  Tidally locked terrestrial planets around low-mass stars are the prime
targets of finding potentially habitable exoplanets. Several atmospheric
general circulation models have been employed to simulate their possible
climates, however, model intercomparisons showed that there are large
differences in the results of the models even when they are forced with the
same boundary conditions. In this paper, we examine whether model resolution
contributes to the differences. Using the atmospheric general circulation model
ExoCAM coupled to a 50-m slab ocean, we examine three different horizontal
resolutions (440 km * 550 km, 210 km * 280 km, and 50 km * 70 km in latitude
and longitude) and three different vertical resolutions (26, 51, and 74 levels)
under the same dynamical core and the same schemes of radiation, convection and
clouds. Among the experiments, the differences are within 5 K in global-mean
surface temperature and within 0.007 in planetary albedo. These differences are
from cloud feedback, water vapor feedback, and the decreasing trend of relative
humidity with increasing resolution. Relatively small-scale downdrafts between
upwelling columns over the substellar region are better resolved and the mixing
between dry and wet air parcels and between anvil clouds and their environment
are enhanced as the resolution is increased. These reduce atmospheric relative
humidity and high-level cloud fraction, causing a lower clear-sky greenhouse
effect, a weaker cloud longwave radiation effect, and subsequently a cooler
climate with increasing model resolution. Overall, the sensitivity of the
simulated climate of tidally locked aquaplanets to model resolution is small.
