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== Implications ==
== Implications ==


The fixed anvil temperature hypothesis has effects on global [[climate sensitivity]], since anvil clouds are the most important source of outgoing radiation linked to tropical convection{{sfn|Hartmann|Larson|2002|pp=1-2}} and their temperature being stable would render the outgoing radiation non-responsive to surface temperature changes.{{sfn|Hartmann|Larson|2002|p=4}} This creates a [[positive feedback]] component of [[cloud feedback]].{{sfn|Del Genio|2016|p=116}} The fixed anvil temperature hypothesis has also been used to argue that climate modelling of high clouds should use temperature rather than pressure to localize{{Clarify|reason=localize = to define the 2D shape and position of the cloud top from above in the model?|date=July 2021}} high clouds.<ref name="KluftDacie2019" />
The fixed anvil temperature hypothesis has effects on global [[climate sensitivity]], since anvil clouds are the most important source of outgoing radiation linked to tropical convection{{sfn|Hartmann|Larson|2002|pp=1-2}} and their temperature being stable would render the outgoing radiation non-responsive to surface temperature changes.{{sfn|Hartmann|Larson|2002|p=4}} This creates a [[positive feedback]] component of [[cloud feedback]].{{sfn|Del Genio|2016|p=116}} The fixed anvil temperature hypothesis has also been used to argue that climate modelling of high clouds should use temperature rather than pressure to model the height of high clouds.<ref name="KluftDacie2019" />


== Alternative views ==
== Alternative views ==

Revision as of 13:48, 4 July 2021

Anvil cloud over tropical Australia

Fixed anvil temperature hypothesis is a physical hypothesis that describes the response of cloud radiative properties to rising surface temperatures. It presumes that the temperature at which radiation is emitted by anvil clouds is constrained by radiative processes and thus does not change in response to surface warming. Since the amount of radiation emitted by clouds is a function of their temperature, it implies that it does not increase with surface warming and thus a warmer surface does not increase radiation emissions (and thus cooling) by cloud tops. The mechanism has been identified both in climate models and observations of cloud behaviour, although some evidence suggests that it may be more correctly formulated as decreased anvil warming rather than no anvil warming.

Background and hypothesis

In the tropics, the radiative cooling of the troposphere is balanced by the release of latent heat through condensation of water vapour lofted to high altitudes by convection. The radiative cooling is mostly a consequence of emissions by water vapour and thus becomes ineffective above the 200 hPa pressure level. Congruently, it is at this elevation that thick clouds and anvil clouds - the topmost convective clouds - concentrate.[1]

The "fixed anvil temperature hypothesis" stipulates that owing to energetic and thermodynamic constraints imposed by the Clausius-Clapeyron relationship, the temperature and thus radiative cooling of anvil clouds does not change much with surface temperature.[1] Specifically, cooling decreases below −73 °C (200 K) as the ineffective radiative cooling by CO
2 becomes dominant below that temperature.[2] Instead, the elevation of high clouds rises with surface temperatures.[3]

A related hypothesis is that tropopause temperatures are insensitive to surface warming; however it appears to have distinct mechanisms from the fixed anvil temperature process.[4] They have been related to each other in several studies,[5] which sometimes find a fixed tropopause temperature a more reasonable theory than fixed anvil temperature.[6]

Evidence

The fixed anvil temperature hypothesis has been widely accepted and even extended to the non-tropical atmosphere. Its strength relies in part on its reliance on simple physical arguments.[7]

Models

The fixed anvil temperature hypothesis was initially formulated by Hartmann and Larson 2002 in the context of the NCAR/PSU MM5 climate model[8] but the stability of top cloud temperatures was already observed in an one-dimensional model by Hansen et al. 1981.[9] It has also been recovered, with limitations, in climate models[10] and in numerous general circulation models.[11] However, some have recovered a dependence on cloud size[12] and on relative humidity[13] or that the fixed anvil temperature is more properly expressed as anvil temperature changing more slowly than surface temperature.[14] Climate models also simulate an increase in cloud top height[15] and some radiative-convective models apply it to the outflow of tropical cyclones.[16]

