|This paper examines the descent of nitric oxide (NO) and water vapour in the northern high latitudes during the stratospheric warming (SSW) of 2013, using a version of WACCM-X nudged to the high-altitude NAVGEM analyses, extending to the mesopause. Results are compared to older simulations with WACCM driven by the MERRA reanalyses extending only to the stratopause region. The total amount of NO deposited in the stratosphere is estimated using various methods.|
In this revised version, the authors have addressed most of previous comments. I still believe that an additional iteration to address my original comment and some minor comments is needed before the paper is in final form for publication.
1) My major comment was about the role of downward transport from the NO main MLT reservoir into the mesosphere that is glimpsed from the observations (Fig 1, 2000 ppb contour in SOFIE). Orsolini et al. (2017) or Limpasuvan et al (2016) showed a short-duration downward transport, diagnosed in the TEM formalism, driven by transient planetary wave activity following SSW onset near 90-100 km.
The authors argue -based on Fig 1- that there is no evidence from NO descent from altitudes above 0.004 hPa during this event. The new Figure 2 tracks the pressure level of the 50 ppb contour, but there is also descent of higher NO values from aloft.
I still believe that it would be of great interest to see w* higher than 0.01 hPa (as shown in Fig 8) from the time of onset onwards. Fig 8 is a monthly mean and the descent from 90-100 km in Orsolini et al. (2017) or Limpasuvan et al (2016) is quite short-lived. Since the actual descent will be zonally asymmetric (as the authors have shown in Harvey et al. 2021 in another case study), can Fig 1 be really a proof that there is no descent of high NO from higher up than 80-85 km?
The consistency with Randall et al. (2001) is mentioned but I believe that the latter is a study of the SH, outside of SSW periods. A short period of downward transport from 90-100 km precisely during SSW events would not be in contradiction with that study.
Although I realize that this is not the main thrust of this paper, but it could nevertheless be an important point if there is a marked difference in the descent near 90-100 km between WACCM (or WACCM-X) driven by conventional re-analyses and by high-altitude NAVGEM re-analyses.
At least, a short caveat about possible transient downward transport from the LT could be indicated.
P4, line 15: In fact, the SD-WACCM simulations of the same 2012/2013 event in Orsolini et al. (2017, cited) were also made with both Pr=2 and Pr=4. Their conclusion was that the high diffusion run did enhance the transport of NO in the mesosphere, where GWs break, but not in the LT.
P14, line 20): the authors argue that the equatorward transport is stronger in WACCM-X than what is inferred from SOFIE or ACE. It is mentioned “as discussed in the context of Figure 8…” Shouldn’t Figs 13 and 14 be rather mentioned here? Or do the authors refer specifically to the WACCM-X equatorward meridional velocities in Fig 8 between 1.0 and 0.1 hPa (incidentally, similar to NAVGEM) ? Please clarify.
P2, line 7: (Langematz and Tully, 2018)
P2, line 24 (also P3, line 6): I wonder if the term “long range transport” is the best choice of terminology since it has a connotation of long-range (e.g., pollutant or aerosol) horizontal transport, esp. for readers of ACP. Here it is meant deep vertical transport across atmospheric layers.
P5, line 15: remove “bf” before Figure 1
P17, line 12: Smith-Johnsen
Caption of Fig 4: Indicate that stars and diamonds taken from Orsolini et al. (2017) are from SMR satellite observations.
Caption of Fig 2: remove “This tracks”