  H$_2$O is a key molecule in characterizing atmospheres of temperate
terrestrial planets, and observations of transmission spectra are expected to
play a primary role in detecting its signatures in the near future. The
detectability of H$_2$O absorption features in transmission spectra depends on
the abundance of water vapor in the upper part of the atmosphere. We study the
three-dimensional distribution of atmospheric H$_2$O for synchronously rotating
Earth-sized aquaplanets using the general circulation model (GCM) ROCKE-3D, and
examine the effects of total incident flux and stellar spectral type. We
observe a more gentle increase of the water vapor mixing ratio in response to
increased incident flux than one-dimensional models suggest, in qualitative
agreement with the climate-stabilizing effect of clouds around the substellar
point previously observed in GCMs applied to synchronously rotating planets.
However, the water vapor mixing ratio in the upper atmosphere starts to
increase while the surface temperature is still moderate. This is explained by
the circulation in the upper atmosphere being driven by the radiative heating
due to absorption by water vapor and cloud particles, causing efficient
vertical transport of water vapor. Consistently, the water vapor mixing ratio
is found to be well-correlated with the near-infrared portion of the incident
flux. We also simulate transmission spectra based on the GCM outputs, and show
that for the more highly irradiated planets, the H$_2$O signatures may be
strengthened by a factor of a few, loosening the observational demands for a
H$_2$O detection.