The fixed anvil temperature hypothesis has also been obtained in simulations of exoplanet climates.[17] At very high CO
2 concentrations approaching a runaway greenhouse however, other physical effects pertaining to cloud opacity may take over and dominate the fixed anvil temperature as surface temperatures reach extreme levels.[18]

Observations

The fixed anvil temperature hypothesis has been backed by observational studies[19] for large clouds. Smaller clouds however have no stable temperature and there are temperature fluctuations of about 5.0 °C (5 K)[20] which may relate to processes involving the Brewer-Dobson circulation.[13] Xu et al. 2007 found that cloud temperatures are more stable for clouds with sizes exceeding 150 kilometres (93 mi).[21] The ascent of cloud top height with warming is also supported by observations.[15]

Implications

The fixed anvil temperature hypothesis has effects on global climate sensitivity, since anvil clouds are the most important source of outgoing radiation linked to tropical convection[22] and their temperature being stable would render the outgoing radiation non-responsive to surface temperature changes.[23] This creates a positive feedback component of cloud feedback.[24] The fixed anvil temperature hypothesis has also been used to argue that climate modelling of high clouds should use temperature rather than pressure to model the height of high clouds.[25]

Alternative views

An alternative hypothesis is the iris hypothesis, according to which the coverage of anvil clouds declines with warming, thus allowing more radiation to escape into space and resulting in slower warming.[26] The proportionate anvil warming hypothesis by Zelinka and Hartmann 2010 was formulated on the basis of general circulation models and envisages a small increase of anvil temperature with high warming.[27] The latter hypothesis was intended as a modification to the fixed anvil temperature hypothesis[20] and includes considerations of atmospheric stability and appears to reflect actual climate conditions more closely.[25] Finally, there is a view that cloud top temperatures could actually decrease with surface warming[28] as convection height rises. This may constitute a non-equilibrium response.[29]

