A turbulent orographic form drag scheme accounting for anisotropy and orientation for kilometer‐ to subkilometer‐scale models

• Published in: Quarterly journal of the Royal Meteorological Society 2023-07, Vol.149 (755), p.2527-2549
• Main Authors: Xue, Haile, Shen, Xueshun
• Format: Article
• Language: English
• Subjects: Anisotropy; Drag; Form drag; large‐eddy simulation; orographic drag; Orography; Parameterization; Parameters; Scale models; Slopes; subkilometer resolution; sub‐grid orography parameterization; Surface wind; Weather forecasting; Wind; Wind observation
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4519

This paper presents a turbulent orographic form drag (TOFD) parameterization scheme that takes into account the directional effects stemming from the angle between the low‐level wind and the principal axis of small‐scale orography. Suitable for models with both kilometer and subkilometer horizontal resolutions, this scheme builds upon prior theoretical and numerical studies to formulate surface TOFD based on the slope and direction of the sinusoidal hills. In this study, we proposed a straightforward function to calculate the surface TOFD for various orographic aspect ratios and directional parameters. The vertical decay of the drag is modeled with a scale twice the standard deviation of the sub‐grid orography. A comparison with numerous large‐eddy simulations featuring a single ellipsoidal hill demonstrates that our scheme effectively captures the dependence of drag on factors such as maximum slope, aspect ratio, the angle between low‐level wind and the principle axis, and hill height. We recommend calculating the sub‐grid orographic parameters using a well‐established method and digital terrain elevation data with a horizontal resolution less than a 100 m. This will allow for representation on orographic scales of both kilometers and subkilometers. Real‐case numerical weather prediction tests are conducted and verified with dense surface wind observations. The proposed scheme improves surface wind simulation compared to a renowned TOFD scheme, and also effectively exhibits the wind response to orographic anisotropy. Abbreviations: LES, large‐eddy simulation; NWP, numerical weather prediction; SRTM, Shuttle Radar Topography Mission data; TOFD, turbulent orographic form drag.