A parametrization scheme accounting for non-hydrostatic effects on vertically propagating orographic gravity waves and implementation in the Model for Prediction Across Scales (MPAS)

 The momentum transport by orographic gravity waves (OGWs) plays an important role in driving the large-scale circulation throughout the atmosphere, which is subject to parameterization in numerical models. Current parameterization schemes commonly assume that the unresolved OGWs are hydrostatic, typically only valid for waves with large horizontal scale, weak winds and high stability. These schemes were originally developed for coarse-resolution numerical models and, as a result, captured the first-order effects of unresolved OWGs. With the increase in the horizontal resolution of state-of-the-art numerical models, unresolved OGWs are of smaller horizontal scales and may be more influenced by nonhydrostatic effects (NHE), thus challenging use of the hydrostatic assumption. Based on the analytical formulae of nonhydrostatic OGWs derived in our recent study, this work revises the orographic gravity wave drag (OGWD) parameterization scheme employed in the Model for Prediction Across Scales (MPAS) by accounting for NHE. Global simulations are conducted to investigate NHE on the momentum transport of parameterized OGWs and their impact on the simulated large-scale circulation. NHE are found to be the most evident in regions of complex terrain where the subgrid-scale orography is narrow. As NHE act to reduce the surface wave momentum flux (WMF) of OGWs, the revised scheme tends to inhibit wave breaking in the lower troposphere and transport more WMF upward, leading to an enhancement of OGWD in the upper troposphere and lower stratosphere. Over Antarctica, where the largest zonal-mean NHE occur, the OGWD-induced meridional circulation is strengthened, which helps reduce the cold pole and westerly wind biases associated with a too strong polar vortex, thereby alleviating the delayed breakdown of the Antarctic polar vortex in late spring and early summer, a bias commonly found in climate models.