Editing Cloud feedback (section)

=== Longwave Feedback ===
The total longwave (LW) part of the high cloud feedback is positive.<ref name="Zelinka-2012" /> Contributions to the LW feedback stem from changes in cloud altitude, optical depth and cloud amount.

==== Cloud Altitude ====
The longwave feedback is dominated by the positive cloud altitude feedback<ref name="Zelinka-2010" /> which is mainly found in the tropics with the mechanisms being identical in the extra tropics.<ref name="Ceppi-2017" /> The LW radiation emitted by the high cloud tops is proportional to the temperature at the cloud top.<ref name="Ceppi-2017" /> The altitude of the high clouds changes with rising temperatures, due to the following mechanisms:<ref name="Ceppi-2017" /> Higher temperatures on the surface force the moisture to rise, which is fundamentally described by the [[Clausius–Clapeyron relation|Clausius Clapeyron]] equation.<ref name="Ceppi-2017" /><ref name="Zelinka-2010" /> The altitude at which the radiative cooling is still effective is closely tied to the humidity and rises equally.<ref name="Ceppi-2017" /><ref name="Zelinka-2010" /> The altitude, at which the [[radiative cooling]] becomes inefficient due to a lack of moisture, then determines the detrainment height of [[Atmospheric convection|deep convection]] due to the [[Conservation of mass|mass conservation]].<ref name="Ceppi-2017" /><ref name="Zelinka-2010" /> The could top height therefore strongly depends on the surface temperature.<ref name="Ceppi-2017" />

There are three theories on how the altitude and thus temperature depends on surface warming.<ref name="Ceppi-2017" /> The [[Fixed anvil temperature hypothesis|FAT]] (Fixed Anvil Temperature) hypothesis argues, that the isotherms shift upwards with [[Climate change|global warming]] and the temperature at the cloud top stays therefore constant.<ref name="Hartmann-2002">{{Cite journal |last1=Hartmann |first1=Dennis L. |last2=Larson |first2=Kristin |date=2002 |title=An important constraint on tropical cloud - climate feedback |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2002GL015835 |journal=Geophysical Research Letters |language=en |volume=29 |issue=20 |page=1951 |doi=10.1029/2002GL015835 |bibcode=2002GeoRL..29.1951H |issn=0094-8276}}</ref> This results in a positive feedback, since no more radiation is emitted while the surface temperature is rising.<ref name="Hartmann-2002" />  According to the FAT hypothesis this leads to a feedback of  0,27 W m<math>^{-2}</math> K<math>^{-1}</math><ref name="Zelinka-2010">{{Cite journal |last1=Zelinka |first1=Mark D. |last2=Hartmann |first2=Dennis L. |date=2010-08-27 |title=Why is longwave cloud feedback positive? |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2010JD013817 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=115 |issue=D16 |doi=10.1029/2010JD013817 |bibcode=2010JGRD..11516117Z |issn=0148-0227}}</ref>. The second hypothesis called PHAT (Proportionally Higher Anvil Temperature) claims a smaller cloud feedback of 0.20 W m<math>^{-2}</math> K<math>^{-1}</math><ref name="Zelinka-2010" />, due to a slight warming of the cloud tops which agrees better with observations.<ref name="Zelinka-2010" /> The static stability increases with higher surface temperatures in the upper troposphere and lets the clouds shift slightly to warmer temperatures.<ref name="Ceppi-2017" /> The third hypothesis is FAP (Fixed Anvil Pressure) which assumes a constant cloud top pressure with a warming climate, as if the cloud top does not move upwards.<ref name="Zelinka-2010" /> This results in a negative LW feedback, which does not agree with observations.<ref name="Zelinka-2010" /> It can be used to calculate the impact of the cloud height change on the LW feedback.<ref name="Zelinka-2010" /> Most models agree with the PHAT hypothesis which also agrees the most with observations.<ref name="Zelinka-2010" />

==== Optical Depth ====
The optical depth feedback is determined by the increasing optical depth of the high clouds with rising temperatures.<ref name="Stephens-1978">{{Cite journal |last=Stephens |first=G. L. |date=1978-11-01 |title=Radiation Profiles in Extended Water Clouds. II: Parameterization Schemes |url=https://journals.ametsoc.org/view/journals/atsc/35/11/1520-0469_1978_035_2123_rpiewc_2_0_co_2.xml |journal=Journal of the Atmospheric Sciences |language=EN |volume=35 |issue=11 |pages=2123–2132 |doi=10.1175/1520-0469(1978)035<2123:RPIEWC>2.0.CO;2 |bibcode=1978JAtS...35.2123S |issn=0022-4928|doi-access=free }}</ref> The optical depth increases the LW emission of the cloud, so that the contribution of the optical depth to the LW feedback is positive.<ref name="Stephens-1978" /> At the same time, the shortwave contribution of increasing optical depth is negative and, because it is larger than the LW  component, dominates. The overall optical depth feedback for high clouds is just below zero.<ref name="Ceppi-2017" />

==== Cloud Amount ====
The [[Cloud cover|area fraction of high clouds]] is also an important part of the LW feedback. A decrease in the area fraction would lead to a more negative feedback.<ref name="Ceppi-2017" />  Two mechanisms can lead to a decrease in the area fraction and therefore a negative feedback.<ref name="Ceppi-2017" /> The warming at the surface decreases the [[Lapse rate|moist adiabat]]  which leads to a decrease of the [[Subsidence (atmosphere)|clear sky subsidence]].<ref name="Jeevanjee-2022">{{Cite journal |last=Jeevanjee |first=Nadir |date=November 2022 |title=Three Rules for the Decrease of Tropical Convection With Global Warming |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022MS003285 |journal=Journal of Advances in Modeling Earth Systems |language=en |volume=14 |issue=11 |doi=10.1029/2022MS003285 |bibcode=2022JAMES..1403285J |issn=1942-2466}}</ref> Since the convective [[mass flux]] has to be equal to the clear sky subsidence it decreases as well and with it potentially the cloud area fraction.<ref name="Jeevanjee-2022" /> Another argument for a smaller area fraction is that the self-aggregation of clouds increases at higher temperatures.<ref name="Ceppi-2017" /> This would lead to smaller convective areas and larger dry areas which increase the radiative longwave cooling, resulting in a negative feedback.<ref name="Ceppi-2017" /> How the area fraction will change is however a topic of ongoing research and discussion.<ref name="Ceppi-2017" /> Since the area fraction of high clouds in models is sensitive, among others to [[Cloud physics|cloud micro physics]]<ref name="Ceppi-2017" />, there are also models which predict an increase in high cloud area fraction<ref name="Zelinka-2010" /> which would lead to a positive feedback.