## 4 Results
### Local Waveform Variability  
Of particular interest is the response of altimeter waveforms to spatial changes of the sea ice surface. In the following we present representative examples over a large ridge, and for different sized MYI floes that are embedded in FYI. Described findings are typical of these features and occurred for many samples in our dataset. The first example in Figure 8 shows the local waveform variation and SAR imagery over a large ridge on 2016-03-15. The time lag between the SAR image and the altimeter track is below five minutes, i.e., for average drift speeds between 6 cm/s and 10 cm/s that means 18 m to 30 m. The ridge is clearly visible in the center of the cross-polarization SAR image by its higher backscatter caused by ice facets oriented towards the radar (Figure 8a-c). The altimeter on the other hand shows a lower peak intensity around the ridge because more energy is scattered away from the nadir-looking instrument (Figure 8d). PP is low over the ridge, while SSD and inverse mean power reach higher values (Figure 8e,f). This is characteristic for more diffuse scattering. Waveforms over FYI have a higher peak power with high PP and low SSD and inverse mean power. Notable is the large negative freeboard occurring for sample 25. Figure 9 shows the waveforms in the vicinity of the negative freeboard and closer inspection of waveform 25 shows a multiple kinked leading edge towards the first peak that significantly delays the retracking point. The smoother sea ice areas, indicated by the darker tone in the SAR image, below the ridge begin to have an impact on the waveform and subsequent waveforms have an increased peak power. Observations over other ridges show that those features need to be sufficiently large, i.e., minimum 3 altimeter footprints, to not be affected by the more specular scattering of the FYI. The freeboard, expected to be larger over the ridge area, does, however, not show any significant changes compared to the surrounding FYI. Figure 10 shows the SAR image and the spatial variation of altimeter waveforms of an old MYI floe embedded in thinner FYI. The data was taken on 22 February 2018 with a time separation of about 20 min, i.e., for average drift speeds between 6 cm/s and 10 cm/s that means 72 m to 120 m. The floe size along-track is approximately 24 km. In the SAR image, the MYI floe is characterized by high backscatter in co- and cross-polarization (Figure 10a) shows co-polarization) while the surrounding FYI has a lower backscatter. In the upper part of the MYI floe, some lead and low backscatter features are visible that are also reflected by reduced mean backscatter intensity and an increased standard deviation (light blue area in Figure 10b,c). The lower part of the floe looks relatively homogeneous. The waveforms in the interior of the floe show a low peak power with an exception around sample number 40 (Figure 10d). In this area the peak power is increased compared to before and after, but no feature is visible in the SAR image. The altimeter is very sensitive to smooth areas within the footprint that are too small to be resolved by the SAR but affect the measured waveform. Over the lower part of the MYI floe the freeboard estimate reflects the higher thickness of the MYI, but in the upper part of the floe, where smoother sea ice features affect the waveforms, freeboard is reduced. Therefore, in areas of mixed ice types sea ice freeboard more likely represents the smoother and thinner part of the ice. Negative freeboard is also observed in areas of changing sea ice surface at the beginning and  
Figure 7: Freeboard distributions for (**a**) FYI and MYI of all winter seasons, (**b**) FYI and (**c**) MYI for the three winter seasons. Solid lines are kernel density estimates of the histograms. end of the MYI floe. This example demonstrates the difficulties of obtaining robust freeboard even over relatively large and apparently homogeneous features, but careful observation of the changes in waveform shape facilitates explanation of the observations.