21 January 2019

FAT or FiTT: Are Anvil Clouds or the Tropopause Temperature-Invariant?

A proposal for Fixed Tropopause Temperature (FiTT) instead of FAT.


It is widely believed that tropical anvil clouds maintain themselves at a fairly constant temperature as Earth warms or cools.  This has become known as the Fixed Anvil Temperature (FAT) hypothesis, and it is argued to stem from basic physics.  But, is the anvil temperature truly temperature-invariant?  If not, where does the logic for the hypothesis fail?  To explore these questions, simulations of radiative-convective equilibrium (RCE) are run in a cloud-resolving model that has been stripped down to the minimal set of physics used to justify the FAT hypothesis.  Even in this best-case scenario, it is found that FAT is violated.  Instead of FAT, a Fixed Tropopause Temperature (FiTT) is found.


Tropical anvil clouds play a large role in Earth's radiation balance, and an important part of their effect is their emission of infrared radiation to space at a rate that is set by their temperature.  Therefore, in order to gain a deep understanding of Earth’s energy balance, it is important to learn what sets the anvil temperature.  This work suggests that a first step in that direction would be to understand what sets the invariant temperature of the overlying tropopause.


As Earth warms, the distribution of clouds changes. Some of those changes generate positive feedback (i.e., producing a warming effect), while others generate negative feedback (i.e., producing a cooling effect). Anvil clouds, which are the high clouds generated by deep convection, would tend to produce positive feedback if their area and temperature do not change as the planet warms. This absence of any change would be positive feedback because they would continue to send energy to space, in the form of longwave radiation, at the same rate regardless of how much the planet warms, thereby reducing the magnitude of the negative Planck feedback.

A widely accepted hypothesis is that anvil clouds do, indeed, remain fixed in temperature as the planet warms, at least to good approximation. This has become known as the Fixed Anvil Temperature (FAT) hypothesis. Furthermore, it has been argued that the FAT hypothesis stems from basic physics. To test this claim, we constructed a cloud-resolving simulation that has only the minimal ingredients upon which the FAT hypothesis rests. In those simulations, we find that FAT is violated: while the changes in the anvil temperatures are damped relative to changes in the sea-surface temperature (SST), they are far from invariant. In fact, the changes in the anvil temperatures are about 40% as large as the changes in the SST. A closer look at the previous literature on FAT shows that this is not an anomalous result: previous studies of FAT have all found changes in anvil temperatures that are 20-50% of the changes in the SST. While anvil temperatures are not invariant, we find that the tropopause temperature is invariant: the change in tropopause temperature is less than 4% of the change in SST. We refer to this phenomenon as FiTT for Fixed Tropopause Temperature.

David Romps
Lawrence Berkeley National Laboratory
Seeley, J, N Jeevanjee, and D Romps.  In Press.  "FAT or FiTT: Are Anvil Clouds or the Tropopause Temperature-Invariant?"  Geophysical Research Letters, doi:IN PRESS, 2019.