A type of one-and-a-half-order turbulence closure that retains forecast equations for mean (first-order statistics) variables such as potential temperature and wind components, and also retains equations for variances (turbulence kinetic energy and temperature variance, symbolized by k) and for molecular dissipation or destruction of variances (symbolized by ε). Compare first-order closure, K-theory, second-order closure, nonlocal closure, Reynolds averaging, closure assumptions.