Glacier-Wide Mass Balance and Compiled Data Inputs: Juneau Icefield Glaciers

Since the 1940s, the Juneau Icefield Research Program (JIRP) has been measuring surface mass balance on the Juneau Icefield. This is the longest ongoing program of its kind in North America. The program nominally occurs between late June and late August, traversing between Juneau, Alaska and Atlin, British Columbia. JIRP has examined the surface mass balance of the Juneau Icefield since 1946, with principal efforts focused on Lemon Creek Glacier and Taku Glacier. Glaciological, geodetic, and meteorological data have been collected by JIRP to characterize the interaction between the climate and glaciers of the Juneau Icefield. Direct field measurements of point glaciological data are combined with weather and geodetic data to estimate the seasonal and annual mass balance at each glacier in both a conventional and reference surface format (Cogley and others, 2011). The analysis framework (O'Neel, 2019; McNeil and others, 2020) is the same at each glacier to enable cross-comparison between output time series. For Taku and Lemon Creek glaciers, temperature lapse rates are optimized using on-icefield weather data. This changes the degree day factor in the melt model, giving small post-geodetic calibration differences on the order of 2-3 cm. Details are described in McNeil (2019). Vocabulary used follows Cogley and others (2011) Glossary of Glacier Mass Balance. This portion of the data release includes glacier wide mass balance solutions for Taku and Lemon Creek Glaciers, as well as the refined inputs used in these calculations. Input data are of three types: 1) time-variable area altitude distribution (AAD); 2) time series of point water balance at long term sites (with secondary sites given in recent years); 3) weather data from nearby stations, either installed along the glacier margins or taken from a nearby site if continuous glacier-adjacent data is unavailable. The USGS runs a coded analysis to transform the three input data types to the output glacier-wide data. Output data represent surface mass balance estimates. The output solution is a geodetically calibrated, conventional glacier-wide mass balance, which represents our preferred solution. Conventional glacier-wide mass balance from direct observations without calibration can be easily derived by using the geodetic calibration coefficients provided, if desired. We do not explicitly account for basal or englacial accumulation or ablation. Mass balances are reported in water equivalent (w.e.) units, and often represent integration of multiple field measurements. Whenever possible, we average multiple field measurements to account for surface roughness and measurement errors. Additional data for Lemon Creek Glacier, part of the Juneau Icefield, is available in a separate data release of USGS Benchmark Glacier data at https://doi.org/10.5066/F7HD7SRF It is not included here to avoid duplication. Preliminary mass balance estimates for the current calendar year are provided, but do not include direct measurements of ablation after the date of the fall visit. Preliminary estimates of mass balance model this winter ablation for the current year. During subsequent field visits in the following calendar year, any ablation that occurred over the winter season is measured and used to revise the previously modeled estimate of mass balance.

自20世纪40年代以来，朱诺冰原研究计划（Juneau Icefield Research Program, JIRP）便持续对朱诺冰原开展表面物质平衡（surface mass balance）观测，这是北美地区同类项目中持续时长最久的一项。该项目通常于6月末至8月末间开展，研究路线横跨阿拉斯加州朱诺与不列颠哥伦比亚省阿特林之间的区域。JIRP自1946年起就对朱诺冰原的表面物质平衡展开研究，核心观测重点为莱蒙溪冰川（Lemon Creek Glacier）与塔库冰川（Taku Glacier）。为解析朱诺冰原的气候与冰川间相互作用，JIRP采集了冰川学、大地测量学与气象学相关数据。研究团队将点位冰川学直接野外观测数据，与气象、大地测量数据相结合，以常规与参考表面两种格式（Cogley等，2011），估算了各冰川的季节与年度物质平衡。本次研究采用的分析框架（O'Neel，2019；McNeil等，2020）在各冰川中保持统一，以便对产出的时间序列进行跨冰川对比。针对塔库冰川与莱蒙溪冰川，研究团队利用冰原上的气象数据优化了温度递减率，这一操作会调整消融模型中的度日因子，仅带来约2至3厘米量级的后大地测量校准差异，相关细节详见McNeil（2019）。本数据集所采用的术语遵循Cogley等（2011）发布的《冰川物质平衡术语表》。本次数据发布的内容包含塔库冰川与莱蒙溪冰川的全域物质平衡估算结果，以及上述计算中使用的精细化输入数据。输入数据分为三类：1）随时间变化的面积-海拔分布（Area Altitude Distribution, AAD）；2）长期监测点位的点尺度水平衡时间序列（近年新增了辅助监测点位）；3）邻近站点的气象数据，这类数据要么安装于冰川边缘，若无法获取连续的冰川邻近区域数据，则采用附近站点的观测数据。美国地质调查局（USGS）通过编码分析程序，将三类输入数据转换为全域冰川尺度的输出数据。输出数据为表面物质平衡估算值，其中经大地测量校准的常规全域冰川物质平衡为推荐最优解。若有需要，也可通过提供的大地测量校准系数，直接推导未经校准的常规全域冰川物质平衡观测结果。本研究未明确考虑基底或冰内的积累与消融过程。物质平衡以水当量（water equivalent, w.e.）为单位报告，通常整合了多组野外观测数据。在条件允许时，研究团队会对多组野外观测数据取平均值，以抵消表面粗糙度与测量误差的影响。朱诺冰原组成部分的莱蒙溪冰川的额外数据，可在美国地质调查局基准冰川数据集的独立发布版本中获取，链接为https://doi.org/10.5066/F7HD7SRF，本次发布未包含该部分内容以避免重复。本次发布还提供了当前日历年的初步物质平衡估算值，但未包含秋季考察之后的消融直接观测数据。当前年份的初步物质平衡模型包含了冬季消融量估算。在后续日历年的野外考察中，研究团队将对整个冬季发生的消融量进行实测，并用该实测数据修正此前通过模型得到的物质平衡估算结果。