On the Pressure Field in the Slope Wind Layer

It has been suggested by some authors that the momentum equation for thermally driven slope flow should contain a horizontal pressure gradient term, in addition to the buoyancy term. It is shown that this suggestion is incorrect and leads to a spurious increase in along-slope forcing unless the vertical component of the perturbation pressure gradient is included as well. Along-slope accelerations due to the horizontal and vertical perturbation pressure gradients cancel each other exactly if the temperature perturbation is constant along the slope. Based on the concept of hydrostatic equilibrium perpendicular to the slope, the error associated with neglecting the vertical component of the pressure gradient, and the error due to the assumption of vertical hydrostatic equilibrium are evaluated. A revised conceptual diagram of the relationship between buoyancy and pressure forces within the slope wind layer is presented.