We examine how cloud cover is determined in shallow-cloud areas by using large-eddy simulation with an extremely wide domain, which covers the transition phase from cumulus-understratocumulus to shallow-cumulus regimes. The relationship between two distances is critical to cloud cover. One characteristic distance is the horizontal distance between cumulus clouds, and the other is the broadening distance of anvil-like stratiform cloud at the top of the boundary layer. High cloud cover occurs with a long distance of broadening and short distances between cumuli. In contrast, low cloud cover appears with a short distance of broadening and a long distance between cumuli. The contrast of the two distances is rooted in aerosol amount and the strength of the surface heat flux. The relationship between these two distances can be applied to estimating the cloud cover below sharp inversions.
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