We present an analytical model for large-scale turbulence, based on a new closure which depends on the growth rate of the instability-generating turbulence. For convection in stars we recover the results of the mixing-length theory and for laboratory convection we recover the $N\sim R^{\frac{1}{3}}$ law. The present model can readily be extended to include magnetic fields and rotation.

• Received 30 May 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.430