Conference Agenda

Part 1, Glacier velocity estimation based on Sentinel-2 observations at Karakoram.

The Sentinel-2A/B Twin satellites provide 5-day repeat observation to the Earth and capable of deriving glacier velocity with high-temporal resolution. In this study, the ‘Karakoram-Pamir anomaly’ region was taken as the study site and a data processing procedure was proposed to derive quasi-monthly glacier flow velocity fields. Each acquisition is performed offset-tracking to its next three almost cloud-free acquisitions to increase number of redundant observations. The detector mosaicking errors are eliminated if offset-tracking is performed between two different Sentinel-2 satellites. Flow speed and direction referenced method is taken to remove the wrong matching of offset-tracking. Then an iterative SVD method solves the glacier velocity and removes the observation with large residual. According to the glacier flow velocity time series between Oct 2017 and Sep 2021, it captures plenty of surged glaciers start and/or end their surging phases across this region. Two types of surging glaciers are identified according to the shape of their high temporal resolution flow rates time series. The first types’ surging phase last for only a few years, and shows no seasonal variation. Rimo’s southern tributary is an example of this type, it experienced a full surging phase during our study period and last for about two years, the maximum speed exceeded 10 m/day. Another type behaves similar to a normal type glacier but with glacier front advancing and much higher summer speed than their stagnation phase, such as Gando at Pamir. Normal type glaciers also presented annually speed up and slow down, with acceleration started usually in late April or earth May, and ends before September.

Part 2, Greenland ice sheet melting and re-freezing status monitoring with Sentinel-1 imagery

First this study introduced a method of incidence angle normalization to the backscatter coefficient of dual-polarized Sentinel-1 images. A multiple linear regression model is trained using the ratio between backscatter coefficient differences and incidence angle differences of quasi-simultaneously observed ascending and descending image pairs. Regression factors include geographical position and elevation. The precision evaluation of the ascending and descending images suggests better normalization results than the widely-used cosine-square correction method for HH images and little improvement for the HV images.

Referring to the 2m air temperature data of AWS, we find that the daily average 2m air temperature higher than 0℃ cannot accurately indicate if the ice sheet melted. The daily maximum 2m air temperature on two consecutive days higher than 0°C and the daily average 2m air temperature exceeds -1°C on the SAR acquisition day that recorded by the AWS find good agreements with the -3dB decrease of the backscatter coefficients. The overall agreement and Kappa coefficients are mostly better than 0.85 and 0.70, respectively. However, at the ablation zone, although backscatter coefficient drops when the melting begins, but it also increases during the melting status, resulting a lower estimation of the melting duration.

Part 3, Glacier velocity estimation based on both Sentinel-1 and -2 observation at Greenland Ice Sheet

Two different methods are designed for deriving glacier velocity fields for Greenland Ice Sheet. The first is designed for area where Sentinel-1 6 or 12 days interferogram show certain level coherence. To overcome the high gradient of phase, a method of re-differential interferometry that employs the result of offset-tracking is designed. The maximum capacity of detecting deformation is ~3.6m for 6-day interferogram than conventional D-InSAR. The second method is designed for quick flowing area, where no coherence can be found for the Sentinel-1 interferogram. This method introduces a small baseline offset-tracking to Sentinel-1 and -2 images, then a least square method based on connective component is applied. SAR images can hardly give acceptable result at wet-snow zone during the melting seasons, while optical images are not obtained from Oct to next Mar. Error propagation theory is employed for precision analysis. Then a weighted least square method based on connective component method combines the time series derived from Sentinel-1 and -2. This method can provide a full time series of glacier velocity fields. The errors of Sentinel-1 images offset-tracking are ~0.4 m while ~2.5 m for Sentinel-2.

Glaciers are crucial sources of freshwater in particular for the arid lowlands surrounding High Mountain Asia. In order to better constrain glacio-hydrological models, annual, or even better, seasonal information about glacier mass changes is highly beneficial. In this study, we test the suitability of very high-resolution Pleiades DEMs to measure glacier-wide mass balance at annual and seasonal scales in two regions of High Mountain Asia (Muztagh Ata in Eastern Pamir and parts of Western Nyainqêntanglha, South-central Tibetan Plateau), where recent estimates have shown contrasting glacier behavior. We find that the average annual mass balance in Muztagh Ata between 2020 and 2022 was -0.11 ±0.21 m w.e. a^-1, suggesting the continuation of a recent phase of slight mass loss following a prolonged period of balanced mass budgets previously observed. The mean annual mass balance in Western Nyainqêntanglha for the same period was highly negative (-0.60 ±0.15 m w.e. a^-1 on average), suggesting increased mass loss rates. The 2022 winter (+0.21 ±0.24 m w.e.) and summer (-0.31 ±0.15 m w.e.) mass budgets in Muztag Ata and Western Nyainqêntanglha (-0.04 ±0.27 m w.e. [winter]; -0.66 ±0.07 m w.e. [summer]) suggest winter and summer accumulation-type regimes, respectively. We support our findings by implementing a Sentinel-1–based Glacier Index to identify the firn and wet snow areas on glaciers and characterize accumulation type. The good match between the geodetic and Glacier Index results demonstrates the potential of very high-resolution Pleiades data to monitor mass balance at short time scales and improves our understanding of glacier accumulation regimes across High Mountain Asia.