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ENVIRONMENTAL SCIENCE: IRON AND POLAR MESOSPHERIC CLOUDS

Notes by ScienceWeek:

In this context, the term "lidar" (light detection and ranging) refers to a laser instrument used to study clouds. A laser beam is directed at the cloud formation, and the degree of scattering or reflection of the beam provides information about the structure of the clouds.

The atmosphere of Earth is divisible into several layers, each layer having a characteristic temperature range, pressure range, and composition. The layers, from the surface of Earth, are (with thicknesses varying at different latitudes): troposphere (0 to approximately 10 kilometers), stratosphere (from approximately 10 to 50 kilometers), mesosphere (approximately 50 to 80 kilometers), thermosphere (approximately 80 to 500 kilometers), and exosphere (above approximately 500 kilometers.

Other layers, essentially meta-layers, are also recognized: a) the "chemosphere" is the region between approximately 32 and 92 kilometers where many important chemical reactions occur; b) the "ionosphere", above approximately 80 kilometers, is a shell of high electron concentration resulting from very short wavelength sunlight stripping electrons from atoms and molecules (mainly oxygen and nitrogen) to create an ionized layer; c) the magnetosphere is the constantly changing magnetic field generated by the Earth's dynamo, this magnetic field influencing the behavior of electrically charged particles and the field extending approximately 10 Earth radii (64,000) kilometers into space on the sunward side.

The following points are made by J.M. Plane et al (Science 2004 304:426):

1) Polar mesospheric clouds (PMCs), also commonly known as "noctilucent clouds", have been intensively studied in recent years because of their potential to provide an early indication of climate change in the upper atmosphere (1) -- although this remains controversial (2). PMCs consist of ice particles that form through the microphysical processes of nucleation, condensation, and sedimentation (3). They occur when the temperature drops below the water frost point, which is about 150 K in the upper mesosphere (4,5).

2) The largest ice particles (radius more than 20 nm) sediment to the base of the cloud layer, where they scatter light sufficiently strongly to be observed by lidar (5) or even with the naked eye during twilight. The meteoric (meteoroid-derived) Fe layer peaks at altitudes around 87 km and has a half-width of about 7 km. Thus, if the removal of iron species on the ice particles in a PMC is rapid relative to the input of fresh iron from meteoric ablation, the vertical transport of Fe into the cloud via eddy diffusion, and the lifetime of the cloud itself, then a local depletion or "bite-out" in the Fe density profile should result.

3) The University of Illinois Fe Boltzmann temperature lidar was installed at the Amundsen-Scott South Pole Station in 1999. This instrument measures the Fe density between about 75 and 110 km. It consists of two lidars operating at the wavelengths of two closely spaced Fe resonance lines (372 and 374 nm). The lidars measure simultaneously the relative populations of the lowest spinorbit multiplets of ground-state Fe. In the presence of a PMC, the lidar return signals consist of resonance backscatter from Fe atoms plus Mie backscatter (the elastic scattering of photons) from the cloud particles. Because the PMC backscatter signals are nearly identical at 372 and 374 nm, whereas the Fe signals are substantially different (the relative population of the higher multiplet is 1.2% at 145 K), the PMC signal can be eliminated from the 372-nm data by subtracting the 374-nm return signal and scaling the difference. The scaling factor was calculated from the known temperature-dependent relationship of the Fe backscatter signals at 372 and 374 nm, assuming the temperature from a South Pole climatology. Hence, the Fe density was determined at the altitude of the cloud layer.

4) In summary: Polar mesospheric clouds are thin layers of nanometer-sized ice particles that occur at altitudes between 82 and 87 kilometers in the high-latitude summer mesosphere. These clouds overlap in altitude with the layer of iron (Fe) atoms that is produced by the ablation of meteoroids entering the atmosphere. Simultaneous observations of the Fe layer and the clouds, made by lidar during midsummer at the South Pole, demonstrate that essentially complete removal of Fe atoms can occur inside the clouds. Laboratory experiments and atmospheric modeling show that this phenomenon is explained by the efficient uptake of Fe on the ice particle surface.

References (abridged):

1. G. E. Thomas, J. Olivero, Adv. Space Res. 28, 937 (2001)

2. U. von Zahn, Eos 84, 261 (2003)

3. G. E. Thomas, Rev. Geophys. 29, 553 (1991)

4. F. J. Loebken, J. Geophys. Res. 104, 9135 (1999)

5. X. Chu, C. S. Gardner, R. G. Roble, J. Geophys. Res. 108 (D8), 10.1029/2002JD002524 (2003)

Science http://www.sciencemag.org

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Related Material:

HISTORY OF ATMOSPHERIC PHYSICS: ON CLOUDS

The following points are made by Graeme L. Stephens (American Scientist 2003 91:442):

1) On a December evening in 1802, Luke Howard (1772-1864), a London pharmacist and amateur meteorologist, aired his ideas about the classification of clouds. These ideas were presented to a small gathering of young science-minded intellectuals who called themselves The Askesian Society. Howard's lecture on that evening was titled "On the Modification of Clouds" and opened as follows:

"My talk this evening concerns itself with what may strike some as an uncharacteristically impractical subject: it is concerned with the modification of clouds. Since the increased attention which has been given to meteorology, the studies of various appearances of water suspended in the atmosphere has become an interesting and even necessary branch of that pursuit. If clouds were the mere result of condensation of vapour in the masses of the atmosphere which they occupy, if their variations were produced by the movements of the atmosphere alone, then indeed might the study be deemed a useless pursuit of shadows...."

2) This was a historic lecture for many reasons. Most importantly, it heralded the beginning of meteorology, a previously unrecognized area of natural science. This lecture was published the following year as an essay and appeared in subsequent publications over a span of almost 20 years. It is a remarkable testimony that Howard's classification, with minor changes, remains in use today by practicing meteorologists. His classification was a revelation, bringing a sense of order and understanding to a subject that had lacked coordinated thought --let alone any documented theories as to how pressure, temperature, rainfall and clouds might be related. Perhaps even more impressive than Howard's classification of clouds, or "modifications" as he referred to them, was his intuition, inspired by the earlier ideas of his close acquaintance John Dalton (1766-1844) that clouds must be considered as "subjects of grave theory and practical research governed by fixed laws." Howard's ideas about the physics of clouds were generally sound despite the poor understanding of the physics of air and water vapor in his time.

3) By contrast, for the past 50 years the modern science of meteorology has fixated on the ever-expanding capability of computer technology and the numerical prediction of the movements of invisible air. To the non-meteorologist this must appear most odd. Clouds, after all, are the most visible manifestation of weather in all its forms, and their prediction should be more than an object of curiosity. However, even the task of numerically integrating forward in time the Navier-Stokes equations governing the behavior of invisible air turned out to be substantially more complex than was originally conceived.

4) Present-day classification of clouds adopted by the World Meteorological Organization:

High clouds (bases > 6 km): cirrus, cirrocumulus, cirrostratus

Middle clouds (bases 2 to 6 km): altocumulus, altostratus, nimbostratus

Low clouds (bases < 2 km) stratocumulus, stratus, cumulus

The 10th type, cumulonimbus, extends through all ranges of altitude.

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