Tuesday, August 23, 2011

Building a Better Discription of the Role of Our Atmosphere

I chose that title because I am going to work on this from time to time. This is related to the last post on the energy budget but I am going to try and stay more focused. The first part is incoming solar radiation and the atmosphere.

Going back to the NASA budget cartoon, the sun provides 100% of the energy to the Earth that is significant. There is energy from the Earth's core and people burn things, some there is other energy, but the most significant source is the sun. Of that energy, about 51% is absorbed by the oceans, 16% absorbed by molecules in the atmosphere and about 3% absorbed by clouds. The total absorbed is 70% of the solar energy available. The 30% not absorbed is reflected by cloud tops and the Earth surface.

Since I want to do comparisons of the in and out of the energy at the surface and hopefully the tropopause eventually, since the Earth system absorbs 70%, 72% of the absorbed energy is at the surface with 28% in the atmosphere. Since the sun only shines in the daytime, at least in my experience, This is the division of the total energy input into the system. Some can debate what the actual value of that energy is, but there is general agreement that this is the division of the energy.

Of the energy absorbed by the atmosphere, approximately half is can be considered transferred via various thermodynamic means to the surface and the rest lost to space. This point can be debated because the diameter of the Earth plus atmosphere is greater than the core diameter. For now we will assume a 50/50 split and possibly revisit the other possible value later. With this assumption, 60.5% of the solar energy at the top of the atmosphere is absorbed by the surface directly or via the atmospheric thermodynamics. So now I am going to give these percentages some numbers, using the 341 Watts/meter squared, the Earth system absorbs 239 W/m^2 with 172 W/m^2 directly absorbed by the surface, 67 W/m^2 directly to the atmosphere with 33.5 W/m^2 radiated to space and 33.5 W/m^2 transferred to the surface, bring the surface solar total impact to 205.5 W/m^2. These numbers vary from the values list on the Trenbert et al 2008 paper with cartoon. That cartoon has 161 W/m^2 to the surface and 78 W/m^2 to the atmosphere with no division of the atmosphere to Earth and space. Since the NASA budget cartoon is more widely accepted, I will continue to use their percentages and compare to Trenberth 2008.

At this point there is an easy comparison to the Postma online paper where Postma argues that the value should be 480 not 240 (~239), which just doesn't work. Since Trenberth uses 161 W/m^2 to the Surface and 78 to the atmosphere,the surface impact including the atmosphere is 200 W/m^2 instead of 205.5 to give you an idea of how close the two are together in this respect. During the day, approximately 480 W/M^2 is absorbed by the Earth/atmosphere with about 400 to 411 W/M^2 being absorbed by the surface assuming half of the energy absorbed by the atmosphere down wells to the surface and half is lost to space during the complex process of heat transfer being mainly radiative near the TOA and becoming a mix of radiative/convective/conductive below the TOA becoming more conductive/convective at the surface. Convection has a major impact because air near the surface rises (this is shown as thermals on the Trenberth drawing)and is replaced by sinking cooler air which is warmer than it would be because of the absorption of solar in the atmosphere. So a portion of the heat in the atmosphere is physically down welling, as in falling, to the surface.

At this point you may see why I am not disagreeable with the term "Down Welling" with respect to a cooler atmosphere contributing to the warming of the Earth or reduced cooling if you prefer. I am not fond of "back radiation" because while the radiate heat from the surface and lower atmosphere contribute to the warmth of the atmosphere, physical "back radiation" plays a very small percentage of the role.

For a second comparison, the NASA drawing shows 6% of the incoming solar directly radiated from the surface to space. This is the infrared energy that has a clear window to space. By NASA's cartoon that 6% is 20.5 W/m^2 with Trenberth showing 40 W/m^2. That is a substantial difference at first glance, but reasonable considering Treberth's goal. Trenberth's diagram better illustrates what is happening because the temperature that causes the up welling infrared is both from the surface and the warmer lower atmosphere and reason for the difference is more understandable with the next comparisons.

