Review of "Balloon drift estimation and improved position estimates for radiosondes" by Voggenberger et al

General

I am pleased to see efforts to calculate and treat historical radiosonde drift in reanalyses. 

This is a step is the right direction but needs tidying up. 

It is known that radiosondes generally drift further in winter than summer (eg Seidel et al, 2011). 

This is mentioned in the abstract but nowhere else and I find it a little strange that most 

of the examples presented are for the summer eg Figure 5 and the assimilation experiments. 

Quality control is an essential part of any data assimilation system (and is still messier than 

we would like). What quality control (if any) was applied in this study to remove 'bad' reported data? 

Complication at the poles: 

The issue is stated in the Provisional 2023 edition of WMO No 8 (see below) and mainly affects the 

South pole station (which still reports in TAC). If dispacements are calculated for this station 

then the wind direction should also be adjusted. 

https://community.wmo.int/en/activity-areas/imop/wmo-no.8/wmo-no-8-provisional-2023-edition

Chapter 13 Measurement of upper wind 

Particular care is needed in reporting wind direction near to the North or South Pole especially

with the reporting of position at each level. A radiosonde crossing the North Pole must be in a

southerly airflow just before crossing the Pole and in a northerly airflow just afterwards. BUFR

reports should give wind direction relative to the longitude at each level. (The older

alphanumeric codes used a special coordinate system for stations within 1° latitude of the pole.)

Detailed comments

19 'alongside the estimated wind' - 'alongside the reported wind'?

29 'so-called representativeness errors' 

There is a recent trend to call them 'representation errors', eg Hodyss and Nichols (2015, Tellus), 

Janjic et al, (2018, QJRMS) although I would regard these references as optional here. 

36 'Even later, when GNSS sensors became available, the information collected was often not transmitted' 

The drift information could not be transmitted in the older alphanumeric codes. 

It was only with the (ongoing) migration to BUFR that this became possible 

along with the reporting of many more levels in the vertical (Ingleby et al., 2016)

Ingleby, B., P. Pauley, A. Kats, J. Ator, D. Keyser, A. Doerenbecher, E. Fucile, J. Hasegawa, E. Toyoda, T. Kleinert, W. Qu, J. St James, W. Tennant, and R. Weedon, 2016: Progress toward High-Resolution, Real-Time Radiosonde Reports. Bull. Amer. Meteor. Soc., 97, 2149-2161, https://doi.org/10.1175/BAMS-D-15-00169.1. 

37 'Only since the mid-2010s is the balloon drift taken into account' 

This needs some rewriting. NCEP have estimated and used radiosonde drift since 2000 (according to 

Laroche and Sarrazin, 2013), but I haven't seen the documentation for the NCEP method. 

Laroche and Sarrazin (I think) implemented it in the Canadian NWP system. 

Ingleby and Edwards (2014, ASL) at the Met Office were early users of the BUFR 

drift positions and quoted an earlier study: 'Macpherson (1995) found little benefit from treating balloon drift and concluded that for simple advection without wind shear the errors due to neglecting the exact time and position of each level cancelled.' 

Suggested text: 'In the 2010s there was a move towards using the balloon drift information 

in modern observation processing of GNSS sondes, with beneficial results (e.g. Laroche and Sarrazin, 2013; Ingleby et al., 2018).' 

60-62 'High-resolution radiosonde data ... in BUFR ... University of Wyoming' 

For information ECMWF BUFR radiosonde data is available (loosely associated 

with IGRA) at the following link:

https://www.ncei.noaa.gov/data/ecmwf-global-upper-air-bufr/archive/ 

There is a separate archive (slightly less comprehensive) of data received by NCEP. 

73 [GNSS sondes] 'can track the horizontal and vertical position of the balloon and the sensor at high frequency' 

Delete 'the balloon and'. The high frequency data, especially the raw data 

(before smoothing), sometimes clearly shows the effect of pedulum motion 

under the balloon (eg Ingleby et al, 2022 https://doi.org/10.5194/amt-15-165-2022 and references, the results there raise questions about how optimal the smoothing is) - worse because 

of the long suspensions often used now (55 m is recommended for the RS41). 

I feel that pendulum motion should be briefly mentioned. 

76 'if a single theodolite was used' I am sure that the vast majority of 

theodolite ascents only use a single theodolite. 

89 '[BUFR] has a much higher vertical resolution (1 second frequency' 

This is potential resolution. Many European ascents report with 2 second 

frequency, there area also many reports from other areas (eg China) with 

fewer than 300 levels. 

NB. Perhaps slightly clearer to replace 'the latter' with 'BUFR'. 

91 '5-degrees'

103 '2.3 Estimation of the balloon trajectory'

There is some overlap with the work of Laroche and Sarrazin (2013). 

Mention this in the text and explain where there are differences. 

