View source for Earth:Fixed anvil temperature hypothesis

{{Short description|Idea that the temperature at the top of anvil clouds does not depend on Earth surface temperature}}
[[File:Hector cloud from Gunn Point.jpg|thumb|Anvil cloud over the Tiwi Islands, Australia]]
'''Fixed anvil temperature hypothesis''' is a [[Hypothesis|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 [[Earth:Climate model|climate model]]s and observations of cloud behaviour, it affects [[Earth:Climate sensitivity|how much the world heats up for each extra tonne of greenhouse gas in the atmosphere]]. However, 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 [[Astronomy:Radiative cooling|radiative cooling]] of the [[Earth:Troposphere|troposphere]] is balanced by the release of [[Physics:Latent heat|latent heat]] through condensation of water vapour lofted to high altitudes by [[Astronomy:Convection|convection]]. The radiative cooling is mostly a consequence of [[Physics:Emission spectrum|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.{{sfn|Hartmann|Larson|2002|p=1}}

The "fixed anvil temperature hypothesis" stipulates that owing to energetic and thermodynamic constraints imposed by the [[Physics:Clausius–Clapeyron relation|Clausius-Clapeyron relationship]], the temperature and thus radiative cooling of anvil clouds does not change much with surface temperature.{{sfn|Hartmann|Larson|2002|p=1}} Specifically, cooling decreases below {{convert|200|K|C|order=flip}} as the ineffective radiative cooling by {{chem|C|O|2|link=Carbon dioxide}} becomes dominant below that temperature.{{sfn|Hartmann|Larson|2002|p=3}} Instead, the elevation of high clouds rises with surface temperatures.<ref name="AlbernVoigt2019" />

A related hypothesis is that [[Earth:Tropopause|tropopause]] temperatures are insensitive to surface warming; however it appears to have distinct mechanisms from the fixed anvil temperature process.<ref name="HuVallis2019" /> They have been related to each other in several studies,<ref name="SullivanSchiro2019" /> which sometimes find a fixed tropopause temperature a more reasonable theory than fixed anvil temperature.{{sfn|Seeley|Jeevanjee|Romps|2019|p=1849}}

== 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.{{sfn|Seeley|Jeevanjee|Romps|2019|p=1842}}

=== Models ===

The fixed anvil temperature hypothesis was initially formulated by Hartmann and Larson 2002 in the context of the NCAR/PSU MM5 [[Earth:Climate model|climate model]]{{sfn|Hartmann|Larson|2002|p=2}} but the stability of top cloud temperatures was already observed in a one-dimensional model by Hansen ''et al.'' 1981.{{sfn|Del Genio|2016|p=107}} It has also been recovered, with limitations, in climate models{{sfn|Igel|Drager|van den Heever|2014|p=10516}} and in numerous [[Earth:General circulation model|general circulation model]]s.<ref name="Maher2019" /> However, some have recovered a dependence on cloud size{{sfn|Noda|Seiki|Satoh|Yamada|2016|p=2313}} and on [[Physics:Relative humidity|relative humidity]]<ref name="ChaeSherwood2010" /> or that the fixed anvil temperature is more properly expressed as anvil temperature changing more slowly than surface temperature.{{sfn|Seeley|Jeevanjee|Romps|2019|p=1848}} Climate models also simulate an increase in cloud top height<ref name="Li2019" /> and some radiative-convective models apply it to the [[Astronomy:Outflow (meteorology)|outflow]] of [[Physics:Tropical cyclone|tropical cyclone]]s.<ref name="Shi2014" />

The fixed anvil temperature hypothesis has also been obtained in simulations of [[Astronomy:Exoplanet|exoplanet]] climates.<ref name="YangLeconte2019" /> At very high {{chem|C|O|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.<ref name="RamirezKopparapu2014" />

=== Observations ===

The fixed anvil temperature hypothesis has been backed by observational studies<ref name="Asrar2013" /> for large clouds. Smaller clouds however have no stable temperature and there are temperature fluctuations of about {{convert|5|C-change|F-change|sigfig=1}}{{sfn|Noda|Seiki|Satoh|Yamada|2016|p=2307}} which may relate to processes involving the Brewer-Dobson circulation.<ref name="ChaeSherwood2010" /> Xu ''et al.'' 2007 found that cloud temperatures are more stable for clouds with sizes exceeding {{convert|150|km}}.{{sfn|Noda|Seiki|Satoh|Yamada|2016|p=2312}} The ascent of cloud top height with warming is also supported by observations.<ref name="Li2019" />