Per NASA, 23% or 78.4 W/m^2 is rising to the atmosphere as latent heat with water vapor. Trenberth's value is 80 W/m^2 by evapo-transpiration. Nasa has 15% or 51 W/m^2 to the atmosphere by surface radiation versus 23 W/m^2 per Trenberth (356 - 333 is the net to the atmosphere). This partially covers the surface up welling difference. Warm moist air rising with cold dry air falling is a common meteorological model. How moist and how dry is not described, so the convection is separated into latent and thermals or conduction.

NASA shows 7% or 24 W/m^2 rising from the surface due to conduction versus 17 by Trenberth labeled as thermals. Here, Trenberth seems to cover the rest showing that the atmosphere donates to the radiation lost in the free path out going radiation.

Going back to the balance in the atmosphere related to the surface, for the surface, Nasa has a total of 74.8 + 51 + 24 = 149.8 to the atmosphere + 20.5 to space I propose, equaling 170.3. Trenberth has 80 + 23 + 17 = 120 rising from the surface to the clouds plus 40 to the surface I point out, equaling 160 W/m^2 with an imbalance of 1 W/m^2. There is an imbalance of 1.7 W/m^2 in the NASA comparison due to rounding errors.

Comparing my calculation of the solar energy absorbed by the atmosphere on the surface, 205.5 - 170.3 = 35.5 W/M^2 NASA and 200 - 160 = 40 W/m^2 Trenberth, the solar energy absorbed by the atmosphere's impact on the surface. The difference is because I am using a frame of reference in the atmosphere, not at the top of the atmosphere. Where that point is located in the atmosphere is roughly where the up welling/ down welling energy is in balance, approximately at the bottom of the average cloud height.

Since NASA doesn't include the greenhouse effect in its budget and Trenberth uses total radiation up of 396 and down of 333 to illustrate the radiative impact, there are questions about the impact and sources of the downward radiation flux. This is where I may screw up so watch closely! The point I have picked, approximates the middle of the green house action. The difference in the net fluxes shown be Trenberth are 396 up - 333 down equals 63 W/m^2 net up. Since the solar impact is not separated 40 W/m^2 (or 35.5 from NASA's drawing) is the solar absorbed greenhouse impact and the rest, 23 W/m^2 (to 27.5)is Earth surface generated greenhouse impact. Remember that this is estimated and there are slight differences between the two budgets.

At the top of the atmosphere the main differences between the two diagrams is the 1 W/m^2 imbalance on the Trenberth drawing which is based on the model estimate of the imbalance due to CO2 (0.8 w/m^2 if you don't like the rounding). NASA assumes energy in equals energy out and Trenberth assume an imbalance due to CO2.

There are several reasons I break the drawings down this way, the main one though is to illustrate the Greenhouse effect or atmospheric gas vibrational mode effect. Some gas molecules are better absorbers/emitters of radiation than others due to the configuration of their molecules. Diatomic gases like nitrogen N2 and oxygen O2 have a simple two atom configuration. When excited by radiation they can only vibrate by stretching their bonds, pulling apart a little and springing back. Water, H20 has three atoms and a vee configuration, both atoms can stretch away at the same time, one can stretch away while the other springs back or the hydrogen atoms can swing toward each other the away from each other. So water has three fundamental vibrational modes. These are shown in this wikipedia article along with more interesting stuff. The "greenhouse gases" are called that because they have more vibrational modes which allow them to interact with electromagnetic radiation more efficiently than diatomic atoms. So there is a difference and because of the difference, so some molecules are much more effective interacting with different wavelengths of electromagnetic radiation (EMR), both incoming from the sun and out going from the surface, than others. Nitrogen and oxygen that make up 99% of the atmosphere are reasonable conductors of heat, but much less effective, even in much higher concentration, interacting with EMR. A simple illustration is right in your kitchen. Put a beef brisket in a pressure cooker with no water and another in a pressure cooker with water, which one do you think will cook better? Now most of the difference is the properties of steam, not just absorption of radiation, but it gives you an idea how differently water behaves when heated. So it is not unreasonable to believe water vapor behaves much differently to radiation than other gases. If you have ever seen dry ice, frozen CO2, you know that it behaves differently to heat than water. Dry ice sublimates, it goes straight from a solid to a gas. Because of their differences, CO2 just happens to interact more with EMR at certain wavelengths than water and much more than nitrogen or oxygen. Different people try to explain the greenhouse gases with different levels of success, I will just let you know there is a difference and you can study more if you like.

