CRYO2ICE radar/laser freeboards, snow depth on sea ice and comparison against auxiliary data during winter season 2020-2021

We investigated the near-coincident radar and laser altimetry observations from the CryoSat-2 and ICESat-2 Resonance (CRYO2ICE) campaign, which was initiated in July 2020, for the winter season of 2020-2021. We extracted Arctic radar and laser freeboard (the elevation of sea ice above the local sea level depending on scattering horizon), and binned the ICESat-2 observations to the CryoSat-2 observations using a search radius of 3500 m and applied inverse-distancing-weightning. From ICESat-2, both strong and weak beams from all three beam pairs were included. For CryoSat-2, we include the baseline-D from the European Space Agency (ESA), and include two further products using two other retrieval methodologies (CCI and LARM). We further derived snow depth from the difference in penetration of the radar and laser (using the freeboards), and this high-resolution orbit-based snow depth estimates were compared with nearest-neighbouring observations from basin-scale snow depth composites using either passive-microwave-radiometry (AMSR2) or a reanalysis-based model estimates (SMLG/SMLG-HS). Orbit-based snow depth estimates were also compared with buoy estimates (acoustic or thermistor string buoys). 

This dataset includes the CRYO2ICE tracks + coincident buoy data computed for Fredensborg Hansen et al. (2023), where the code is available here): https://github.com/reneefredensborg/CRYO2ICE_freeboards_snow_depths_roughness_auxiliary_v1

The files/folders are as follows: 

• File: cryo2ice_tracks.csv ---- information (IDs of CryoSat-2/ICESat-2 tracks along with start/end times of the tracks) for others to compare and identify CRYO2ICE tracks for future studies. 
• Files (...)_smooth.h5 in CRYO2ICE_individual ZIP-folder (778 tracks --> 389 individual CRYO2ICE tracks identified): CRYO2ICE individual tracks after binning (search radius of 3500 m, inverse-distancing-weighting etc., following the methdology presented in Fredensborg Hansen et al. (2023)) available as .h5-files. Two files pr. track is provided, one with all the filtered data available and one track with additional filtering applied (related to filtration on waveform parameters SSD (stack standard deviation) and PPK (pulse peakiness)). 
• Files (...)_smooth_test.csv in CRYO2ICE_individual_comparison ZIP-folder (358 tracks tracks identified with data for comparison): Includes all of the CRYO2ICE track with additional re-trackers (LARM (Landy et al, 2020) and CCI+ (Rinne and Hendricks, 2023)), along-track snow depth of Warren et al. (1999)/W99 extracted from ESA's CryoSat-2 product, as well as nearest-neighbour daily composite observations from SMLG (Liston et al., 2018)/SMLG-HS (Merkouriadi et al., 2020) and AMSR2 (Meier et al., 2018), available as .csv-files.
• Files: (....)_gridded_smooth_25km.csv  in CRYO2ICE_25km_segments ZIP-file (358 tracks) ---- all CRYO2ICE tracks (after processing, filtration and binning) computed as 25-km orbit segments.
• Files: (...)_semivariogram.csv in CRYO2ICE_semivariogram ZIP file (76 tracks investigated) ---- the analysis of tracks to identify the search radius to use when binning CS2 and IS2 to comparable observations. 
• SIMBA buoy comparison (ZIP folder): Computed daily snow depth estimates are available for both SIMBA buoys (FRIC0607 and PRIC0906) (and a buoy estimated created by combining the two original buoy observations). Coincidident CRYO2ICE observations are available for each of these three daily buoy estimates. 
• ASSIST comparison: one example of coincident CRYO2ICE/ASSIST data (presented in Supporting Information in Fredensborg Hansen et al., 2023) using IceWatch observations, https://icewatch.met.no/, last access: 06-01-2023 (observations-108/109/110 were investigated, but very little coincident data. ) 
• AWI buoy comparisons (ZIP folder): buoy data from the following buoys 2020S98, 2020S105-108 with the acoustic snow depth buoy data from the Alfred Wegener Institute (AWI) by Nicolaus et al. (2017) were used. Averaged daily snow depth estimates (buoyID_daily.csv) using all four sensors and the initial snow depth provided in the buoy deployment documents for each buoy are provided along with coincident CRYO2ICE/AWI buoy observations. 

CRYO2ICE tracks

We refer to the code/READ ME-files/additional descriptions available in the Github repository, where descriptions of what the data entails are provided. 

References 

Fredensborg Hansen et al. (2023):  Arctic freeboard and snow depth from near-coincident CryoSat-2 and ICESat-2 (CRYO2ICE) observations: A first examination during winter 2020-2021. ESS Open Archive. https://doi.org/10.22541/essoar.168614619.96485405/v1 

Landy, J. C., Petty, A. A., Tsamados, M., & Stroeve, J. C. (2020). Sea ice roughness overlooked as a key source of uncertainty in CryoSat-2 ice freeboard retrievals. Journal of Geophysical Research: Oceans, 125, e2019JC015820. https://doi.org/10.1029/2019JC015820 

Liston, G. E., Polashenski, C., Rösel, A., Itkin, P., King, J., Merkouriadi, I., & Haapala, J. (2018). A distributed snow-evolution model for sea-ice applications (SnowModel). Journal of Geophysical Research: Oceans, 123, 3786- 3810. https://doi.org/10.1002/2017JC013706 

Meier, W. N., T. Markus, and J. C. Comiso. (2018). AMSR-E/AMSR2 Unified L3 Daily 12.5 km Brightness Temperatures, Sea Ice Concentration, Motion & Snow Depth Polar Grids, Version 1 [Data Set]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/RA1MIJOYPK3P . Date Accessed 06-01-2023. 

Merkouriadi, I., Liston, G. E., Graham, R. M., & Granskog, M. A. (2020). Quantifying the potential for snow-ice formation in the Arctic Ocean. Geophysical Research Letters, 47, e2019GL085020. https://doi.org/10.1029/2019GL085020 

Nicolaus, M.; Hoppmann, M.; Arndt, S.; Hendricks, S.; Katlein, C.; König-Langlo, G.; Nicolaus, A.; Rossmann, L.; Schiller, M.; Schwegmann, S.; Langevin, D.; Bartsch, A. (2017): Snow height and air temperature on sea ice from Snow Buoy measurements. Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven,https://doi.org/10.1594/PANGAEA.875638

Rinne, E., & Hendricks, S. (2023). CCI+ Sea Ice ECV Sea Ice Thickness Product User Guide (PUG) . (Reference D4.2, Phase 1).