== Implications ==

Clouds are the second biggest uncertainty in future climate change after human actions, as their effects are complicated and not properly understood.<ref name=":0">{{Cite web|last=Irfan|first=Umair|date=2021-05-19|title=Scientists aren't sure what will happen to clouds as the planet warms|url=https://www.vox.com/22430792/cloud-science-mystery-unexplainable-podcast-climate-change|access-date=2021-07-05|website=Vox|language=en}}</ref> The fixed anvil temperature hypothesis has effects on global [[Earth:Climate sensitivity|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|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 should use temperature rather than pressure to model the height of high clouds.<ref name="KluftDacie2019" />

== Alternative views ==

A hypothesis which would have the opposite effect on climate is the [[Earth:Iris hypothesis|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.<ref name="SeeleyJeevanjee2019" /> The ''proportionate anvil warming hypothesis'' by Zelinka and Hartmann 2010 was formulated on the basis of [[Earth:General circulation model|general circulation model]]s and envisages a small increase of anvil temperature with high warming.<ref name="ZelinkaHartmann2011" /> The latter hypothesis was intended as a modification to the fixed anvil temperature hypothesis{{sfn|Noda|Seiki|Satoh|Yamada|2016|p=2307}} and includes considerations of atmospheric stability and appears to reflect actual climate conditions more closely.<ref name="KluftDacie2019" /> Finally, there is a view that cloud top temperatures could actually decrease with surface warming{{sfn|Igel|Drager|van den Heever|2014|p=10530}} as convection height rises. This may constitute a non-equilibrium response.{{sfn|Igel|Drager|van den Heever|2014|p=10531}}

== Research ==
{{As of|2020}} further research is needed to properly understand the physics of some cloud feedbacks,<ref>{{Cite web|title=Cooling effect of clouds 'underestimated' by climate models, says new study|url=https://www.weforum.org/agenda/2021/06/cooling-effect-clouds-climate-models-study/|access-date=2021-07-05|website=World Economic Forum|date=10 June 2021 |language=en}}</ref> as they differ between models,<ref>{{Cite journal|last1=Yoshimori|first1=Masakazu|last2=Lambert|first2=F. Hugo|last3=Webb|first3=Mark J.|last4=Andrews|first4=Timothy|date=2020-04-01|title=Fixed Anvil Temperature Feedback: Positive, Zero, or Negative?|journal=Journal of Climate|language=EN|volume=33|issue=7|pages=2719–2739|doi=10.1175/JCLI-D-19-0108.1|bibcode=2020JCli...33.2719Y|issn=0894-8755|doi-access=free|hdl=10871/121112|hdl-access=free}}</ref> and progress on properly modelling clouds globally is very slow.<ref name=":0" />

