from 1.4 for temperatures over 350 K.

That being said the Venus model is doing ok, but I've discovered the learning curve on modelling atmospheric absorption of solar radiation is quite steep. The (z,T) curve for Venus rapidly approaches a dT/dz of ~0 at a certain altitude, I guess due to increased heat gain from radiation. Now I could throw in an empirical fit, just like VIRA, but I'd like to read more of my references and come to some understanding of what's going on. I'm guessing that the atmosphere is becoming stable against convection forming an isothermal 'lid' just like the temperate zone tropopause on Earth.

If so there should be some literature around on just how that works, at least for Earth, and by extrapolation for Venus too.

Adam Crowl aka qraal
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Guest_DonPMitchell_*
post Jul 5 2006, 10:49 PM
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Guests






You're looking at Mark Bullock's thesis I assume. The only problem there is he is doing a 1-dimensional climate simulation, just the radiation balance calculation. That's the simplest of models, and doesn't take into account the important and complex effect of heat transport by convection. As soon as you try to take that into account, you must begin simulating general circulation, and the calculation becomes a lot more complex and expensive.

There are probably also latent heat issues due to phase changes and chemical changes, which are not fully understood yet on Venus. In Earth simulations, the phase changes in water are important, but I don't know if that's true for Venus. The role of water vapor in Earth climate modelling is still one of the biggest unknown factors -- water vapor is a greenhouse gas, but when it increases so does cloud formation, which then increases the reflection of heat.
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MichaelT
post Jul 6 2006, 06:27 PM
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QUOTE (qraal @ Jul 5 2006, 10:32 PM) *
If so there should be some literature around on just how that works, at least for Earth, and by extrapolation for Venus too.

Adam Crowl aka qraal

On Earth the absorption of UV radiation by the ozone layer is mainly responsible for the increasing temperature in the stratosphere, and, thus for the existence of the tropopause (dT/dz = 0). So there should be plenty of literature around on how that radiation absorption works on our planet.

Michael
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qraal
post Jul 10 2006, 07:56 AM
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Hi Michael

There's quite a lot of literature, but nothing giving a general overview - yet.

I've got a set of course notes about the issue I need to read in depth.

Don, the cloud chemistry and advective processes are complex but simpler than Earth in some ways. Must be because of a lack of Coriolis forces and insignificant surface friction effects. Ralph Lorenz has some papers of relevance on atmospheres self-organising to run at the Carnot limit.

Adam

QUOTE (MichaelT @ Jul 7 2006, 06:27 AM) *
On Earth the absorption of UV radiation by the ozone layer is mainly responsible for the increasing temperature in the stratosphere, and, thus for the existence of the tropopause (dT/dz = 0). So there should be plenty of literature around on how that radiation absorption works on our planet.

Michael
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ljk4-1
post Sep 26 2006, 02:36 PM
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Venus Atmosphere Profile from a Maximum Entropy Principle

http://arxiv.org/abs/astro-ph/0609649


--------------------
"After having some business dealings with men, I am occasionally chagrined,
and feel as if I had done some wrong, and it is hard to forget the ugly circumstance.
I see that such intercourse long continued would make one thoroughly prosaic, hard,
and coarse. But the longest intercourse with Nature, though in her rudest moods, does
not thus harden and make coarse. A hard, sensible man whom we liken to a rock is
indeed much harder than a rock. From hard, coarse, insensible men with whom I have
no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

- Henry David Thoreau, November 15, 1853

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qraal
post Jul 12 2007, 11:05 PM
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Hi All

I haven't bothered with this for some time, but just the other day I sat down and re-did the integration - and found my silly mistake. I had set up the integral wrong at the very first. The equation now gives results in line with the numerical simulation.

Funny how problems can solve themselves after leaving them be for a while.
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rlorenz
post Jul 13 2007, 12:08 PM
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QUOTE (DonPMitchell @ Jul 2 2006, 01:56 PM) *
The U of Arizona books are essential. There are actually two Venus books, and you want them both (Venus and Venus II). Also check out their Mars book.


I would add, by the way, the excellent book by Marov and Grinspoon, which covers a lot of
the findings of the Russian probes in a rather accessible form.


QUOTE (DonPMitchell @ Jul 2 2006, 01:56 PM) *
There have been many descent probes on Venus. The first really detailed information came from Venera-9 and 10 in 1975, which had nephelometers, spetrometers, and various other instruments.


A good compilation of solar system missions is Andrew Wilson's Solar System Log (Janes, 1987)
sadly out of print (and somewhat out of date).
A recent volume to which I made some contribution, which attempts a textbook treatment of
probes and landers (rather than orbiting/flyby spacecraft, which many texts already do well)
covering parachutes, landing dynamics etc. is Ball et al, 'Planetary Landers and Entry Probes'
Cambridge, 2007. Sadly, it isnt cheap
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rlorenz
post Jul 13 2007, 12:16 PM
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QUOTE (ljk4-1 @ Sep 26 2006, 10:36 AM) *
Venus Atmosphere Profile from a Maximum Entropy Principle

http://arxiv.org/abs/astro-ph/0609649


Hmm, interesting.

Without specifying the radiative deposition and emission profile, which the atmosphere
(at least, without the scary prospect of invoking clouds) cannot do much about, I
think this approach is not hugely different from saying the atmospheric temperature profile
with pressure or altitude is adiabatic (which for much of Venus, it is)

Rather more intriguing is the work by Ozawa ( Ozawa, H., and A. Ohmura,
Thermodynamics of a global-mean state of the atmosphere ––A state of maximum entropy increase, J. Clim., 10, 441–445, 1997)
which shows that given an opacity profile, the vertical structure of the (earth's) atmosphere
may adjust convectively to produce entropy at the maximum rate (so-called Maximum
Entropy Production principle, MaxEP).

Although the physical/thermodynamic foundations of the application of these ideas has
improved in recent years, it is still controversial (unloved by dynamicists) and really useful
applications have yet to be found...
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edstrick
post Jul 14 2007, 06:03 AM
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"....Sadly, it isnt cheap..."
I'd say European scientific publishers are rapacious, but then I do have some idea of the horrendous overhead costs that pushes their prices up.

The all time bargains in quality-info-per-dollar are the University of Arizona Press conference books, like "Venus", "Protostars and Planets", .... etc. etc. etc.

I need to get the more recent ones.. I have all or most of the older ones up to the early 90's, including the prototype: "Planets, stars and asteroids observed with Photopolariimitry" <something like that>
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