= Coastal vorticity tendency from variable surface drag
{title2=$[\nabla\times(-r\mathbf u)]_z=-r\zeta-(\nabla r\times\mathbf u)_z$}
Spatially varying drag has a vorticity tendency beyond simple damping of existing relative vorticity. The cross-gradient term depends on wind direction and the gradient of surface resistance. Increasing roughness on moving from ocean to land does not alone specify a universal cyclonic or anticyclonic sign. Seasonal heating also changes stratification and isentropic mass thickness, so it cannot generally be replaced by adiabatic PV conservation.
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