You could say that maybe when TPW is high the differences might be proportionately higher, but the TPW changes are proportionately small, and are also unlikely to be globally distributed in any one month. In fact, I suspect the fluctuations are due to monsoons.

You’ve mentioned some spatial arguments from SB08, but there’s no spatial data here. I just can’t see it.

A better regressor might be total cloudiness. But it would need careful interpretation.

I suspect it has to do with the author attempting to make a quick point or counter point. As long as the authors involved are willing to rationally discuss the results of these tests, I think the science progresses. If the authors, on the other hand, appear to fear that admitting a mistake (and correcting it) puts doubt into a general thesis that they might hold, and thus ignore or obscure the critics, I think the science suffers.

Re: the actual significant of the cloud feedback from the change in clear-sky – all-sky (and cloudiness weighting), this is an interesting point to consider (I’ve been pondering it myself), but as you know this is the method that Dessler10 uses, claiming that “this definition of the cloud feedback is a standard approach for quantifying feedbacks [26]“. That reference 26 might be a good place to start when looking more in-depth at it, and while I’m happy to pursue this avenue for interest, understand that it is unrelated to the critiques I’ve made of Dessler10 up to this point.

Re: the whole-globe water vapor thing, if you look at the figures in the SB08 “dry-bias” paper (particularly figure 5), the effect seems to be global, and we should be able to detect the difference in global water vapor. Yes, there are areas where the effect is stronger (northern hemisphere mid-latitudes), yet it seems rather impossible that the dry-bias would show up in such a way as to be correlated with temperature (as it must if it going to create a bias in the cloud feedback calculations) but uncorrelated with global water vapor, particularly when global water vapor and temperature are so strongly correlated. Still, if you want to propose a different test, such as using the precipitable water for the midlatitudes of the northern hemisphere, I welcome your input.

I would also note that while the “perfect” test may discover some greater percentage of the difference between ERA-clear sky and CERES-clear sky coming from a humidity correction, the bulk of the difference still comes from the short-wave clear-sky fluxes, unrelated to any dry bias referenced.