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Hubble (HST) spectroscopic transit observations of the temperate sub-NeptuneK2-18b were interpreted as the presence of water vapour with potential waterclouds. 1D modelling studies also predict the formation of water clouds at someconditions. However, such models cannot predict the cloud cover, driven byatmospheric dynamics and thermal contrasts, and thus their real impact onspectra. The main goal of this study is to understand the formation,distribution and observational consequences of water clouds on K2-18b and othertemperate sub-Neptunes. We simulated the atmospheric dynamics, water cloudformation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. Weanalysed the impact of atmospheric composition (with metallicity from 1*solarto 1000*solar), concentration of cloud condensation nuclei and planetaryrotation rate. Assuming that K2-18b has a synchronous rotation, we show thatthe atmospheric circulation in the upper atmosphere essentially corresponds toa symmetric day-to-night circulation. This regime preferentially leads to cloudformation at the substellar point or at the terminator. Clouds form formetallicity >100*solar with relatively large particles. For 100-300*solarmetallicity, the cloud fraction at the terminators is small with a limitedimpact on transit spectra. For 1000*solar metallicity, very thick clouds format the terminator. The cloud distribution appears very sensitive to theconcentration of CCN and to the planetary rotation rate. Fitting HST transitdata with our simulated spectra suggests a metallicity of ~100-300*solar. Inaddition, we found that the cloud fraction at the terminator can be highlyvariable, leading to a potential variability in transit spectra. This effectcould be common on cloudy exoplanets and could be detectable with multipletransit observations.

Formation and dynamics of water clouds on temperate sub-Neptunes: The example of K2-18b