New Computer Fund

Sunday, August 25, 2013

It is Greenhouses all the way Down

Dr. Roy Spencer is doing a redo of the redos of the Woods greenhouse experiment.  It would seem that after more than 100 years of Greenhouse gas theory and more that 100 years of actual greenhouse use that people would tire of reinventing the wheel so to speak.  One of the problems Dr. Spencer will have is his greenhouse experiment will be conducted in a greenhouse.  Woods had the same problem and installed a layer of glass above his experiment so Woods had a greenhouse in a greenhouse in a greenhouse.  The drawing above attempts to illustrate one of the atmosphere's greenhouses, the Atmospheric Boundary Layer (ABL).

Solar in yellow on the left is based on our current best estimate of the actual average annual energy absorbed during the day portion of our 24 hour day.  The orange on the right is based on our current best estimate of the purely thermal or infrared portion of the energy transfer during the night portion of our 24 hour day.  The blue lines sloping down represent the ABL.

While the total energy absorbed in the ABL by all sources is large, the net energy, what really matters, averages about 88Wm-2 or about the amount of the 24 hour average of the latent energy transferred from the actual surface, the floor of the ABL greenhouse to the roof of the greenhouse the ABL capping layer.  You should noticed that I have divided the net energy of the ABL into 39 up and 39 down with a question mark in the middle.  That question mark is about 20 Wm-2 or about the range of estimates for the total latent energy transferred to the atmosphere, 78Wm-2 to 98 Wm-2.  That is how well we understand our atmosphere.

This graphic, borrowed from an Ohio State University server which unfortunately doesn't have a good attribution, the author remains nameless so far, though he does thank Nolan Atkins, Chris Bretherton and Robin Hogan in the at least three versions that pop up while Googling atmospheric boundary layer inversion capping, has a neat graphic that I plan to critique in an educational way.  "The Stable boundary layer has stable stability" is something that might be better worded, but makes a fair tongue twister.  There, critique over.  I need to be careful because the course is on lightning, I don't want sparks flying because of my questionable use of intellectual property. 

The power point presentation that this graphic was lifted from looks like it has good potential to simplify explanation of the complex processes that take place in the ABL convective mixing layer, so I hope the author fine tunes and publishes since most of the other discussions I have seen are dry as all hell.

The  gist of the graphic for my use right now is the transition from an entrainment to inversion capping layer.  During the day mode, moist air rises with convection mixing the layer between surface and capping layers.  As the moisture begins to condense, buoyancy decreases at a stable capping surface where further cooling, mainly radiant, increases the air density which lead to subsistence or sinking air mass that replace the mass that convected from the surface.  As night falls, the thermal energy driving the convective mixing decreases leaving an inversion layer that produces the true surface greenhouse effect.  For those less than radiantly inclined, the temperature of the roof is greater than air above and below producing a higher than "normal" sink temperature for the greenhouse floor source.  The ABL "roof" gets most of its energy from solar, the 150 and floor latent and sensible energy the 224 in the first drawing.

The reason I have "Day" values for the solar powered portion is because the ABL can only contain so much energy.  The ABL expands upward and outward increasing the effective surface area allowing greater heat loss to the "space"/atmosphere above the ABL capping layer.  That expansion can be towering deep convective deep in summer or just clear skies with a little haze, but the ABL "envelop" expands in the day and contracts at night producing what is known as a residual layer between the entrainment layer and the stable nocturnal layer.  If there is advective or horizontal winds below the capping layer, the impact changes.  A sea breeze is a good example that tends to cool sometimes and warm others, lake effect another and then you can have massive fronts move through which can destroy the ABL stability layers all together.  Considering all those possibilities is a challenge, so let's stick with a static version so heads don't explode.

If you look at the orange arrow with the 59 and thin 20 arrow, that is the current best estimate of annual average radiant surface cooling, where that surface is the floor of our greenhouse.  That 20 Wm-2 represents the imperfection of our greenhouse.  If the average final or lowest average floor energy is 335 Wm-2 which would be a temperature of about 4 C degrees, then the greenhouse floor would start the night at about 355 Wm-2, about 8C and ramp down to about 4C producing a 4C range of temperature.  If the 20 Wm-2 were less, the range would be less and either the start temperature would be lower or the end temperature higher.  Since the ABL has a limited capacity, the initial temperature would tend to be about the same and the final temperature would increase reducing the diurnal temperature range of our ABL greenhouse.  If the 20 Wm-2 window was completely shut, the capacity of the ABL would still limit the total amount of initial warmth, just the diurnal temperature range would approach zero.

Now let's imagine we reduce the 20 Wm-2 by 4 Wm-2.  Our final energy would be about would be about 339Wm-2 which has an effective temperature of 4.9C or about 0.8 to 0.9 C warmer than the less efficient ABL greenhouse at the lowest point of operation.  That increased efficiency would likely increase the latent heat loss a little during the day mode, increasing the initial temperature of the ABL greenhouse by about 0.8C.  The overall impact would be about 0.8C warmer conditions or less.  Nothing is perfectly efficient so the 0.8 -0.9 C per 4 Wm-2 would be an upper limit for the ABL greenhouse.

The ABL greenhouse though is inside a free atmosphere greenhouse.  Adding the 4 Wm-2 at the ABL roof may impact the performance of the free atmosphere greenhouse.  What would keep the free atmosphere greenhouse from impacting the ABL greenhouse and vice versa?  When the roof of the ABL greenhouse is produced by water vapor which has a limited temperature of condensation and set freezing point, adding energy doesn't change the temperature only the rate of energy transfer.  The ABL volume has a limited heat capacity so convective mixing would increase with added energy expanding the ABL envelop more or producing more deep convection loss if the ABL capping layer becomes less stable.

So why is the globally averaged annual floor of the ABL greenhouse about 335 Wm-2 or about 4 C degrees?  If the roof is limited by the condensation or freezing point of fresh water, 0C or about 315 Wm-2 and the efficiency of the ABL greenhouse is limited by the 20 Wm-2 of window radiant energy, what else would it be? 

No comments:

Post a Comment