Limitations of one-dimensional mesoscale PBL parameterizations in reproducing mountain-wave flows

Mesoscale models are considered to be the state of the art in modeling mountain-wave flows. Herein, the authors investigate the role and accuracy of planetary boundary layer (PBL) parameterizations in handling the interaction between large-scale mountain waves and the atmospheric boundary layer. To that end, recent large-eddy simulation (LES) results of mountain waves over a symmetric two-dimensional bell-shaped hill are used and compared to four commonly used PBL schemes. It is found that one-dimensional PBL parameterizations produce reasonable agreement with the LES results in terms of vertical wavelength, amplitude of velocity, and turbulent kinetic energy distribution in the downhill shooting-flow region. However, the assumption of horizontal homogeneity in PBL parameterizations does not hold in the context of these complex flow configurations. This inappropriate modeling assumption results in a vertical wavelength shift, producing errors of approximately 10ms^-1 at downstream locations because of the presence of a coherent trapped lee wave that does not mix with the atmospheric boundary layer. In contrast, horizontally integrated momentum flux derived from these PBL schemes displays a realistic pattern. Therefore, results from mesoscale models using ensembles of one-dimensional PBL schemes can still potentially be used to parameterize drag effects in general circulation models. Nonetheless, three-dimensional PBL schemes must be developed in order for mesoscale models to accurately represent complex terrain and other types of flows where one-dimensional PBL assumptions are violated.