Abstract
An investigation of the unclosed horizontal turbulent flux budget in compressible large-eddy simulations (LESs) of shallow cumulus (ShCu) clouds has inadvertently turned up omnipresent pressure waves. These waves originate from the simulated ShCu clouds and ripple radially outwards in all directions at the speed of sound. They manifest as isolated spectral spikes in the pressure spectrum. Similar wave features are found with three compressible LES models and in two classic ShCu cases, suggesting their potential commonality. Spatial and temporal convergence tests show that the pressure wave is a spatially resolved yet spurious numerical artifact. Systematic sensitivity studies with respect to both numerical modeling and physical processes are conducted to ascertain the cause of the waves. Their effects on the simulated clouds and flow fields are found to be small, except for the pressure variance and the horizontal pressure-gradient interaction term. For wave removal, traditional numerical techniques designed for stabilizing grid-scale acoustic noises are ineffective due to the resolved nature of the pressure waves. Two practical wave-suppression approaches are to either limit the model time step or to switch to a soundproof pressure solver.
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