What is radiation divergence, in the context of atmospheric physics? For example, the phrase is used in this paper.
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The way I understand it, the net longwave radiation $L^\star:=L^\downarrow-L^\uparrow$ (maybe with different sign), i.e. the difference between downwelling and upwelling radiation, can be viewed as a vector field (with vanishing horizontal components under the assumption of homogeneous horizontal radiation distribution). This vector field has a divergence $\nabla\cdot L^\star=\frac{\partial L^\star}{\partial z}$.
This term occurs in equation (1) of the paper you referenced as part of the heating rate. Since non-vanishing divergence is in a vector field is associated with the field having a source or sink, this corresponds to heating/cooling of the atmosphere.
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$\begingroup$ Ah, of course, thanks. I was thinking in terms of a single layer land-surface model. That'll teach me to read the paper first :/ $\endgroup$ – naught101 Mar 7 '16 at 10:12
