40 Years of Surface Warming in the Northern US Rocky Mountains: Implications for Snowpack Retreat

The northern US Rocky Mountains are experiencing rapid warming. Combined analysis of Ground Temperature (GT) measurements at two high-fidelity boreholes with Surface Air Temperature (SAT) measurements near Helena Montana spanning the past 40 years indicate the northern US Rockies have warmed on average 0.12°C - 0.32°C/decade since 1975, at least a factor of ~5 higher than the predicted 500-year-average. Warming appears to be accelerating, with warming rates since 2013 4 - 7 times higher than the 40 year average. Though uncertainty exists, the most significant GT warming appears to occur at higher elevation. Warming estimates are consistent with modelling predictions, snowpack observations, and stream temperature studies, all suggesting rapid surface temperature change in this region during the past ~40 years. The analysis indicates GT warming measured at remote borehole sites is slightly lower than regional SAT measurements collected near urban environments. We associate the discrepancy between GT/SAT measurements to both anthropogenic effects (urban development) that increase warming at the nearest SAT measurement station and a 14-year period of anomalously low snowfall that reduces surface insulation and GT warming. Using a derived average forty-year surface warming rate of 0.22°C/ decade and regional temperature-elevation trends, we calculate that the elevation of the winter freeze line during the three coldest months of the year (December, January, and February) in the northern US Rocky Mountains is retreating upward, on average, 33 m/decade. This implies a 21% reduction in freeze-line area since 1974. If this trend continues, we estimate that within the next 40 years (by 2060), the total area where ground freeze occurs during the three coldest months of the year will be ~60% of 1974 values. Since GT measurements indicate accelerated warming, this may be an underestimate. The analysis has important implications for the snowpack-water budget for Montana and the northern US Rocky Mountains.

Kilometer-resolution three-dimensional crustal deformation of Tibetan Plateau from InSAR and GNSS

Located at the forefront of the collision between the Indian and Eurasian Plates,the Tibetan Plateau experiences intense crustal movement.Traditional ground-based geodetic monitoring,such as GNSS and leveling,is challenging,due to factors such as high altitude and harsh climate,making it difficult to accurately determine a high-resolution crustal deformation field of the plateau.Unaffected by ground observation conditions,InSAR technique has key advantages for obtaining extensive and high-resolution crustal deformation fields.This makes it indispensable for crustal deformation monitoring on the Tibetan Plateau.This study used Sentinel-1 data from 2014 to 2020 to compute the ascending and descending InSAR deformation fields for the Tibetan Plateau.This was conducted with a measurement accuracy of approximately 3 mm/yr.Building upon this,we integrated InSAR and GNSS data to yield kilometer-resolution three-dimensional(3D)crustal deformation and strain rate fields for the Tibetan Plateau.A spherical wavelet analysis was used to decompose the 3D deformation field and separate the nontectonic crustal deformation to increase the strength of the tectonic deformation signal.Short-wavelength(<110 km)deformations match the distribution of fault movement,post-seismic deformations,and other non-tectonic factors.Long wavelength(>110 km)deformation mainly results from subsidence in the central plateau and uplifts along the Himalayan Arc.This indicates that the Tibetan Plateau may have stopped the entire uplift and entered a local collapse stage.Furthermore,the deformation fields at different spatial scales reveal that the plateau exhibits discontinuous deformation in short wavelengths and continuous deformation in long wavelengths.The findings of this study contribute to resolving the controversy between the Block and Continuum Deformation models of the Tibetan Plateau.

Factors Contributing to the 2005-Present, Rapid Rise in Lake Levels, Dominican Republic and Haiti (Hispaniola)

Lakes Enriquillo and Azuei, the two largest lakes in Hispaniola and in the Caribbean, have risen 10 and 5 m respectively within the last 8 years. Higher lake levels have submerged towns, road systems, agricultural lands and utilities, and have threatened to submerge the major overland highway that connects the Dominican Republic and Haiti. In this study, we use CHIRP seismic data, satellite imagery, and regional meteorological data to quantify and assess controls on the recent lake level rises. Although data are limited, the analyses indicate that the lakes’ water level changes may be attributed to a combination of increased rainfall and natural or man-made changes to the hydraulic connectivity of the various water bodies within the drainage basin. We show that a weak correlation exists between changes in Lake Enriquillo’s and Azuei’s water levels and precipitation rates (0.2 and 0.08 respectively, 1984-2012) and that both lakes experience periods of anti-correlation where, for example, water level drops at Lake Azuei (~20 masl) coincide with water level rises at Lake Enriquillo (41 mbsl). From these observations, we propose that the lakes experience intermittent periods of hydraulic connectivity along reactivated or newly developed stratigraphic-controlled sub-surface transport pathways. We also note that moderately small earthquakes along the large active fault system that extends through both lakes may promote or limit hydraulic conductivity on decadal or shorter time scales. The extents to which recent earthquakes have triggered changes in groundwater flow at this site remain unclear but represent an important topic of future research.

