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Sub-Neptunes (planets with radii between 2 and 4 R$_{\oplus}$) are abundantaround M-dwarf stars, yet the atmospheric dynamics of these planets isrelatively unexplored. In this paper, we aim to provide a basic underpinning ofthe dry dynamics of general low mean molecular weight, temperate sub-Neptuneatmospheres. We use the ExoFMS GCM with an idealised grey gas radiation schemeto simulate planetary atmospheres with different levels of instellation androtation rates, using the atmosphere of K2-18b as our control. We find that theatmospheres of tidally-locked, temperate sub-Neptunes have weak horizontaltemperature gradients owing to their slow rotation rates and hydrogen-dominatedcomposition. The zonal wind structure is dominated by high-latitudecyclostrophic jets driven by the conservation of angular momentum. At lowpressures we observe superrotating equatorial jets, which we propose are drivenby a Rossby-Kelvin instability similar to the type seen in simulations ofidealised atmospheres with axisymmetric forcing. By viewing the flow intidally-locked coordinates, we find the predominant overturning circulation tobe between the dayside and nightside, and we derive scaling relations linkingthe tidally-locked streamfunction and vertical velocities to instellation.Comparing our results to the only other GCM study of K2-18b, we findsignificant qualitative differences in dynamics, highlighting the need forfurther collaboration and investigation into effects of different dynamicalcores and physical parameterizations. This paper provides a baseline forstudying the dry dynamics of temperate sub-Neptunes, which will be built on inpart II with the introduction of moist effects.

Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics