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. In this study, we implement a vertical grid refinement scheme in the radiationroutine of the global aerosol–climate model ECHAM-HAM, aiming to improve therepresentation of stratocumulus clouds and address the underestimation oftheir cloud cover. The scheme is based on a reconstruction of the temperatureinversion as a physical constraint for the cloud top. On the refined grid, theboundary layer and the free troposphere are separated and the cloud's layer ismade thinner. The cloud cover is recalculated either by conserving the cloudvolume (SC-VOLUME) or by using the Sundqvist cloud cover routine on the new grid representation (SC-SUND). In global climate simulations, we find that theSC-VOLUME approach is inadequate, as there is a mismatch, in most cases, betweenthe layer of the inversion and the layer of the stratocumulus cloud, which prevents itsapplication and is itself likely caused by an overly low verticalresolution. Additionally, we find that the occurrence frequency ofstratocumulus clouds is underestimated in ECHAM-HAM, limiting a priori thepotential benefits of a scheme like SC-VOLUME targeting only cloud amount whenpresent. With the SC-SUND approach, the possibility for new clouds to beformed on the refined grid results in a large increase in mean total cloudcover in stratocumulus regions. In both cases, however, the changes exerted inthe radiation routine are too weak to produce a significant improvement in thesimulated stratocumulus cloud cover. We investigate and discuss the reasonsbehind this. The grid refinement scheme could be used more effectively forthis purpose if implemented directly in the model's cloud microphysics andcloud cover routines, but other possible ways forward are also discussed.

Vertical grid refinement for stratocumulus clouds in the radiation scheme of the global climate model ECHAM6.3-HAM2.3-P3