FiTT or FAT? Anvil Clouds and the Tropopause in Radiative-Convective Equilibrium

The Fixed Anvil Temperature (FAT) hypothesis proposes that tropical cloud fraction peaks at a special isotherm that is insensitive to surface temperature. It is widely believed that FAT should result from simple ingredients such as longwave radiative emission from water vapor, Clausius-Clapeyron, and tropospheric energy and mass balance. Here, the first cloud-resolving simulations of radiative-convective equilibrium (RCE) designed to contain only these basic ingredients are presented. This minimal-recipe setup does not produce FAT: the anvil temperature varies by almost 50% of the 50-K range in simulated surface temperature. However, the temperature at the tropopause—where radiative cooling rates go to zero—changes by only 5% of the surface temperature range. Thus, the results support the existence of a Fixed Tropopause Temperature (FiTT) rather than a Fixed Anvil Temperature. In full-complexity RCE simulations, the spreads in both anvil and tropopause temperatures are smaller by about a factor of two, but the basic story is unchanged: the tropopause temperature is more invariant than the anvil temperature by an order of magnitude. We explore whether simple models for the tropopause can explain its invariant temperature.