|Review #2 of Lan et al., 2017.|
This is the first time I am seeing a response to my original review comments, so I will respond to the author comments here. My main concern is that two of the three original reviewers requested comparisons with TCCON, and the authors have done so, but have not included this in the revised manuscript nor in the supplementary materials.
Author comments are in quotes and my responses are interspersed.
"We agree with the reviewer that it will be interesting to compare TCCON with aircraft + CT based total column CO2. The following figure can give us some ideas about this comparison. TCCON values in this figure are daily averages computed from mid-day data (10 to 15 LST). Outliers in TCCON data are filtered out, by using 3.S.D threshold of residuals (with iteration) from a quadratic fit on data for each day. However, it is not an apple-to-apple comparison between our aircraft-based column CO2 and TCCON. The sampling time and location are different. Aircraft sampling typically takes ~ 30 min to get the whole profile through downward spiral, mostly within 0.1⁰ of the site location."
I agree that what you have plotted is not an apples-to-apples comparison. I would suggest that instead, you average the TCCON data during the 30 minute aircraft sampling times instead of simply taking mid-day averages. I also do not see the need for your additional 3-sigma filtering.
"In addition, the aircraft and tower measurements are calibrated on WMO scale; however, direct calibrations of remote sensing instruments are not available. It makes more sense to compare TCCON against calibrated measurements for accuracy instead of the other way around. Our study focuses on the long-term mean spatial patterns of column CO2 over North America and the influence of large-scale transport..."
The overall scaling of the two data products is not of prime interest, here. As you mention, the long-term mean spatial pattern are primarily important, which I believe you can assess using a comparison with TCCON data. This might give you a way of assessing the CT stratosphere, and improving your product!
"For TCCON that sampling the whole column instantly at an angle, the actual sampling location, especially at high altitudes, may be far away from our aircraft inlets even though the distance on the ground is much smaller."
Near noon, this should be a less significant problem. At the very least, this is a quantifiable problem, and you have not demonstrated the problem in a quantitative manner. Also, you base much of your paper on the fact that the stratosphere does not change significantly in time or space, so this argument is not strong. (For example, you later state: “There is little spatial variation of atmospheric CO2 in the stratosphere over mid-latitude region (Andrews et al., 2001).” and “We cannot properly assign an exact number as the uncertainty for the top 1/3 of the atmosphere without routine in-situ measurements. However, spatial gradient and atmospheric variability is very small in this part of the atmosphere, and thus it is not important in our study.”)
My other requests were mostly satisfactorily dealt with.