See pdf file.

review of 'Extreme springs in Swizerland since 1763 in climate and phenological indices' by Noemi Imfeld, Koen Hufkens and Stefan Bronnimann

Based on an existing reconstruction of daily mean temperature and daily precipitation amounts, an overview is given of the variations in sping climate over the period since 1763 for Switzerland. The analysis is based on climatological indices and two phenological indices. In the study, the impact of climate change on these indices is documented and the climatic variability in earlier times is highlighted. A few exceptional springs are discussed in detail and in relation to the atmospheric situation.

The manuscript is well written and a joy to read. There are - as far as I can say - no methodological errors. The main concerns relate to the presentation of results which could be a bit more clear, and the selection and presentation of the climate and phenological indices needs some further thoughts. In addition, the assessment of uncertainty in the results deserves more attention.

My advise to the editor is to accept with minor revisions.

Main concerns

*) In contrast to the warming spring, the warming of the winter climate has a delaying effect on spring phenology and Wang et al. (2020) argue that existing winter chilling model underestimate the effects of winter chilling, leading to substantial overestimates of the advance of spring phenology under climate change. A similar concern relates to the cherry flowering model and the beech leaf unfolding model. The model used to relate the cherry flowering and beech leaf unfolding is not completely clear to me, but it seems that winter chilling is not part of the equation. Motivate why the winter chilling is left out or explicitly comment on this aspect - if possible with an assessment of the consequence of not using winter chilling.

The description of the phenological model is very terse. As this is an important - and interesting! - part of the work, the description of the model should be expanded a bit to guide the readers through the model that are less well acquainted with these models.

*) the selection of climatic indices is strongly biased towards the temperature-related indices. The only two indices which are precipitation-based are the number of Wet Days and Snowfall Days (the latter is a mix between precip and temperature). Although these two indices are relevent, it would be interesting to add indices that relate to droughts or pluvials - like the Consecutive Dry Days or Consecutive Wet Days indices. This would contribute to earlier studies on droughts where a propagating signal from spring drought into summer drought is observed, and might give some perspective on e.g. the drought in the mid 1940s in central Europe (Brazdil et al. 2016; Hirschi et al. 2013)

*) In the discussion of the quality of the reconstruction, it was noted that the skill in the temperature reconstruction is higher than that of precipitation. This is perfectly understandable, but what is missing is a view how this uncertainty propagates into the indices. It would have been very nice if the authors would be able to assess the uncertainty in the indices, and therefore in the conclusions. I briefly went through the paper that documents the reconstruction, but I understand that this rerconstruction does not come with an uncertainty estimate in terms of an ensemble? That would have made the assessment of uncertainty not too difficult (it only requires quite a bit of computations). The uncertainty assessment in the manuscript is now based on using various sources reconstructions (like 20CR, ModE-RA and long observational records). The spread in the various reconstructions is demonstrated in fig. 2, but the sometimes large deviations between the estimates is not discussed. Particularly the cherry flowering in the Liestal deserves some attentiuon as the observations show much stronger variability and show for many years much earlier flowering. Can you indicate if this discrepancy relates to the temperature reconstruction or is there an issue with the phenological model?

Interesting is also that the GGD and mean temperature reconstructions are spot-on with the 20CR after ±1840, but for the earliest decades, there is a bit of a bias. Can you comment on this?

other concerns the authors may want to look at

*) line 95: The group involved in the ETCCDI also prescribed levels of missing data that are allowed in the aggregation of seasonal/annual values. Can you argue why you deviate from their approach by selecting the 10% threshold?

*) Figure 1: Except for the warm spell duration index and cold spell duration index (figs. e and f), the climatologies of the 30-yr periods are quite similar. This has been noted in the text. The figure would be a bit more interesting if you would show one reference

period (e.g. your favourite 1871-1900 period) and deviations from this reference for the other periods.

*) figure 5: it seems that in the figures for the Last Frost Day, something is wrong. I see purple vertical stripes and I wonder if some detail is lacking in figure a?

smallish concerns

*) line 81: for completeness, you could mention that your definition is considered colder than what is usually called a frost, but also warmer than an ice day (where the Tmax drops below zero)

*) line 197: the use of two digits for temperature is not in-line with what is used in the rest of the manuscript.

*) figure 5: the labels of the colour bars are difficult to read with this size of the figure (I needed to zoom to read it). The a) and b) labels are set twice.

H. Wang, C. Wu, P. Ciais, J. Pe ̃nuelas, J. Dai, Y. Fu, Q. Ge, Overestimation of the effect of climatic warming on spring phenology

due to misrepresentation of chilling, Nat. Commun. 11 (4945) (2020).  doi:10.1038/s41467-020-18743-8.

Brázdil, Rudolf, et al. "The Central European drought of 1947: causes and consequences, with particular reference to the Czech Lands." Climate Research 70.2-3 (2016): 161-178.

Hirschi, E., R. Auchmann, O. Martius, and S. Brönnimann (2013) The 1945-1949 droughts in 

Switzerland. In: Brönnimann, S. and O. Martius (Eds.) Weather extremes during the past 140 years. 

Geographica Bernensia G89, p. 81-90, DOI: 10.4480/GB2013.G89.09. 

This is overall well-written, and the approaches are appropriate, I have provided an annotated copy of the manuscript to assist the authors in making corrections. I have three substantive points below which require addressing.

1. The snow days approach presented is not actually documenting snow days – you are presenting ‘potential snow days’.  What you are presenting is a ‘potential snowfall days’, as no observation of snowfall are made. This may seem rather pedantic but is important from an accuracy perspective, and the method you apply is likely to result in substantial overestimation of snow days. I recommend rephrasing the snow days to potential or removing this from the paper as it is likely to result in an ill-informed discussion. The sensitivity of such an approach in a country with extensive elevation differences is also liable to result in high uncertainties.  I would remove this section from the paper as it I fear detracts from the paper overall, an alternative approach would be to demonstrate that the method you apply provides a good proxy for snow days for the different regions using observational records.   

1. I am concerned by your use of the period 1871-1900 as pre-industrial, I appreciate that parts of Switzerland may not have been industrial, but certainly neighbouring regions and countries had extensive industry by this stage. I think using any period post-1750/1800 as pre-industrial is fraught with risk, particularly when dealing with climate and climatic parameters which are influenced beyond regional boundaries.

1. The discussion would benefit from a section explaining how the results of this paper relate to studies in neighbouring regions/countries, can you provide a little further context please of how this fits with other studies in Central Europe.

You may also be interested in considering:

Adamson, G., Nash, D. & Grab, S., (2022), Quantifying and reducing researcher subjectivity in the generation of climate indices from documentary sources, Climate of the Past. 18, 5, p. 1071-1081 11 p.  

Manley, G., 1969. Snowfall in Britain over the past 300 years. Weather 24, 428–437. https://doi.org/10.1002/j.1477-8696.1969.tb03117.x

Nash, D., Adamson, G., Ashcroft, L., Bauch, M., Camenisch, C., Degroot, D., Gergis, J., Jusopović, A., Labbé, T., Lin, K-H. E., Nicholson, S., Pei, Q., Prieto, M. D. R., Rack, U., Rojas, F. & White, S., (2021) Climate indices in historical climate reconstructions: a global state of the art, Climate of the Past. 17, 3, p. 1273–1314 42 p 

Neil Macdonald 

University of Liverpool