References

  1. ^ a b Hartmann & Larson 2002, p. 1.
  2. ^ Hartmann & Larson 2002, p. 3.
  3. ^ Albern, Nicole; Voigt, Aiko; Pinto, Joaquim G. (2019). "Cloud-Radiative Impact on the Regional Responses of the Midlatitude Jet Streams and Storm Tracks to Global Warming". Journal of Advances in Modeling Earth Systems. 11 (7): 1949. doi:10.1029/2018MS001592. ISSN 1942-2466.
  4. ^ Hu, Shineng; Vallis, Geoffrey K. (2019). "Meridional structure and future changes of tropopause height and temperature". Quarterly Journal of the Royal Meteorological Society. 145 (723): 2709. doi:10.1002/qj.3587. ISSN 1477-870X.
  5. ^ Sullivan, Sylvia C.; Schiro, Kathleen A.; Stubenrauch, Claudia; Gentine, Pierre (2019). "The Response of Tropical Organized Convection to El Niño Warming". Journal of Geophysical Research: Atmospheres. 124 (15): 8490. doi:10.1029/2019JD031026. ISSN 2169-8996.
  6. ^ Seeley, Jeevanjee & Romps 2019, p. 1849.
  7. ^ Seeley, Jeevanjee & Romps 2019, p. 1842.
  8. ^ Hartmann & Larson 2002, p. 2.
  9. ^ Del Genio 2016, p. 107.
  10. ^ Igel, Drager & van den Heever 2014, p. 10516.
  11. ^ Maher, Penelope; Gerber, Edwin P.; Medeiros, Brian; Merlis, Timothy M.; Sherwood, Steven; Sheshadri, Aditi; Sobel, Adam H.; Vallis, Geoffrey K.; Voigt, Aiko; Zurita-Gotor, Pablo (2019). "Model Hierarchies for Understanding Atmospheric Circulation". Reviews of Geophysics. 57 (2): 267. doi:10.1029/2018RG000607. ISSN 1944-9208.
  12. ^ Noda et al. 2016, p. 2313.
  13. ^ a b Chae, Jung Hyo; Sherwood, Steven C. (1 January 2010). "Insights into Cloud-Top Height and Dynamics from the Seasonal Cycle of Cloud-Top Heights Observed by MISR in the West Pacific Region". Journal of the Atmospheric Sciences. 67 (1): 259. doi:10.1175/2009JAS3099.1. ISSN 0022-4928.
  14. ^ Seeley, Jeevanjee & Romps 2019, p. 1848.
  15. ^ a b Li, R. L.; Storelvmo, T.; Fedorov, A. V.; Choi, Y.-S. (15 August 2019). "A Positive Iris Feedback: Insights from Climate Simulations with Temperature-Sensitive Cloud–Rain Conversion". Journal of Climate. 32 (16): 5306. doi:10.1175/JCLI-D-18-0845.1. ISSN 0894-8755.
  16. ^ Shi, Xiaoming; Bretherton, Christopher S. (September 2014). "Large-scale character of an atmosphere in rotating radiative-convective equilibrium". Journal of Advances in Modeling Earth Systems. 6 (3): 616. doi:10.1002/2014MS000342.
  17. ^ Yang, Jun; Leconte, Jérémy; Wolf, Eric T.; Merlis, Timothy; Koll, Daniel D. B.; Forget, François; Abbot, Dorian S. (April 2019). "Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: A 3D Model Intercomparison". The Astrophysical Journal. 875 (1): 11. doi:10.3847/1538-4357/ab09f1. ISSN 0004-637X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  18. ^ Ramirez, Ramses M.; Kopparapu, Ravi Kumar; Lindner, Valerie; Kasting, James F. (August 2014). "Can Increased Atmospheric CO2 Levels Trigger a Runaway Greenhouse?". Astrobiology. 14 (8): 723. doi:10.1089/ast.2014.1153. ISSN 1531-1074.
  19. ^ Asrar, Ghassem R.; Hurrell, James W., eds. (2013). Climate Science for Serving Society. Dordrecht: Springer Netherlands. p. 406. doi:10.1007/978-94-007-6692-1. ISBN 978-94-007-6691-4.
  20. ^ a b Noda et al. 2016, p. 2307.
  21. ^ Noda et al. 2016, p. 2312.
  22. ^ Hartmann & Larson 2002, pp. 1–2.
  23. ^ Hartmann & Larson 2002, p. 4.
  24. ^ Del Genio 2016, p. 116.
  25. ^ a b Kluft, Lukas; Dacie, Sally; Buehler, Stefan A.; Schmidt, Hauke; Stevens, Bjorn (1 December 2019). "Re-Examining the First Climate Models: Climate Sensitivity of a Modern Radiative–Convective Equilibrium Model". Journal of Climate. 32 (23): 8121. doi:10.1175/JCLI-D-18-0774.1. ISSN 0894-8755.
  26. ^ Seeley, Jacob T.; Jeevanjee, Nadir; Langhans, Wolfgang; Romps, David M. (2019). "Formation of Tropical Anvil Clouds by Slow Evaporation". Geophysical Research Letters. 46 (1): 492. doi:10.1029/2018GL080747. ISSN 1944-8007.
  27. ^ Zelinka, Mark D.; Hartmann, Dennis L. (16 December 2011). "The observed sensitivity of high clouds to mean surface temperature anomalies in the tropics: Temperature Sensitivity of High Clouds". Journal of Geophysical Research: Atmospheres. 116 (D23): 1. doi:10.1029/2011JD016459.
  28. ^ Igel, Drager & van den Heever 2014, p. 10530.
  29. ^ Igel, Drager & van den Heever 2014, p. 10531.

Sources

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