## Friday, February 1, 2013

### What the Heck this About? Condensation Driven Winds?

Condensation Driven Winds Update: is currently being re-torn apart.  It is a big deal only because the paper got published after folks said they didn't understand what the heck it was all about.  This got me doing a little review and I may have things wrong, but I so far think that the concept in the paper has some merit.  They want to use mole fractions, but being a pressure kinda guy, I am going to try and explain my under or mis-under standing, with pressure.

Dalton's law of partial pressure is really complicated :) Ptotal=Pa+Pb+Pc etc., you just add all the partial pressures to get the total pressure.  Being lazy I found a blog, Diatronic UK, that has a brief discussion of the atmospheric partial pressures.  This one is in mmHg, not a big deal.

They list the approximate sea level pressure as 760 mm Hg with Nitrogen providing 593 mm Hg, Oxygen 159 mm Hg, Carbon Dioxide 0.3 mm Hg and water vapor variable.  Since water vapor varies it is the PITA.  At 4%, which water vapor can possibly reach, 30 mm Hg.

Since Diatronic  is discussion a medical air device, they maintain a constant pressure of 760 mm Hg.  In the real atmosphere, pressure varies.  Since H2O, has two light H molecules, it is the light weight of the atmospheric gases being discussed.

If there is a lot of H2O in the atmosphere, say 50 mm Hg locally, and that H2O condensates, it would release energy as the latent heat of evaporation which is reversible.  The H2O would precipitate out of the volume of gas reducing the local partial pressure of H2O in that volume of air.   Does that have any significant impact on local or global climate?

That would depend now wouldn't it?  At first blush you would say no.  All that is cover perfectly well with what we know we don't need no mo. Well, the authors tend to tick some folks off by assuming that condensation happens at saturation, so far so good, but then solving for condensation as a function of molar density and the relative partial pressure of the dry air left over. Instead on moles, I will use the pressure.

So with the 50 mm Hg for H2O and 760 mm Hg as the total pressure, the pressure differential after condensation could be as much as 760-50 or 710 mm Hg.  That is a local pressure differential of 50 mm Hg which is quite a lot.  710 mm Hg is about 946 millibars which is about what a Category 3 hurricane central pressure would be.  With the released heat from condensation warming the air above the condensation layer and the water vapor pressure differential, there would be convection and advection.  Pretty common knowledge, they just think they have a better way of figuring out the energy involved.

Where the authors are going with all this is that vegetation would produce a fairly constant local humidity that would tend to "drive" winds due to the partial pressure of reforested moles of water vapor available to flow to dry air.  Basically, a biological version of a sea breeze.

I have absolutely no heartburn at all over their conclusions or the basic premise of their math. There are still a lot of other issues, Relative Humidity is just that relative, it is hard to predict, clouds tend to form when they like not when we want them to, and condensation with real precipitation that makes it to the surface is also a tad unpredictable to name a few.  Possibly, their method could improve the predictability somewhat, I don't know that is a modeling thing.  As far as the Biota impact, I am all over that.

A major part of the land use impact is changing the water cycle.  Water is heat capacity which tends to buffer temperature range.  Since the majority of warming is still over land area that have a great deal of land use and water uses changes, a large portion of that is likely due to the land use change with some amplification due to CO2 and other anthropogenic stuff.  That by the way is becoming a much more popular view, though the Ice Mass Balance which I think is totally under appreciated is still under appreciated.

Anyway, as usual I see something more than the in your face conclusion of the paper.  Improved modeling of clouds and storm intensity is likely a better possibility for the results than telling us what we pretty much already know.

Time will tell.