Onset dates from annual snowmelt on Antarctic sea ice from satellite scatterometer observations from 1992 to 2014

The timing and intensity of snowmelt processes on sea ice are key drivers determining the seasonal sea-ice energy and mass budgets. In the Arctic, satellite passive microwave and radar observations have revealed a trend towards an earlier snowmelt onset during the last decades, which is an important aspect of Arctic amplification and sea ice decline. Around Antarctica, snowmelt on perennial ice is weak and very different than in the Arctic, with most snow surviving the summer.

Here we compile time series of snowmelt-onset dates on seasonal and perennial Antarctic sea ice from 1992 to 2014/15 using active microwave observations from European Remote Sensing Satellite (ERS-1/2), Quick Scatterometer (QSCAT) and Advanced Scatterometer (ASCAT) radar scatterometers. We define two snowmelt transition stages: A weak backscatter rise indicating the initial warming and metamorphism of the snowpack (pre-melt), followed by a rapid backscatter rise indicating the onset of thaw-freeze cycles (snowmelt).

Results show large interannual variability with an average pre-melt onset date of 29 November and melt onset of 10 December, respectively, on perennial ice, without any significant trends over the study period, consistent with the small trends of Antarctic sea ice extent. There was a latitudinal gradient from early snowmelt onsets in mid-November in the northern Weddell Sea to late (end-December) or even absent snowmelt conditions in the southern Weddell Sea.

We show that QSCAT Ku-band (13.4 GHz signal frequency) derived pre-melt and snowmelt onset dates are earlier by 25 and 11 days, respectively, than ERS and ASCAT C-band (5.6 GHz) derived dates. This offset has been considered when constructing the time series. Snowmelt onset dates from passive microwave observations (37 GHz) are later by 13 and 5 days than those from the scatterometers, respectively.

Based on these characteristic differences between melt onset dates observed by different microwave wavelengths, we developed a conceptual model which illustrates how the evolution of seasonal snow temperature profiles affects different microwave bands with different penetration depths. These suggest that future multi-frequency active/passive microwave satellite missions could be used to resolve melt processes throughout the vertical snow column.

海冰表面融雪过程的发生时机与强度，是决定季节性海冰能量与质量收支的关键驱动因子。在北极地区，过往数十年间的卫星被动微波与雷达观测结果显示，融雪起始时间呈提前趋势，这是北极放大效应（Arctic amplification）与海冰消退的重要表征之一。而在南极地区，多年海冰上的融雪过程较弱，与北极情况差异显著，大部分积雪可在夏季留存。

本研究基于欧洲遥感卫星（European Remote Sensing Satellite, ERS-1/2）、快速散射计（Quick Scatterometer, QSCAT）与先进散射计（Advanced Scatterometer, ASCAT）的主动微波观测数据，构建了1992年至2014/15年间南极季节性与多年海冰的融雪起始日期时间序列。本研究定义了两个融雪过渡阶段：首先是后向散射微弱上升，表征雪层的初始升温与变质过程（融雪前期，pre-melt）；随后是后向散射快速上升，表征冻融循环的起始（融雪期，snowmelt）。

研究结果显示，多年海冰上的融雪前期与融雪起始日期存在显著年际变率，其平均起始日期分别为11月29日与12月10日；在本研究时段内未观测到显著趋势，这与南极海冰范围的微弱变化趋势相一致。威德尔海北部的融雪起始时间在11月中旬较早，而威德尔海南部则较晚（至12月末）甚至无融雪过程，整体呈现纬向梯度特征。

研究表明，基于快速散射计Ku波段（信号频率13.4 GHz）反演得到的融雪前期与融雪起始日期，分别比欧洲遥感卫星与先进散射计C波段（信号频率5.6 GHz）的反演结果提前25天与11天。在构建时间序列时已考虑了该系统偏差。而基于被动微波观测（37 GHz）得到的融雪起始日期，分别比散射计观测结果滞后13天与5天。

基于不同微波波长观测得到的融雪起始日期间的特征差异，本研究构建了一个概念模型，用以阐释季节性雪层温度廓线的演变如何对不同穿透深度的微波波段产生影响。该模型表明，未来的多频主动/被动微波卫星任务可用于解析垂直雪柱内的完整融雪过程。