FTP, identify this quantity, often given as the curl of the velocity field, that is related to the amount of circulation in a fluid.
The barotropic equation describing this quantity is used for forecasting the movement of Rossby waves in the atmosphere.  In aerodynamics, the total amount of this quantity is used in the Kutta-Joukowski theorem to calculate the lift on an airfoil. The Biot-Savart law gives the velocity induced by it and the lack of this quantity, along with incompressibility, leads to d'Alembert's paradox. For a fluid with rigid rotation, its value is twice the angular velocity of the fluid.  FTP, identify this quantity, often given as the curl of the velocity field, that is related to the amount of circulation in a fluid.
The barotropic equation describing this quantity is used for forecasting the movement of Rossby waves in the atmosphere.  
vorticity