Ice-sheet mass-balance estimates derived from repeat satellite-altimetry observations require accurate calculation of spatiotemporal variability in firn-air content (FAC). However, firn-compaction models remain a large source of uncertainty within mass-balance estimates. In this study, we investigate one process that is neglected in FAC estimates derived from firn-compaction models: enhanced layer thinning due to horizontal divergence. We incorporate a layer-thinning scheme into the Community Firn Model. At every time step, firn layers first densify according to a firn-compaction model and then thin further due to an imposed horizontal divergence rate without additional density changes. We find that horizontal divergence on Thwaites (THW) and Pine Island Glaciers can reduce local FAC by up to 41% and 18%, respectively. We also assess the impact of temporal variability of horizontal divergence on FAC. We find a 15% decrease in FAC between 2007 and 2016 due to horizontal divergence at a location that is characteristic of lower THW. This decrease accounts for 16% of the observed surface lowering, whereas climate variability alone causes negligible changes in FAC at this location. Omitting transient horizontal divergence in estimates of FAC leads to an overestimation of ice loss via satellite-altimetry methods in regions of dynamic ice flow.

中文翻译：

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水平发散对冷空气含量估计的影响

从重复卫星测高观测得出的冰盖质量平衡估计需要准确计算冷空气含量 (FAC) 的时空变异性。然而，冰层压实模型仍然是质量平衡估计中不确定性的主要来源。在这项研究中，我们研究了一个在 FAC 估计中被忽略的过程，该过程来自于 firn-compaction 模型：由于水平发散而增强的层变薄。我们将层细化方案纳入社区 Firn 模型。在每一个时间步，冰层首先根据冰层压实模型致密，然后由于施加的水平发散率而进一步变薄，而没有额外的密度变化。我们发现 Thwaites (THW) 和 Pine Island 冰川的水平分歧可以分别降低高达 41% 和 18% 的当地 FAC。我们还评估了水平背离的时间变异性对 FAC 的影响。我们发现 2007 年至 2016 年间 FAC 下降了 15%，原因是在一个具有较低 THW 特征的位置的水平分歧。这种下降占观测到的地表下降的 16%，而仅气候变化导致该位置 FAC 的变化可以忽略不计。在 FAC 的估计中忽略瞬时水平发散会导致在动态冰流区域通过卫星测高方法高估冰损失。而仅气候变率会导致该位置 FAC 的变化可以忽略不计。在 FAC 的估计中忽略瞬时水平发散会导致在动态冰流区域通过卫星测高方法高估冰损失。而仅气候变率会导致该位置 FAC 的变化可以忽略不计。在 FAC 的估计中忽略瞬时水平发散会导致在动态冰流区域通过卫星测高方法高估冰损失。