Estimation of Sea Ice Thickness from SARAL/AltiKa in Drifting Orbit Phase

Abstract

Intra and inter-annual variations in the sea ice thickness are highly sensitive indicators of climatic variations undergoing in the earth’s atmosphere and oceans. This paper describes the method of estimating sea ice thickness using radar waveforms data acquired by SARAL/Altika mission during its drifting orbit phase from July 2016 onwards yielding spatially dense data coverage. Based on statistical analysis of return echoes, classification of the surface has been carried out in three different types, viz. floe, lead and mixed. Time delay correction methods were suitably selected and implemented to make corrections in altimetric range measurements and thereby freeboard. By assuming hydrostatic equilibrium, freeboard data were converted into sea ice thickness. Results show that sea ice thickness varies from 4 to 5 m near ice shelves and 1 to 2.5 m in the marginal sea ice regions. Freeboard and sea ice thickness estimates were also validated using NASA’s Operation Ice Bridge (OIB) datasets. Freeboard measurements show very high correlation (0.97) having RMSE of 0.13. Overestimation of approximately 1–2 m observed in the sea ice thickness, which could be attributed to distance between AltiKa footprint and OIB locations. Moreover, sensitivity analysis shows that snow depth and snow density over sea ice play crucial role in the estimation of sea ice thickness.

Keywords:

• Drifting orbit phase
• Ka-band altimeter
• snow density
• snow depth
• waveform retracking

Acknowledgement

The SARAL/AltiKa mission is an achievement of a very fruitful cooperation between ISRO and CNES. Authors express their gratitude towards Director, Space Applications Centre (SAC) for providing an opportunity to contribute in the challenging field of polar cryosphere. We acknowledge the support of National Centre for Polar and Ocean Research (NCPOR), Goa for providing an opportunity to the SAC scientists for participating in the Indian Scientific Expedition to Antarctica for the realization of the dynamics of sea ice through voyage. We extent our gratefulness towards Dr. S. Chander for fruitful discussion on SARAL/AltiKa. We are thankful to Dr. B Kartikeyan, Outstanding Scientist, SAC and Dr. Rashmi Sharma, Scientist-G, SAC for reviewing this manuscript critically and suggesting very useful modifications.