From AMS Glossary
A type of turbulence closure where the shape of the vertical profile of a variable is assumed to be known or idealized a priori, leaving only the parameters of that profile shape to be modeled or forecast.
For example, in the convective mixed layer, the potential temperature profile is idealized as uniform with height, requiring only a single value of mixed-layer potential temperature to specify the potential temperature at every height within that layer. A different example is the potential temperature in the stable boundary layer, which when idealized as an exponential shape requires only two variables (temperature decrease at the surface and e-folding depth) to specify the potential temperature at all heights within that layer. This type of turbulence closure is less than first-order statistical closure and is sometimes called half-order closure.