Use of satellite altimetry for monitoring river ice state and ice jams

In regions with low density of ground observational network, hydrodynamic models  and satellite observations are able to reproduce the river water level regime and inundated areas with an accuracy sufficient for  monitoring climate change. However, very limited number of studies demonstrated the capacity of synergy of satellite altimetry and hydrodynamic modelling in complex Arctic environment due to lack of certainty in interpretation of altimetric measurements over river ice and high errors in calculation of winter water levels. However, it is the winter regime of arctic and boreal rivers that experiences the most significant climate change. These changes are seen as a decrease in ice duration and thickness, ice jam occurrence and an increase in winter water levels.  We hypothesized that the river ice properties varying in space and time may affect the performance of water level retrievals from satellite altimetry and investigated the validity of our hypothesis using the synergy of satellite measurements and numerical experiment with hydrodynamic models. Two  models MIKE 11DHI and STREAM 2D were adapted for 90-300 km-long river reaches located on the Severnaya Dvina, Lena and Kolyma Rivers. The models were run for winter periods with ice modules switched on/off. Comparison of simulation results showed that in case of smooth ice of the Kolyma River the altimetry retrievals rather showed the elevation of the ice bottom, while in case of rough S.Dvina and Lena ice the satellite measurements were close to the elevation of the ice top. The latter allowed to reproduce with the altimetric measurements the mid-winter ice jam conditions occurred in 2019 on the S.Dvina River and to validate the model in ungauged sections.   
The verification of the altimetry return signal (waveform) over the test sites demonstrated that over the Kolyma's smooth ice,  the waveforms expressed two distinct peaks. The second peak dominated in power and was used by the SAMOSA 2 retracker algorithm for the range and elevation  estimation. In cases of rough ice, the return signals had one peak, which, according to comparison with the modeled levels, was produced by reflection of the signal from the ice top. Our finding can have an important implication for future adaptation of satellite altimetry for high latitude river hydrology and can explain the variable in space and time performance of the satellite altimetry over frozen rivers. 
The new interferometric altimetric instrument installed on the SWOT satellite (on orbit since 2023) globally maps the river water surface topography and potentially may indicate the locations and severity of the ice jams. However, the low instrument incidence angle makes surface elevation retrievals unrealistic in many locations characterized by low water/ice roughness. We investigated the performance of the SWOT surface elevation retrievals in winter period in our test sites and demonstrated that its measurements may have precision compared with the model simulations. This makes the SWOT measurements extremely valuable for hydrodynamic model runs in winter.  

The study was supported by CNES TOSCA SWIRL project; Water Problems Institute, RAS Governmental Order FMWZ-2025-0003 and RSF project №24-17-00084.