Hypercane

Hypercane er en hypotetisk tropisk syklon.^[1] Kerry Emanuel med flere har forslått at slike ekstreme sykloner kan dannes dersom havstemperaturen når 50°C.^[2] ^[3] En slik økning kan skyldes en stor asteroide- eller kometpåvirkning, en stor undersjøisk flombasalt eller på grunn av ekstrem global oppvarming.

Ifølge A Dictionary of Weather er hypercane en dårlig definert term.^[4] Emmanuel har etter den opprinnelige artikkelen ble publisert påvist feil i beregningene og anbefalt at det må gjøres nye beregninger.^[2]

Hypercane brukes også noen ganger av avisene om en eksepsjonelt sterk tropisk syklon.^[4]

Forklaring[rediger | rediger kilde]

For at en «hypercane» skal kunne oppstå, må havtemperaturen være minst 45°C. En betraktelig forskjell mellom en «hypercane» og dagens tropisk syklon er at en «hypercane» ville ha strukket seg inn i den øvre stratosfæren, mens dagens tropiske sykloner strekker seg ut i bare den nedre stratosfæren.

Om en «hypercane» hadde oppstått, ville de ha hatt vindhastigheter på over 600 km/t.^[3] Skulle en slik tropisk syklon nå frem til et land, ville den ha ødelagt alt i sin vei. Slike tropiske sykloner ville ha et trykk på under 700 hPa, noe som gir dem en enorm levetid.

De ekstreme forholdene som kreves for å skape en «hypercane», kan tenkes å produsere et system opp til størrelse med Nord-Amerika, og skape stormflod på 18 meter og et øye på nesten 300 km. Vannet kan forbli varmt nok i flere uker, slik at flere «hypercaner» blir dannet. Skyene i en «hypercane» vil nå fra 30 km til 40 km inn i stratosfæren. En slik intens tropisk syklon vil også skade jordens ozon. Vannmolekyler i stratosfæren vil reagere med ozon for å akselerere forfall til oksygen og redusere absorpsjon av ultrafiolett lys.

Referanser[rediger | rediger kilde]

^ Kerry A. Emanuel, Kevin Speer, Richard Rotunno, Ramesh Srivastava, Mario Molina (20. juli 1995). «Hypercanes: A possible link in global extinction scenarios». Journal of Geophysical Research. American Geophysical Union. 100 (D7): 13.755–13.765. doi:10.1029/95JD01368.
^ ^a ^b Kerry Emanuel (10. februar 2003). «Tropical Cyclones». Annual Review of Earth and Planetary Sciences. 31: 75–104. doi:10.1146/annurev.earth.31.100901.141259. «A closer look at the expression for maximum wind speed, Equation 8, reveals that it is an implicit expression. This is because the saturation enthalpy of the sea surface, k0*, is a function of pressure as well as temperature, increasing with decreasing pressure. As winds increase, the surface pressure in the eyewall falls, increasing the value of k0*. This is a positive feedback that, under typical tropical conditions, contributes approximately 40% to the maximum wind speed. However, if the sea surface temperature is substantially greater, this feedback becomes strong enough that Equation 8 no longer has a solution (Emanuel 1988). This implies that the extra heat input from the ocean can no longer be balanced by surface-based dissipation, and any storm that might occur would be of a very different character. The author dubbed such storms hypercanes. Emanuel et al. (1995) proposed that hypercanes might occur after the impact of a large comet or asteroid with the ocean. Numerical simulations show that seawater temperatures of at least 50°C are necessary to generate hypercanes. These simulations also suggest that hypercanes are relatively narrow, with eye dimensions of a few kilometers, and owing to extremely high specific entropy in the core, they can extend into the middle stratosphere. The authors proposed that such events might inject enough water into the middle stratosphere to destroy the ozone layer, thereby contributing to the mass extinctions that are thought to follow large asteroid impacts. The numerical code used in this work was designed for subsonic flows, but the simulated storms contained supersonic wind speeds. Thus, there is a need to revisit this problem using a numerical code capable of simulating flows with Mach numbers of order one.»
^ ^a ^b «ENVIRONMENT: The Dawn of the Hypercane?». web.archive.org. 17. mai 2008. Archived from the original on 17. mai 2008. Besøkt 16. mars 2019. CS1-vedlikehold: Uheldig URL (link)
^ ^a ^b Dunlop, Storm (2008). «hypercane». A Dictionary of Weather (2 ed.). Oxford University Press. ISBN 9780191726903. «A poorly defined term for a hypothetical, intense tropical cyclone that it has been suggested might result from extreme heating produced by the impact of a kilometre-sized meteorite in the ocean. The term is also occasionally used in the media for an exceptionally violent tropical cyclone.»

[Emmanuel1995-1] Kerry A. Emanuel, Kevin Speer, Richard Rotunno, Ramesh Srivastava, Mario Molina (20. juli 1995). «Hypercanes: A possible link in global extinction scenarios». Journal of Geophysical Research. American Geophysical Union. 100 (D7): 13.755–13.765. doi:10.1029/95JD01368.

[Emmanuel2003-2] Kerry Emanuel (10. februar 2003). «Tropical Cyclones». Annual Review of Earth and Planetary Sciences. 31: 75–104. doi:10.1146/annurev.earth.31.100901.141259. «A closer look at the expression for maximum wind speed, Equation 8, reveals that it is an implicit expression. This is because the saturation enthalpy of the sea surface, k0*, is a function of pressure as well as temperature, increasing with decreasing pressure. As winds increase, the surface pressure in the eyewall falls, increasing the value of k0*. This is a positive feedback that, under typical tropical conditions, contributes approximately 40% to the maximum wind speed. However, if the sea surface temperature is substantially greater, this feedback becomes strong enough that Equation 8 no longer has a solution (Emanuel 1988). This implies that the extra heat input from the ocean can no longer be balanced by surface-based dissipation, and any storm that might occur would be of a very different character. The author dubbed such storms hypercanes. Emanuel et al. (1995) proposed that hypercanes might occur after the impact of a large comet or asteroid with the ocean. Numerical simulations show that seawater temperatures of at least 50°C are necessary to generate hypercanes. These simulations also suggest that hypercanes are relatively narrow, with eye dimensions of a few kilometers, and owing to extremely high specific entropy in the core, they can extend into the middle stratosphere. The authors proposed that such events might inject enough water into the middle stratosphere to destroy the ozone layer, thereby contributing to the mass extinctions that are thought to follow large asteroid impacts. The numerical code used in this work was designed for subsonic flows, but the simulated storms contained supersonic wind speeds. Thus, there is a need to revisit this problem using a numerical code capable of simulating flows with Mach numbers of order one.»

[IPS-3] «ENVIRONMENT: The Dawn of the Hypercane?». web.archive.org. 17. mai 2008. Archived from the original on 17. mai 2008. Besøkt 16. mars 2019. CS1-vedlikehold: Uheldig URL (link)

[Dictweather-4] Dunlop, Storm (2008). «hypercane». A Dictionary of Weather (2 ed.). Oxford University Press. ISBN 9780191726903. «A poorly defined term for a hypothetical, intense tropical cyclone that it has been suggested might result from extreme heating produced by the impact of a kilometre-sized meteorite in the ocean. The term is also occasionally used in the media for an exceptionally violent tropical cyclone.»

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