  The majority of potentially habitable exoplanets detected orbit stars cooler
than the Sun, and therefore are irradiated by a stellar spectrum peaking at
longer wavelengths than that incident on Earth. Here, we present results from a
set of simulations of tidally-locked terrestrial planets orbiting three
different host stars to isolate the effect of the stellar spectra on the
simulated climate. Specifically, we perform simulations based on TRAPPIST-1e,
adopting an Earth-like atmosphere and using the UK Met Office Unified Model in
an idealised 'aqua-planet' configuration. Whilst holding the planetary
parameters constant, including the total stellar flux (900 W/m$^2$) and orbital
period (6.10 Earth days), we compare results between simulations where the
stellar spectrum is that of a quiescent TRAPPIST-1, Proxima Centauri and the
Sun. The simulations with cooler host stars had an increased proportion of
incident stellar radiation absorbed directly by the troposphere compared to the
surface. This, in turn, led to an increase in the stability against convection,
a reduction in overall cloud coverage on the dayside (reducing scattering),
leading to warmer surface temperatures. The increased direct heating of the
troposphere also led to more efficient heat transport from the dayside to the
nightside and, therefore, a reduced day-night temperature contrast. We inferred
that planets with an Earth-like atmosphere orbiting cooler stars had lower
dayside cloud coverage, potentially allowing habitable conditions at increased
orbital radii, compared to similar planets orbiting hotter stars for a given
planetary rotation rate.
