  Synchronously orbiting, tidally-locked exoplanets with a dayside facing their
star and a permanently dark nightside orbiting dim stars are prime candidates
for habitability. Simulations of these planets often show the potential to
maintain an Earth-like climate with a complete hydrological cycle. Here, we
examine the sensitivity of the atmospheric water cycle to changes in stellar
flux and describe the main underlying mechanisms. In a slowly-rotating,
tidally-locked Earth-like atmospheric model, the response to a small (about
10%) increase in stellar irradiance from a habitable-zone control simulation is
examined. The water cycle is enhanced in response to the increased stellar
irradiance. While the evaporation increase behaves similarly to the stellar
radiation increase, the day-to-night energy transport by the mean circulation
is critical to the planet's precipitation changes. Increased efficiency of the
energy transport in a warmer climate shapes the substellar precipitation
increase. On the nightside, precipitation changes are weak as a result of the
large cancellation between the increased energy transport and the increased
longwave emission. The day-to-night energy transport efficiency is sensitive to
the variation of the atmosphere's vertical stratification. Due to weak
temperature gradients in upper troposphere and a moist adiabat maintained in
the substellar region, variations in the substellar surface temperature and
specific humidity govern the increase of the planet's stratification with
warming. This suggests a scaling of nightside's precipitation based on the
substellar surface thermodynamic changes, a sensitivity that holds over a wider
range of stellar irradiance changes.
