Analysis of pressure‐strain and pressure gradient‐scalar covariances in cloud‐topped boundary layers: A large‐eddy simulation study

A detailed analysis of the pressure‐scrambling terms (i.e., the pressure‐strain and pressure gradient‐scalar covariances) in the Reynolds‐stress and scalar‐flux budgets for cloud‐topped boundary layers (CTBLs) is performed using high‐resolution large‐eddy simulation (LES). Two CTBLs are simulated — one with trade wind shallow cumuli, and the other with nocturnal marine stratocumuli. The pressure‐scrambling terms are decomposed into contributions due to turbulence‐turbulence interactions, mean velocity shear, buoyancy, and Coriolis effects. Commonly used models of these contributions, including a simple linear model most often used in geophysical applications and a more sophisticated two‐component‐limit (TCL) nonlinear model, are tested against the LES data. The decomposition of the pressure‐scrambling terms shows that the turbulence‐turbulence and buoyancy contributions are most significant for cloud‐topped boundary layers. The Coriolis contribution is negligible. The shear contribution is generally of minor importance inside the cloudy layers, but it is the leading‐order contribution near the surface. A comparison of models of the pressure‐scrambling terms with the LES data suggests that the more complex TCL model is superior to the simple linear model only for a few contributions. The linear model is able to reproduce the principal features of the pressure‐scrambling terms reasonably well. It can be applied in the second‐order turbulence modeling of cloud‐topped boundary layer flows, provided some uncertainties are tolerated.