== References ==
{{Reflist|refs=
<ref name="Shi2014">{{cite journal |last1=Shi |first1=Xiaoming |last2=Bretherton |first2=Christopher S. |title=Large-scale character of an atmosphere in rotating radiative-convective equilibrium |journal=Journal of Advances in Modeling Earth Systems |date=September 2014 |volume=6 |issue=3 |page=616 |doi=10.1002/2014MS000342|bibcode=2014JAMES...6..616S |doi-access=free }}</ref>
<ref name="ChaeSherwood2010">{{cite journal |last1=Chae |first1=Jung Hyo |last2=Sherwood |first2=Steven C. |title=Insights into Cloud-Top Height and Dynamics from the Seasonal Cycle of Cloud-Top Heights Observed by MISR in the West Pacific Region |journal=Journal of the Atmospheric Sciences |date=1 January 2010 |volume=67 |issue=1 |page=259 |doi=10.1175/2009JAS3099.1 |bibcode=2010JAtS...67..248C |language=EN |issn=0022-4928|doi-access=free }}</ref>
<ref name="SullivanSchiro2019">{{cite journal |last1=Sullivan |first1=Sylvia C. |last2=Schiro |first2=Kathleen A. |last3=Stubenrauch |first3=Claudia |last4=Gentine |first4=Pierre |title=The Response of Tropical Organized Convection to El Niño Warming |journal=Journal of Geophysical Research: Atmospheres |date=2019 |volume=124 |issue=15 |page=8490 |doi=10.1029/2019JD031026 |bibcode=2019JGRD..124.8481S |language=en |issn=2169-8996|doi-access=free }}</ref>
<ref name="KluftDacie2019">{{cite journal |last1=Kluft |first1=Lukas |last2=Dacie |first2=Sally |last3=Buehler |first3=Stefan A. |last4=Schmidt |first4=Hauke |last5=Stevens |first5=Bjorn |title=Re-Examining the First Climate Models: Climate Sensitivity of a Modern Radiative–Convective Equilibrium Model |journal=Journal of Climate |date=1 December 2019 |volume=32 |issue=23 |page=8121 |doi=10.1175/JCLI-D-18-0774.1 |bibcode=2019JCli...32.8111K |url=https://journals.ametsoc.org/view/journals/clim/32/23/jcli-d-18-0774.1.xml |language=EN |issn=0894-8755|hdl=21.11116/0000-0002-A35E-D |s2cid=135038760 |hdl-access=free }}</ref>
<ref name="YangLeconte2019">{{cite journal |last1=Yang |first1=Jun |last2=Leconte |first2=Jérémy |last3=Wolf |first3=Eric T. |last4=Merlis |first4=Timothy |last5=Koll |first5=Daniel D. B. |last6=Forget |first6=François |last7=Abbot |first7=Dorian S. |title=Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: A 3D Model Intercomparison |journal=The Astrophysical Journal |date=April 2019 |volume=875 |issue=1 |page=11 |doi=10.3847/1538-4357/ab09f1 |language=en |issn=0004-637X|arxiv=1912.11329 |bibcode=2019ApJ...875...46Y |s2cid=146053272 |doi-access=free }}</ref>
<ref name="Li2019">{{cite journal |last1=Li |first1=R. L. |last2=Storelvmo |first2=T. |last3=Fedorov |first3=A. V. |last4=Choi |first4=Y.-S. |title=A Positive Iris Feedback: Insights from Climate Simulations with Temperature-Sensitive Cloud–Rain Conversion |journal=Journal of Climate |date=15 August 2019 |volume=32 |issue=16 |page=5306 |doi=10.1175/JCLI-D-18-0845.1 |bibcode=2019JCli...32.5305L |url=https://journals.ametsoc.org/view/journals/clim/32/16/jcli-d-18-0845.1.xml |language=EN |issn=0894-8755|hdl=10852/83101 |s2cid=198420050 |hdl-access=free }}</ref>
<ref name="Asrar2013">{{Cite book|url=http://link.springer.com/10.1007/978-94-007-6692-1|title=Climate Science for Serving Society|date=2013|publisher=Springer Netherlands|isbn=978-94-007-6691-4|editor-last=Asrar|editor-first=Ghassem R.|location=Dordrecht|language=en|doi=10.1007/978-94-007-6692-1|editor-last2=Hurrell|editor-first2=James W.|page=406|s2cid=131478611}}</ref>
<ref name="HuVallis2019">{{cite journal |last1=Hu |first1=Shineng |last2=Vallis |first2=Geoffrey K. |title=Meridional structure and future changes of tropopause height and temperature |journal=Quarterly Journal of the Royal Meteorological Society |date=2019 |volume=145 |issue=723 |page=2709 |doi=10.1002/qj.3587 |url=https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/qj.3587 |language=en |issn=1477-870X|arxiv=1902.08230 |bibcode=2019QJRMS.145.2698H |s2cid=118967908 }}</ref>
<ref name="ZelinkaHartmann2011">{{cite journal |last1=Zelinka |first1=Mark D. |last2=Hartmann |first2=Dennis L. |title=The observed sensitivity of high clouds to mean surface temperature anomalies in the tropics: Temperature Sensitivity of High Clouds |journal=Journal of Geophysical Research: Atmospheres |date=16 December 2011 |volume=116 |issue=D23 |page=1 |doi=10.1029/2011JD016459}}</ref>
<ref name="SeeleyJeevanjee2019">{{cite journal |last1=Seeley |first1=Jacob T. |last2=Jeevanjee |first2=Nadir |last3=Langhans |first3=Wolfgang |last4=Romps |first4=David M. |title=Formation of Tropical Anvil Clouds by Slow Evaporation |journal=Geophysical Research Letters |date=2019 |volume=46 |issue=1 |page=492 |doi=10.1029/2018GL080747 |bibcode=2019GeoRL..46..492S |s2cid=134486980 |language=en |issn=1944-8007|doi-access=free }}</ref>
<ref name="Maher2019">{{cite journal |last1=Maher |first1=Penelope |last2=Gerber |first2=Edwin P. |last3=Medeiros |first3=Brian |last4=Merlis |first4=Timothy M. |last5=Sherwood |first5=Steven |last6=Sheshadri |first6=Aditi |last7=Sobel |first7=Adam H. |last8=Vallis |first8=Geoffrey K. |last9=Voigt |first9=Aiko |last10=Zurita-Gotor |first10=Pablo |title=Model Hierarchies for Understanding Atmospheric Circulation |journal=Reviews of Geophysics |date=2019 |volume=57 |issue=2 |page=267 |doi=10.1029/2018RG000607 |bibcode=2019RvGeo..57..250M |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018RG000607 |language=en |issn=1944-9208|hdl=10871/36644 |s2cid=146704580 |hdl-access=free }}</ref>
<ref name="RamirezKopparapu2014">{{cite journal |last1=Ramirez |first1=Ramses M. |last2=Kopparapu |first2=Ravi Kumar |last3=Lindner |first3=Valerie |last4=Kasting |first4=James F. |title=Can Increased Atmospheric CO2 Levels Trigger a Runaway Greenhouse? |journal=Astrobiology |date=August 2014 |volume=14 |issue=8 |page=723 |doi=10.1089/ast.2014.1153 |pmid=25061956 |bibcode=2014AsBio..14..714R |url=https://www.liebertpub.com/doi/full/10.1089/ast.2014.1153 |issn=1531-1074}}</ref>
<ref name="AlbernVoigt2019">{{cite journal |last1=Albern |first1=Nicole |last2=Voigt |first2=Aiko |last3=Pinto |first3=Joaquim G. |title=Cloud-Radiative Impact on the Regional Responses of the Midlatitude Jet Streams and Storm Tracks to Global Warming |journal=Journal of Advances in Modeling Earth Systems |date=2019 |volume=11 |issue=7 |page=1949 |doi=10.1029/2018MS001592 |bibcode=2019JAMES..11.1940A |s2cid=182771431 |language=en |issn=1942-2466|doi-access=free }}</ref>
}}

