An important point often made by critics of climate models is that they often represent our best guess at specific phenomena. One of the biggest uncertainties in climate models is in proper modeling of atmospheric moisture. Considering that H2O is widely accepted to be the strongest of all greenhouse gasses, water is fairly important component of climate models. As is often the case at tAV, I’m not the guy who figured this out but am the one who will attempt to translate the deficiency in models as I currently understand it.
Based on Makarieva et al. recent multi-author paper (M10) on the driving force behind winds several here at the Air Vent have discovered that the climate model CAM3.0 linked here doesn’t include precipitation condensation based pressure loss in its cloud parametrization. The model doesn’t even attempt a simulation for what I believe will soon be accepted as the primary driver of most winds including tornadoes and hurricanes, not just globally on earth but solar system wide. Jupiter’s red spot, bands in Saturn, all of it powered by condensation based pressure changes. Why is that so important? Because the additional energy stored in water vapor which translates into lower than modeled pressure and higher than modeled windspeeds in hadley cell updrafts.
Ask yourself to explain what powers a tornado, and soon you’ll find yourself describing strong temperature inversions where the hot surface air breaks through the cold upper air or something of that sort. In fact, that is what we’ve all been taught since junior high. M10 teaches that there is a component of basic gas physics missing from this explanatoin — condensation.
The water vapor component of saturated air at 30C has a Water vapor is about 0.6PSI where as standard air pressure at sea level is 14.7 PSI. When the vapor condenses, it no longer contributes to the gas pressure in the region of condensation much as it takes up nearly zero volume at that point. The air pressure in the saturated volume would drop by the vapor pressure amount when condensed. The pressure doesn’t sound like much but remember, we already have a model which creates some convection without it and 0.6PSI over a square yard is 780 lbs of additional upward force. That’s not all though, the heat release during condensation creates an additional pressure loss warming the surrounding air reducing the air pressure even further. The net effect is all toward powerful amplification of updrafts in a condensation environment.
I’m not intending to do the calculations any further here, because it is basic knowledge. None of what I’ve written above is in any way new and all of it has been known longer and more completely than we have known about CO2 absorbing infrared radiation. In fact, 20 years ago in my undergraduate thermodynamics class we were forced to calculate all of these factors for a variety of mixed gasses. So when I first read Anastassia’s paper on what powers hurricanes, it made perfect sense to me. Except for the part where she claimed it wasn’t part of mainstream literature.
Nick Stokes, who takes too much criticism some times, summed it up best, brackets are mine.
I have to say that it’s still not clear to me where condensation comes in in 3.3.6. However, I remain sure that they haven’t just forgotten about it. This stuff [models] has been around for thirty years, reviewed by thousands.
For some background reading Nick linked to the Zhang paper from which the cloud parametrization for CAM3.0 was adopted. Paper here and he also pointed out some very similar equations to M10 right from the CAM model there is a difference though.
From the CAM3.0 global climate model, chapter 3.3.2 paraphrased style, it’s not a direct quote and I’m not the original author ;)
The conservation of total air mass using as the prognostic variable can be written as
Similarly, the mass conservation law for tracer species (or water vapor) can be written as
There is no “law” of conservation of water vapor to my knowledge. Water vapor can condense, especially when V can be in the vertical direction toward lower pressure. This is clearly a simplification of the situation as mass of water vapor is obviously not conserved during condensation in an air volume.
Read the rest of this entry »