119-120 ', but temporal information is not available for all the reported pressure levels.' - ': time information is not available for any of the reported pressure levels' (slightly clearer)

130 'a piecewise polytropic atmosphere' I had to look up 'polytropic' and I am still not quite 

sure what it means in this context. I suspect that you mean that temperature is a piecewise linear 

function of height - please say this if so. My preference is to avoid the word polytropic. 

155-156 'This is true for the differences within a single ascent, but also for the differences between different ascents.' - 'This is true both within a single ascent and also between different ascents.' 

192 'The software necessary for the creation of calculated balloon trajectories can be found in:' 

Is it the same software in each of the three repositories? 

215 '4. Verification with GNSS radiosondes' 

'Validation' might be better than 'Verification' 

216 'indisputable reference' I suggest replacing indisputable with 'good'. 

I agree that GNSS data is usually very good quality, but there can be problems with interference or 

only a few GPS satellites above the horizon. As mentioned above pendulum motion complicates a detailed 

view of the positions/raw winds. 

230 'Figure 5 ...' 

The longitude displacements (in degrees) are larger than the latitude displacements - presumably 

the longitude displacements are dominated by high latitude ascents. 

I recommend showing values in km instead and ideally show summer and winter displacements separately. 

At the lowest level there is a local maximum in latitude RMS - probably caused by a minor error in 

the reported launch position, I have seen such errors. 

Y-axis - personally I would prefer to see more levels labelled. 

246 'Figure 8' Again, I think that displacements in km would be better. 

333-334 'The second experiment assimilates the radiosonde observations as slanted profiles' 

Is each level given a separate latitude/longitude/time or is it done (for efficiency reasons) in 

15-minute sub-profiles as in Ingleby et al (2018). 

334 'the balloon trajectory, when it is available' ie when there is a reasonably complete wind profile? 

364 'Laroche, 2013' - 'Laroche and Sarrazin, 2013

370-371 'we also expect that any positive impact of sonde displacement be amplified when 

satellite observations are used alongside radiosondes' I'm not convinced of this - more satellite 

data usually means less impact of radiosonde data (especially tempertures). 

384 'experiments, covering a two-month period in summer 1980' 

There is usually more drift in winter. 

419-420 'reconstructing displacements based on the reversed calculation of wind speed and direction' 

'reconstructing displacements based on the wind profile' would be better in my opinion. 

With GNSS the winds are sometimes derived from the Doppler effect rather than differentiating the 

displacements. 

Figure 1. Perhaps clarify that the wind rose gives the direction from which the wind is coming. 

Figure 9. At first I didn't understand what the green dashed line was. I now think it is the 

RMS difference between ERA5 temperatures 'vertical' and 'slanted' - I presume that there are time 

differences included too (please confirm). Does the dashed green line use the upper or lower axis? 

Figures 11. The 10 hPa differences from ERA5 in 1970 are very large, presumably due to the 

radiosonde type used at the time and possibly a lack of quality control. 

I wondered if figures 10 and 11 add much or would be better omitted. 

Figure 15. The 10 hPa obs-bg RMSE values are huge (15 degrees). 

My guess is that there has been little or no attempt to quality control the reported values. 

573 'Favà et al' drop 'Discuss.' ?

I am glad to see this work being done so that rawinsonde data can be better assimilated into re-analyses.  One thing that should be clarified is the two different datasets (TAC and BUFR), that one contains time and three-dimensional position information and one does not, and the fact that perhaps not all data are yet transmitted in BUFR.  Therefore, this technique could also be useful in operations until the time that all data are available in BUFR.

I note that similar work has been done for downsondes (as opposed to the current upsondes), https://doi.org/10.1175/JTECH-D-17-0023.1, and that this should be referenced.

Some of the writing is confusing and could use some editing for improvement.  ines 240-242 are a good example.

Some additions to the current study would be helpful:

1.  Mean statistics with height over all sondes in both TAC and BUFR of the differences between the calculated and actual trajectories.  Figure 5 shows the statistics for the "summer months" (undefined) for one year, which is helpful, but incomplete.  In addition, it isn't clear why the displacements are separated into latitudinal and longitudinal components instead of total distance.  If there is a physical reason to do so, this should be explained.  If there is no physical reason, then total differences should be shown.

2.  Perhaps examples of particularly accurate and particularly inaccurate displacement calculations (trajectories) can be shown to show the user how this may work in practice.

Additional comments:

Figure 1 is not referenced in the text.  

Significant levels are defined starting on line 92.  There are also significant levels for thermodynamic variables, and these should also be defined here for completeness.

The text says that PILOT/PIBAL have height data.  All observations with mandatory levels have height data (if there are thermodynamic data), but it seems that these values are not used in the calculations.  If this is correct, why not?  If this is incorrect, clarify in the text.

Line 198:  Is the station height also necessary for the calculation?