From AMS Glossary
The total cooling since the time in the evening when the turbulent heat flux near the ground produces a net heat flow from the earth to the atmosphere; used to measure or predict evolution of the stable boundary layer.
It is defined as the integral of the surface kinematic heat flux
over time, starting from the time t0 just before sunset when the flux changes from positive to negative, and ending at any time te before the flux changes sign back to positive. Dimensions are temperature times length, such as (K·m). In the absence of advection and direct radiative cooling of the air, accumulated cooling (AC) also equals the area under the potential temperature profile z(θ), integrated from the surface potential temperature θs to the residual-layer potential temperature θRL, that is,