Geostrophic drag law for conventionally neutral atmospheric boundary layers revisited

• Published in: Quarterly journal of the Royal Meteorological Society 2021-01, Vol.147 (735), p.847-857
• Main Authors: Liu, Luoqin, Gadde, Srinidhi N., Stevens, Richard J.A.M.
• Format: Article
• Language: English
• Subjects: Atmospheric boundary layer; Boundary layers; conventionally neutral atmospheric boundary layer; Coriolis parameters; Drag; Drag coefficient; friction velocity; geostrophic drag law; lapse rate; large‐eddy simulation; Latitude; Parameterization; Wind power
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.3949

The geostrophic drag law (GDL), which predicts the geostrophic drag coefficient and the cross‐isobaric angle, is relevant for meteorological applications such as wind energy. For conventionally neutral atmospheric boundary layers (CNBLs) capped by an inversion, the GDL coefficients A and B are affected by the inversion strength and latitude, expressible via the ratio of the Brunt–Väisälä frequency (N) to the Coriolis parameter (f). We present large‐eddy simulations (LES) covering a wider range of N/|f| than considered previously, and show that A and B obtained from carefully performed LES collapse to a single curve when plotted against N/|f|. This verifies the GDL for CNBLs over an extended range of N/|f| within LES. Additionally, in agreement with atmospheric observations, we show that using A = 1.9 and B = 4.4 accurately predicts the geostrophic drag coefficient in the limit of weak inversion strength or high latitude (N/|f|≲300). However, due to the strong dependence of B on N/|f|, corresponding predictions for the cross‐isobaric angle are less accurate. As we find significant deviations between the LES results and the original parameterization of the GDL for CNBLs, we update the corresponding model coefficients. By performing a series of high‐fidelity large‐eddy simulations (LES), we reevaluate the empirical constants involved in the geostrophic drag law by Zilitinkevich and Esau (2005) for conventionally neutral atmospheric boundary layers. We find significant differences between the drag law curves based on existing low‐resolution data and updated curves based on our high‐resolution LES database. This new insight will be beneficial for improving predictions in meteorological applications.