One-and-a-half order closure

From Glossary of Meteorology

one-and-a-half order closure

A higher-order closure for turbulence where forecast equations are retained for mean variables (e.g., first-order
or first moment mean values of potential temperature and wind components), and for variance of selected variables (e.g., selected second statistical moments such as turbulence kinetic energy or potential temperature variance).

It is not second-order closure because forecast equations are not retained for covariances, which are also second moments statistically. Any other higher-order statistics remaining in the equations are approximated by the mean and variance values. One type of one-and-a-half-order closure is k-ε closure.
Compare first-order closure, K-theory, second-order closure, nonlocal closure, Reynolds averaging, closure assumptions.