The comparison of results from four different centers was based on large vertical layers, large latitudinal zones, and some time-smoothing of the data. Yet there remain small but non-zero differences in the four refractivity time series, and it seems important to note that these are not retrieved meteorological data, but raw refractivity. What are the sources of these differences and can they be eliminated? If not, why not?
Response from Kevin Trenberth:
See also answer above. Issues of how retrievals are made especially the homogeneity of the ionosphere seem to be important. Given that the technique is a limb sounding to produce a vertical profile, the rays do not just sample the vertical column; a full 4D var system might account for the structure. This issue can be better quantified and maybe reduced, but will remain and leave some residual effects, as I understand it. However, many of these effects should be random and thus average out for climate purposes.