Multi-dimensional and long-term time series monitoring and early warning of landslide hazard with improved cross-platform SAR offset tracking method

Multi-dimensional, long-term time series displacement monitoring is crucial for generating early warnings for active landslides and for mitigating geohazards. The synthetic aperture radar(SAR) interferometry method has been widely applied to achieve small-gradient landslide displacement monitoring;however, measuring the landslide displacement with a steep gradient has posed certain challenges. In comparison, the SAR offset tracking method is a powerful tool for mapping large-gradient landslide displacement in both the slant-range and azimuth directions. Nevertheless, there are some limitations in the existing SAR offset tracking approaches:(i) the measurement accuracy is greatly reduced by the extreme topography relief in high mountain areas,(ii) a fixed matching window from expert experience is usually adopted in the calculation of cross-correlation,(iii) estimating the long-term displacements between the SAR images from cross-platforms and with longer spatiotemporal baselines is a challenging task, and(iv) it is difficult to calculate the three-dimensional(3D) landslide displacements using a single SAR dataset.Additionally, only a few studies have focused on how to realize early warning of landslide deformation using SAR measurements.To address these issues, this paper presents an improved cross-platform SAR offset tracking method, which can not only estimate high-precision landslide displacements in two and three dimensions but also calculate long-term time series displacements over a decade using cross-platform SAR offset tracking measurements. Initially, we optimize the traditional SAR offset tracking workflow to estimate high-precision ground displacements, in which three improvements are highlighted:(i) an“ortho-rectification” operation is applied to restrain the effect of topography relief,(ii) an “adaptive matching window” is adopted in the cross-correlation computation, and(iii) a new strategy is proposed to combine all the possible offset pairs and optimally design the displacement inversion network based on the “optimization theory” of geodetic inversion. Next, robust mathematical equations are built to estimate the two-dimensional(2D) and 3D long-term time series landslide displacements using cross-platform SAR observations. The M-estimator is introduced into the 2D displacement inversion equation to restrain the outliers, and the total least squares criterion is adopted to estimate the 3D displacements considering the random errors in both the design matrix and observations. We take the Laojingbian landslide, Wudongde Reservoir Area(China), as an example to demonstrate the proposed method using ALOS/PALSAR-1 and ALOS/PALSAR-2 images. The results reveal that the proposed method significantly outperforms traditional methods. We also retrieve the movement direction of each pixel of the Laojingbian landslide using the proposed method, thus allowing us to understand the fine-scale landslide kinematics. Finally, we capture and analyze the acceleration characteristics of the landslide, perform an early warning of hazard, and forecast the future displacement evolution based on the 3D displacement time series coupled with the physical models of the rocks.

联合InSAR和GNSS构建青藏高原千米分辨率三维地壳形变场

青藏高原位于印度板块与欧亚板块碰撞的前沿地带,地壳运动十分活跃.受高海拔、高寒气候等因素的限制,以地面观测站为代表的传统大地测量监测较为困难,因此至今青藏高原的高空间分辨率地壳形变场尚未精确测定. InSAR技术不受地面观测条件限制,能够获得大范围、高空间分辨率的地壳形变场,在青藏高原地壳形变监测领域具有不可替代的优势.本文利用2014~2020年的Sentinel-1卫星SAR数据,计算了青藏高原升降轨InSAR形变场,测量精度约为3mm/a.在此基础上,融合InSAR与GNSS数据,获取了覆盖青藏高原的千米分辨率三维地壳形变场和应变率场.通过球面小波多尺度分析,将三维形变场分解出不同波长的形变,其中波长小于110km的形变主要来自断层运动、震后形变和其他非构造形变,大于110km的形变主要来自大范围的构造运动.研究结果显示,高原中部地区呈现大范围下沉,而喜马拉雅弧地区则呈现明显隆升,这表明青藏高原可能已经停止了整体隆升并进入了局部坍塌阶段.此外,不同空间尺度的三维形变场表明,青藏高原在短波长变形中表现为非连续形变,在长波长中则表现为连续形变.本文研究结果有助于调节青藏高原块体变形模型和连续变形模型之间的争议.