Wave and form drag: their relation in the linear gravity wave regime

The linear, asymptoticwork of Belcher andWood (Belcher, S. E. andWood, N., 1996. Q. J. R. Meteorol. Soc. 122, 863–902), for the surface pressure drag over orography of small amplitude, is revised and expanded. A new equation is obtained that incorporates the pressure drag due to gravity waves (wave drag) and pressure drag due to turbulence (form drag) and shows how these mechanisms operate together. In its general form, the equation is in the Fourier space and can be solved numerically for any orography. An analytical example is given for sinusoidal orography.