An explanation for the sensitivity of the mean state of the Community Atmosphere Model to horizontal resolution on aqua-planets.

Abstract

he sensitivity of the mean state of the Community Atmosphere Model to horizontal resolutions typical of present-day General Circulation Models is investigated in an aqua-planet configuration. Non-convergence of the mean state is characterized by a progressive drying of the atmosphere and large reductions in cloud coverage with increasing resolution. Analyses of energy and moisture budgets indicate that these trends are balanced by variations in moisture transport by the resolved circulation, and a reduction in activity of the convection scheme. In contrast, the large-scale precipitation rate increases with resolution, which is approximately balanced by greater advection of dry static energy associated with more active resolved vertical motion in the ascent region of the Hadley Cell.An explanation for the sensitivity of the mean state to horizontal resolution is proposed, based on linear Boussinesq theory. We hypothesize that an increase in horizontal resolution in the model leads to a reduction in horizontal scale of the diabatic forcing arising from the column physics, facilitating fine scale flow and faster resolved convective updrafts within the dynamical core, and steering the coupled system towards a new mean state. Our hypothesis attempts to explain the underlying mechanism driving the variations in moisture transport observed in the simulations. Through considering the process ordering at the level on an individual time-step, our hypothesis also provides a logical explanation for the progressive deactivation of the convection scheme with increasing resolution.