If you accept that greenhouse gases are good absorbers and emitters of EMR, then you can think about how they have an effect on the surface. First think about the warming of the air by the sun. A good deal of that warming is lost to space but about half wants to move to the surface both in the day and at night. In the day, you can probably believe that it helps both warm the Earth and slow the rate of warming some. The evaporation of water from the surface happens mainly in the middle part of the day, which also helps slow the rate of warming. That rising water vapor with its latent heat increases the amount of solar energy absorbed in the day by increasing the amount of water molecules that can interact with the solar radiation. Most of the "warming" effect of the atmosphere containing greenhouse gases is directly do to solar energy directly, not energy radiated from the surface. Since the NASA diagram did not try and the Trenberth diagram lumps its down welling radiation together, that point is not obvious. So remember that the greenhouse effect is a two way street.

A perfect illustration of the atmospheric impact is here in the tropics. Where I live is surrounded by the ocean. Today's high temperature will be 90 degrees F with the low today of 83 degrees F. Brownsville, Texas, just a little north, near sea level and not surrounded by water will have a high of 97 and a low of 79. Water vapor does make a difference.

Getting back to the estimated solar contribution not included in the drawings I calculated earlier, the 23 to 27.5 W/m^2 is about a third of the atmospheric warming. If you take the solar energy absorbed by the air and clouds plus the rising latent heat, that works out to about two thirds so up welling would be about a third. While not a perfect check, it somewhat confirms my estimate at the surface using my frame of reference. CO2 in the atmosphere is well mixed, unlike water vapor which mainly stays in the lower atmosphere. This makes CO2's impact a little more complicated because it can help cool by blocking a portion of the down welling radiation high in the atmosphere and warm lower in the atmosphere by absorbing more outgoing radiation. Twice as much CO2 is estimated to increase the total down welling radiative impact by about 1 percent. Because of the interaction with wavelengths associated with water vapor and that an increase will increase the amount of water the atmosphere can hold, an estimate is about the best anyone can hope for. Clouds, made of water vapor, absorb about 3% of the incoming solar and reflect about 20 percent. So a one percent increase in clouds would erase the one percent increase in down welling due to doubled CO2. There would be an increase in the solar absorbed by clouds, but only about half that increase would impact the surface. The cooling effect of CO2 should offset much of the increase in incoming solar absorption. That may limit CO2's impact to mainly the one third out going radiation to the atmosphere and a fraction of the direct surface to space radiation, but the warming will also increase the amount directly radiated to space from the surface. So what heats where, by how much and at what wavelength all have to be considered.

While I don't have a problem with down welling radiation, the sky has a temperature after all, I do have an issue with causes warming by directly radiating the surface or if it is better thought of as slowing the rate of cooling. Since the Earth system tries to balance radiative flow, energy in equals energy out, the Earth will become warmer if something tries to reduce energy out and cooler if something reduces energy in. So it is more a matter of semantics, but common thinking in thermodynamics is heat flows from warm to cold. Radiation is a little more complicated because of its relationship with its atmospheric window. Radiation from a colder object can flow through a warmer object or layer, that happens in our atmosphere. Radiation can flow from a colder object to a warmer object. That happens also. In a vacuum, a colder object can cause a warm object to grow warmer by reducing the warmer object's net out going radiation flux. In our lower atmosphere though,the path of the flux in both directions is pretty well blocked, so conduction plays a large role in the heat transfer. Conduction is a one way street, so I am incline to stick to the old school rational. It doesn't much matter because the end result is Earth is warmer because of our atmosphere.

So without argument, I can accept that a doubling of CO2 will likely cause a one percent increase in down welling which results in a one percent reduction of the net out going radiation, which will cause some degree of warming for some time period. I am not convinced that change will cause feedbacks that increase the degree of warming or that the Earth system will not compensate for the change with more cloud cover, precipitation and/or changes in circulation patterns to reduce the increase in temperature. How the Earth responds to the change is the major issue of the global warming debate, not if the atmosphere contributes to the warmth of the surface.

