COAMPS®-LES:: Model evaluation and analysis of second- and third-moment vertical velocity budgets

The Naval Research Laboratory Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS (R)) has been extended to perform as a large-eddy simulation (LES) model. It has been validated with a series of boundary-layer experiments spanning a range of cloud nighttime, and includes a nighttime stratocumulus case, a trade wind cumulus layer, shallow cumulus convection over land, and a mixed regime consisting of cumulus clouds under broken stratocumulus. COAMPS-LES results are in good agreement with other models for all the cases simulated. Exact numerical budgets for the vertical velocity second (over bar(w'(2))) and third moment (over bar(w'(3))) have been derived for the stratocumulus and trade wind cumulus cases. For the over bar(w'(3)) budget in the stratocumulus, the buoyancy contribution from the updraughts and downdraughts largely cancel each other due to their similar magnitudes but opposite signs. In contrast, for the cumulus layer, the negative buoyancy contribution from the environmental downdraughts is negligible and the positive contribution from the updraughts completely dominates due to the conditional instability in the environment. As a result, over bar(w'(3)) is significantly larger in the cumulus than in the stratocumulus layer.