The diffusivity factor re-examined

The issue of the optimum diffusivity factor for use in atmospheric heating calculations is re-examined in detail. It is shown that the value of this factor depends on the method by which the heating is calculated. In one method, the flux is assumed equal to π times the specific intensity I, and the heating is obtained by differentiating the flux. In another method, the mean intensity J is assumed equal to I and the heating is obtained by the explicitly subtracting the total emission from the total absorption. When the first method is used, a diffusivity factor of 1.81 is demonstrated to be the preferred choice in heating calculations. When the second, intensity-based method is used, the best choice is 2.07. While the traditionally used value of 1.66 is not substantially inferior to the above choices, it is clearly not optimal.