Geostrophic drag law for conventionally neutral atmospheric boundary layers revisited

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-Vaisala 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|less than or similar to 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.

Automated summary

The study shows the applicability of the geostrophic drag law to conventionally neutral atmospheric boundary layers over a wider range of N/|f|. Results from large-eddy simulations reveal that coefficients A and B collapse to a single curve when plotted against N/|f|, but predictions for the cross-isobaric angle are less accurate due to strong dependence of B on N/|f|.