torsdag 3 januari 2013

How to Interprete an Atmospheric Radiance Spectrum

The thermal emisson spectrum of the Earth + atmosphere measured by the IRIS Michelson interferometer instrument on the Nimbus 4 space craft over three different regions, takes the following form:

The curves shows the radiance (in mW/m2 and 1/cm) per unit wave number as function of wavenumber, and are computed from interferometer measurements of temperature using Planck's law of blackbody radiation.

The basic measurement thus concerns temperature and not radiance, but the computed radiance spectra  are commonly taken as observed rates of emission of energy. For example, the dip in the spectrum around wavenumber 700 is interpreted as "blocking" energy transfer or "trapping" energy by atmospheric CO2 as a Greenhouse Gas Effect GHE causing global warming, and the size of the dip is used to suggest alarm. We argue below that this inflates the role of the trace gas CO2.

But sticking to the true the measurements of temperature gives the following somewhat different interpretation:

The temperature of 220 K at the bottom of the dip around wavenumber 700 is the temperature of the tropopause, suggesting that the Earth + tropopause can be be viewed as a blackbody with a spectrum following the 220 K curve with total radiation of 140 W/m2. The radiation above the 220 K curve of a total of 100 W/m2  would then correspond to 40 W/m2 directly radiated to outer space from the Earth surface through the atmospheric window (wave numbers larger than 800), and 60 W/m2 from water vapor in the atmosphere (wave numbers smaller than 550).

The total would be 240 W/m2 absorbed by the Earth+troposphere with 180 W/m2 absorbed by the Earth surface. The role of combined thermodynamics and radiation within the troposphere would then be to transport 140 W/m2 from the Earth surface to the tropopause, with an estimated 120 by thermodynamics of convection and evaporation/condensation, thus mainly by thermodynamics.

The effect of the thermodynamics would then be a reduction of the dry adiabatic lapse rate of 10 C/km to the observed 6.5 C/km, with the drop increasing with the vigor of the thermodynamics.

A change of the radiative properties of the atmosphere could then have an effect on (i) the temperature of the tropopause as the outer boundary of the Earth + atmosphere system, and (ii) the lapse rate, which together would determine the Earth surface temperature.

A decrease of the transparency of the atmosphere would decrease the atmospheric window and thus demand an increase of the temperature of the tropopause, while at the same time demand more vigorous thermodynamics decreasing the lapse. The net effect could be interpreted as radiative warming with negative feedback cooling from thermodynamics, thus with potentially a small net effect. In other words, the climate sensitivity could very well be small.

The purpose of the argument is to seek to assess the Earth surface temperature from lapse rate and temperature of the tropopause as an (effective) outer boundary of the Earth + atmosphere as a blackbody, without more precise (difficult) modeling of the combined thermodynamics/radiation within the atmosphere. This analysis indicates that the Earth surface temperature may be insensitive to even quite large perturbations of atmospheric composition and insolation.

It is also an attempt to get away from the notions of "effective emission altitude" (at 5 km) and "effective emission temperature" (255 K) as fictitious entities which cannot be measured and have no physical correspondence (as compared to the tropopause which has a physical reality).

Notice that with an ideal blackbody as a universal thermometer, the temperature of a given body would be measured by radiative equilibrium with the universal thermometer at a certain temperature defined by no radiative transfer of energy between the body and the thermometer. The absorptivity/emissivity of the body would then not affect the temperature reading, but would be directly connected to the radiance of the body. Without knowing the absorptivity/emissivity, it would thus be impossible to tell the effective radiance of a body from its temperature. The above radiance plots constructed from temperature readings assuming absorptivity = emissivity = 1, may thus not represent reality. In particular, the emissivity of CO2 as a trace gas must be very small, and the radiation from the troposphere even within the 550-800 band of CO2, must emanate from the whole atmosphere, not only CO2. In other words, the radiance dip between 550 and 800 would be due to CO2 only to a small extent, and the alarm balloon would collapse.

1 kommentar:

  1. I have in the past made some comments on the definition of black/gray bodies which must in consideration of the Stefan-Boltzman (S-B) equation have a surface. Also for the S-B equation to apply the emitting surface must be in a vacuum.
    However, in this case you should note that the "window of earths emission" is 66 w/m2 not 40 w/m2 as in the cartoon in Trenberth et al 's poor paper (Global Energy Budget) see slide 26 in this presentation to KNMI by Dr Noor Van Andel
    I suggest that this post and comments are also worth reading as are many posts on this blog.

    Happy, healthy and prosperous New Year