Improved sea ice motion estimates from enhanced resolution passive microwave brightness temperatures

Abstract:

Sea ice motion is an important component of the evolution of sea ice. An increasing trend in the speed of the Arctic ice has contributed to increased advection of ice out of the Arctic and the loss of older ice. Passive microwave imagery is a primary source for sea ice motion estimates because of its complete daily coverage and all-sky capabilities. However, a significant limiting factor for passive microwave derived ice motions is the low spatial resolution. The motion is derived using feature tracking algorithms based on correlation peaks between features in two spatially coincident images separated by a time interval. The size of the feature and the displacement discretization is a function of the resolution of the imagery. Thus, passive microwave motion fields are coarse and show substantial noise at daily time steps. Using a longer period or averaging over several days reduces the noise, but at the expense of capturing temporal variability.

Recently, a new enhanced resolution passive microwave brightness temperature (TB) product has been created. It uses overlapping footprints and signal processing techniques to synthesize a higher spatial resolution. Compared to the nominal 25 km gridded resolution, the enhanced TBs have gridded resolutions as high as 3.125 km. The signal processing method yields a gridded resolution higher than the effective resolution, but the enhanced TBs are at least double the resolution of the standard fields. The motion fields from the enhanced resolution TBs are denser (more estimates) and more coherent (less noise). Comparisons of the motions with buoys show a ~15% lower RMSe in the enhanced motions and biases are reduced by more than 50%. Further improvement is also possible because the enhanced TBs are gridded by local time of day (LOTD) as two (morning and evening) fields per day. The standard TBs are a 24-hour composite. Twice-per-day fields grouped by LOTD provides a more precise temporal interval, reducing uncertainties. It also opens up the possibility to retrieve ~12-hour twice daily motions, which would give insight into sub-daily kinematics of the ice cover. Thus, the enhanced resolution source has the potential to substantially improve ice motion estimates and create a more accurate, higher resolution long-term climate record of sea ice motion.