Different glacier surge patterns revealed by Sentinel-2 imagery derived quasi-monthly flow velocity at west Kunlun Shan, Karakoram, Hindu Kush and Pamir

The twin optical Sentinel-2 A/B satellites, with their 5-day repeat observations, have proven to be capable of deriving high temporal resolution glacier velocity fields. This study proposes a data processing procedure for deriving quasi-monthly glacier flow velocity fields for the “Karakoram-Pamir anomaly” region. Each Sentinel-2 acquisition is performed offset-tracking (OT) to its next three almost cloud-free acquisitions to increase number of redundant observations. The detector mosaicking errors are eliminated for the two different Sentinel-2 satellites OT. A preliminary flow speed and direction referenced method is taken to remove the wrong matching of OT, and an iterative SVD (singular value decomposition) method solves the glacier velocity time series and removes the observation with large residual. Between Oct 2017 and Sep 2021, the derived results capture several surged glaciers initiating and/or ending their surging phases throughout the region. Additionally, two types of surging glaciers are identified solely on the basis of their high temporal resolution flow rates time series. The first type exhibits a short surging phase of just a few years and exhibited no seasonal variation in their flow rates, as exemplified by Rimo's southern tributary, which experienced a full surging phase lasting for approximately two years and reaching maximum speeds exceeding 10 m/day within the study period. The second type behaves similarly to normal glaciers, albeit with a glacier front that advances and exhibits much higher summer speeds (>3.5 m/day) than during stagnation, such as Gando at western Pamir. Normal glaciers exhibit annual speed-ups and slowdowns, with acceleration typically beginning in late April or early May and ending before September.