A type of higher-order turbulence closure in which prognostic equations are retained for mean variables (such as potential temperature and wind), as well as for some of the higher-order statistics, including variance (such as turbulence kinetic energy or temperature variance) and covariance (kinematic fluxes such as for heat and momentum). Any third- and higher-moment statistics remaining in the equations are approximated as functions of lower-order statistics and independent variables. Compare first-order closure, K-theory, one-and-a-half order closure, nonlocal closure, closure assumptions.