  Rossby wave turbulence, as modelled by the Charney-Hasegawa-Mima (CHM)
equation, is nonlocal in scale. As a result, the formal stationary
Kolmogorov-Zakharov solutions of the Rossby wave kinetic equation, which
describe local cascades, are not valid. Rather the solution of the kinetic
equation is dominated by interactions between the large and small scales. This
suggests an alternative analytic approach based on an expansion of the
collision integral in a small parameter obtained from scale separation. This
expansion approximates the integral collision operator in the kinetic equation
by anisotropic diffusion operators in wavenumber space as first shown in a
series of papers by Balk, Nazarenko and Zakharov in the early 1990's. In this
note we summarize the foundations of this theory and provide the technical
details which were absent from the original papers.
