From AMS Glossary
A method to approximate the surface energy budget that accounts for storage of heat in the top layer of soil.
In this model the ground is conceptually split into two layers, a relatively thin layer near the top with uniform temperature, and a deep soil layer also of uniform but different temperature. The thickness of the top layer is carefully chosen based on the region where a periodic temperature surface forcing has appreciable effect. The net result is that flux from the deep soil layer tends to restore the top layer, opposing any radiative forcing from the atmosphere. This method is an alternative to a multilayer soil model, which is computationally more expensive.