EOC scientists are setting up measuring equipment in the Atacama Desert, one of Earth’s most arid regions. Their purpose is to find climate signals and indicators to improve present tsunami early warning systems. The European Southern Observatory (ESO) operates there, on Cerro Paranal, a mountain in northern Chile, the Very Large Telescope array (VLT), whose infrastructure can be used by EOC for this purpose. On 24 Oct. 2017 a corresponding cooperation agreement was signed at ESO in Garching.
Cooperation with ESO makes it possible to operate the equipment at an ideal location. At more than 2,600 metres altitude the dry climate of the Atacama Desert allows a very clear view of the heavens at night, and also makes it possible for the instruments to record the sky over the Pacific Ocean, only 12 kilometres away. In addition to the climate signals, these advantages could mean a considerable time advantage when giving early warnings of tsunami. The cooperation also benefits ESO. DLR scientist Prof. Michael Bittner explains: "We anticipate being able to improve our astronomy weather forecasts with this new data. This is crucial information for astronomers, who need to know what kinds of atmospheric disruptions to expect during their observations". Prof. Stefan Dech, EOC director, is enthusiastic about the opportunities provided by this agreement for earth observation: "If we can integrate the instruments also in a tsunami early warning system for Chile, then this agreement can mark the beginning of an entirely new line of technology that might one day also be used on satellites".
The measurement equipment is comprised of a spectrometer (GRIPS) and an infrared camera (FAIM) that record the near-infrared glow that is emitted in the atmosphere at night. This radiation is invisible to the human eye, has wavelengths of ca. 1.0 to 1.5 micrometres, and arises primarily at an elevation of about 90 kilometres. A fragile layer of hydroxyl (OH), only a few kilometres thick, envelops the planet there. Hydroxyl is produced in photochemical reactions mainly involving ozone, atomic hydrogen and atomic oxygen. What is special is that in the process of formation the two-atom hydroxyl molecule goes into complex rotating and pulsating modes. This inner-molecular movement causes the OH layer to glow. This so-called "airglow" is particularly intensive in the spectral range referred to, and thus relatively easy to measure. The GRIPS and FAIM measuring equipment record this radiation with high temporal and horizontal resolution. The ambient temperature at 90 km altitude can be derived from these recorded emission spectra since the excited hydroxyl molecules are in thermodynamic equilibrium with the local atmosphere there.
Continuous temperature measurements in the mesopause, far from the turbulent weather in the stratosphere, also make it possible to predict temperature changes near Earth’s surface. More carbon dioxide in the lower levels of the atmosphere leads to more warming. At higher altitudes, by contrast, which is where the EOC instruments measure, the carbon dioxide leads to cooling. The reason is, to keep it simple, that when each carbon dioxide molecule collides with other atmospheric molecules, it takes over some of their kinetic energy and releases it in the form of heat. At higher atmospheric levels this results in a “cooling-to-space” effect. The atmosphere cools here because the heat radiation can immediately escape into space due to the low air density (at 100 kilometres altitude it is about one hundred thousand times lower than on the earth’s surface).
The interesting aspect is that this cooling is about one order of magnitude greater than the warming at the earth’s surface. Thus even minor changes in the temperature mix in the atmosphere can be detected in good time by monitoring the temperature trend in the mesopause at an altitude of 90 kilometres. These temperature changes are not uniform globally and do not follow a simple linear trend. The variations take place over various temporal scales and have diverse causes. These include dynamic processes in the Earth System that take place every two years as well as cycles with periods of several decades, which are more likely caused by extra-terrestrial events. In addition, the temperatures vary from cycle to cycle. The geographic latitude also seems to play a role. Measurements at Cerro Paranal will help us to improve our understanding of the interrelationships and to decode these “climate signals”. The measurements are one of the contributions to the international Network for the Detection of Mesospheric Change, NDMC.
The wafer-thin hydroxyl layer enveloping Earth not only supplies indications of global temperature changes but also reacts to pressure changes in the underlying atmosphere. If a horizontal airstream on Earth’s surface encounters a barrier like the Andes, it is forced to rise. These rising air masses gradually cool as a result, becoming relatively heavier than their environment. This causes them to sink down again, warming up in the process, and then when they become lighter than the ambient air they once again rise, and so forth, repeatedly. In this way a pulsating airstream is produced in the atmosphere, a “gravity wave”. Because of the low air density in the upper stories of the atmosphere the vertical deviation of the wave peaks and valleys becomes larger with increasing altitude, where it can be clearly measured.
These waves are comparable to those we know from the ocean. They transport energy and momentum over large distances. When they break they release their energy to the surrounding atmosphere. These processes are, however, not explicitly incorporated in any current numeric models of the atmosphere. Their influence has to be estimated ("parametrized"), which makes current models open to attack.
Cerro Paranal is, so to speak, close to a birthplace of gravity waves. By making local measurements there, gravity waves, their interactions with each other, and emerging turbulence can be analysed.
Zu einer Druckveränderung in der Atmosphäre kommt es auch durch einen Tsunami. Die vertikale Anhebung des Meeresbodens und des Wasserkörpers staucht die Luftmassen über dem Ozean zusammen. Dieser Druckimpuls breitet sich dann als Infraschall-Welle aus und erreicht in nur sechs bis acht Minuten auch die luftleuchtende Schicht. Die Druckschwankung führt dort zu einem "Flackern" der Intensität, das durch die Messgeräte detektiert werden kann. Bei einem Seebeben mit horizontaler Auslenkung bleibt diese Druckschwankung aus. Dadurch liefern die Messungen binnen weniger Minuten nach einem Seebeben einen Hinweis, ob ein Tsunami ausgelöst wurde. So könnten künftig Fehlalarme vermieden werden und die Vorwarnzeit erheblich verkürzt werden. In Chile benötigt ein Tsunami weniger als eine halbe Stunde, bis er die Küste erreicht.
Tsunami can also cause pressure changes in the atmosphere when the vertical lifting of the seafloor and the mass of water above it compresses the air masses over the ocean. This pressure impulse spreads as an infrasound wave and reaches the airglow level in just six to eight minutes. The pressure fluctuation there causes the intensity to “flicker”, which can be detected by the measurement instruments. In the case of a submarine earthquake with horizontal deviation there is no accompanying pressure fluctuation in the atmosphere. Thus the measurements indicate within a very few minutes after a seismic quake whether a tsunami was triggered. In this way false alarms can be avoided in the future and the time before an advance-warning considerably shortened. In Chile a tsunami needs less than half an hour to reach the coast.
(see links at upper right)
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