Missing drag due to orographic gravity waves in a global numerical weather prediction model

Various studies have shown that low-level drag in the atmosphere is parametrised inconsistently across the world’s numerical weather prediction and climate models, ultimately due to a lack of constraints on the underlying physical processes and the overlap in scale between them. Trapped lee waves (TLWs) are not parametrised in most models but have been shown in theoretical and case studies to produce significant drag (necessarily at low levels) on the atmosphere under the right conditions. To investigate whether TLWs contribute to low-level drag consistently, the resolved momentum fluxes in the archived analyses of the TLW-resolving UKV model are calculated and compared to the resolved plus parametrised gravity wave fluxes in the coarse-resolution, global version of the Met Office Unified Model (MetUM), which does not resolve TLWs.

The comparison between the models reveals that gravity wave momentum fluxes in the UKV model are about double that of the global MetUM in the mid-troposphere and up to four times that in the boundary layer. Only a portion of this discrepancy in momentum fluxes can be explained by the presence of trapped lee wave modes, which are found using a numerical solver of the Taylor-Goldstein equation. The other part is likely to be caused by orographic gravity waves that are reflected due to the general decrease of the Scorer parameter with altitude (and are distinct from the resonant TLWs). This work therefore demonstrates that the inclusion of the drag produced by both reflected and trapped lee waves would alleviate the current issues with low-level drag parametrisation.