Newtonian cooling in the middle atmosphere: Where does it work and where does it fail?

In mechanistic modeling studies of the middle atmosphere, radiative heating is often approximated with Newtonian cooling, i.e. a local, linear relaxation of the temperature to a prescribed value T_r. The validity of this approximation is examined using the empirical relationship between instantaneous heating rates and temperatures from a realistic GCM with interactive chemistry. It is found that Newtonian cooling is a good approximation throughout most of the middle atmosphere -- although T_r is not necessarily the radiative equilibrium temperature -- provided the cooling rates and T_r vary with season, height and latitude. These values are described and may be of interest to mechanistic modelers. However, the approximation breaks down in the lower Antarctic stratosphere during the spring, where non-local radiative effects play a significant role. This suggests the need to use more detailed radiation codes in mechanistic modeling studies of the final warming of the Antarctic polar vortex.