Most of the debate recently on the Climate Etc. blog, centers around the issue of down welling energy, specifically radiation. I think that my taking a slightly lower frame of reference may reduce the differences there somewhat, at least among the more reasonable factions. The Postma skeptics are a whole different issue.

The Postma crew does not believe that the averaging of the incoming solar radiation is correctly calculate, that it should be twice as much. Postma considers the day side with its 480 W/m^2 and integrates the distribution of the atmosphere resulting in 610 W/m^2. This is were I have difficulty following his argument. I can see integrating the incoming solar at a point in the atmosphere where the atmosphere outside the radius of the Earth is warmed that can contribute to Earth atmosphere system. The sun's rays do pass through the atmosphere around the edge of the Earth's surface creating some beautiful sunrises and sunsets. This impact was brought up is 1996 in a paper by Albert Arkin in Science Magazine;

"An atmospheric general circulation model, which assimilates data from daily observations of temperature, humidity, wind, and sea-level air pressure, was compared with a set of observations that combines satellite and ground-based measurements of solar flux. The comparison reveals that the model underestimates by 25 to 30 watts per square meter the amount of solar energy absorbed by Earth's atmosphere. Contrary to some recent reports, clouds have little or no overall effect on atmospheric absorption, a consistent feature of both the observations and the model. Of several variables considered, water vapor appears to be the dominant influence on atmospheric absorption."

Since this issue was brought up by this and other papers well before the Trenberth budget, I don't think it unreasonable to assume it has been addressed. In any case, that small of a discrepancy does not resolve Postma's situation.

Postma accurately calculates the total solar input power at 1.22 x 10^17 Watts, since the area of the Earth is 5.1x10^14, the Watts per meter squared would be 239.2 or 478.4 for just the day side. He back calculate the total energy based on an average surface temperature of 1.99 x 10^17 W which implies an average surface radiating average temperature of 15 degrees C resulting in 390 W/m^2 or 780 W/m^2 for just the day side which be radiating at an effective temperature or 30 degrees C, twice the average of 15 degrees C. This also agrees well with the mainstream calculations considering the greenhouse effect. Here though, Postma concludes that that is impossible within the laws of physics. To reconcile his perceived energy deficit he concludes that the earth system or aggregate spherical ensemble (Earth plus atmosphere) has to be considered. This is not an outrageous way to consider the Earth energy budget, it is after all a combination of the Earth and atmosphere.

Unfortunately, Postma notes that the average surface temperature of the sunlit side does not achieve 30 degrees C. That is true, because the average surface temperature is 15 degrees C, the sunlit only hemisphere does not exist in reality. It is a concept of mathematics. The Earth rotates. Then Postma concludes that if the concept of mathematics radiated at 30 degrees C that it would emit 1.22 x 10^17 Watts which equals the total absorbed by the spherical ensemble from the sun, which is exactly what the mathematical concept should do. For the real world average temperature to not equal that of a mathematical concept of a hemisphere receiving "instantaneous" solar energy is not an unreasonable concept for most people to grasp. Postma is not most people so he determines that this concept would have a maximum temperature of 87.5 degrees C, a temperature not found on the real Earth's surface. Half of the 87.5 is 43.8 degrees C, pretty damn hot still. While 43.8 is hot, it is less than the warmest temperatures ever recorded.

Except for the scientific brain fart, Postma's math is very good, his visualizing of concepts not so good. His confusing mathematical conceptual models with reality should limit his scientific career options. I, being a fishing guide, can have all the scientific brain farts I like without suffering loss of career options I am not pursuing.

Back to my ideas based on the lower frame of reference. With two thirds of the atmospheric warming due to the solar input absorbed by the atmosphere and latent heat rising, one third would be due to conduction and outgoing radiation which are related. The temperature of the Earth causes both. Separating latent makes a great deal of sense because latent heat is much greater than sensible heat which is the majority of the conduction.

This is a work in progress that will probably die, but I may continue with the Sky Dragons if the hurricane season insists.

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