Horizontal momentum friction parameterizes the exchange of horizontal momentum from the SGS processes to the grid scale. An eddy-viscosity hypothesis is the basis for the form of the friction. In general, this friction acts to dissipate kinetic energy without introducing spurious sources of angular momentum. MOM provides options in which the friction can be second order (Laplacian) or a fourth order (biharmonic). Details concerning the derivation of these operators are provided in Chapter 9.