Radiative forcings at top of the atmosphere (TOA) (def. positive down)

Note: Forcing definitions based on Ghan (2013) for SW have been extended to apply also for LW fluxes.
Note that all LW fluxes are positive downwards, opposite of the SW fluxes, so that the sum of SW & LW
should be (and is for "SW + LW") calculated as SW - LW.