=== Sources ===
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* {{cite journal |last1=Igel |first1=Matthew R. |last2=Drager |first2=Aryeh J. |last3=van den Heever |first3=Susan C. |title=A CloudSat cloud object partitioning technique and assessment and integration of deep convective anvil sensitivities to sea surface temperature: CloudSat Objects and Sensitivity to SST |journal=Journal of Geophysical Research: Atmospheres |date=16 September 2014 |volume=119 |issue=17 |pages=10515–10535 |doi=10.1002/2014JD021717|doi-access=free }}
* {{cite journal |last1=Noda |first1=A. T. |last2=Seiki |first2=T. |last3=Satoh |first3=M. |last4=Yamada |first4=Y. |title=High cloud size dependency in the applicability of the fixed anvil temperature hypothesis using global nonhydrostatic simulations: Cloud Size Dependency of FAT Hypothesis |journal=Geophysical Research Letters |date=16 March 2016 |volume=43 |issue=5 |pages=2307–2314 |doi=10.1002/2016GL067742|doi-access=free }}
* {{cite journal |last1=Seeley |first1=Jacob T. |last2=Jeevanjee |first2=Nadir |last3=Romps |first3=David M. |title=FAT or FiTT: Are Anvil Clouds or the Tropopause Temperature Invariant? |journal=Geophysical Research Letters |date=2019 |volume=46 |issue=3 |pages=1842–1850 |doi=10.1029/2018GL080096 |bibcode=2019GeoRL..46.1842S |s2cid=135276405 |language=en |issn=1944-8007|doi-access=free }}
{{refend}}{{Climate change}}

[[Category:Clouds]]
[[Category:Climatology]]
[[Category:Climate change feedbacks]]

{{Sourceattribution|Fixed anvil temperature hypothesis}}