The impact of the rotation rate on an aquaplanet's radiant energy budget: Insights from experiments varying the Coriolis parameter

Abstract. We investigate the effect of changes in the Coriolis force caused by changes in the rotation rate on the top-of-atmosphere (TOA) radiant energy budget of an aquaplanet general circulation model with prescribed sea surface temperatures. We analyse the effective radiative forcing caused by changes from Earth-like rotation to values between 1/32 and 8 times the Earth's rotation rate. The forcing differs by about 60 Wm-2 between the fastest and slowest rotation cases, with a monotonically increasing positive forcing for faster than Earth-like rotations and a non-monotonically increasing negative forcing for slower rotations. The largest contributions to the forcing are, in that order, due to changes in the shortwave cloud radiative effect (SWCRE) and the clear-sky outgoing longwave radiation (OLR). From the fastest to the slowest rotation, the Hadley cell expands and the troposphere becomes drier, increasing the OLR. This contributes to negative forcing at slower and positive forcing at faster than Earth-like rotations. The SWCRE is influenced by changes in the low-level cloudiness within the Hadley cell and the baroclinic regime. With the expansion of the Hadley cell, the area of enhanced tropospheric stability increases, resulting in more low-level clouds, higher SWCRE, and increased negative forcing. The non-monotonicity results from an intermediate decrease in the SWCRE caused by the disappearance of baroclinic eddies as the Hadley cell reaches global extension. At rotations faster than Earth-like, the decrease in SWCRE, mainly due to the weakening of baroclinic eddies and storm systems, leads to an increase in positive forcing. In summary, changes in the SWCRE, driven by different circulation responses at slower and faster than Earth-like rotations, strongly influence the TOA radiant energy budget. These effects, along with a substantial contribution from the clear-sky OLR, could impact the habitability of Earth-like rotating planets.