Snowfall and snow accumulation processes during the MOSAiC winter and spring season The Cryosphere

Data from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition allowed us to investigate the temporal dynamics of snowfall, snow accumulation and erosion in great detail for almost the whole accumulation season (November 2019 to May 2020). We computed cumulative snow water equivalent (SWE) over the sea ice based on snow depth and density retrievals from a SnowMicroPen and approximately weekly measured snow depths along fixed transect paths. We used the derived SWE from the snow cover to compare with precipitation sensors installed during MOSAiC. The data were also compared with ERA5 reanalysis snowfall rates for the drift track. We found an accumulated snow mass of 38 mm SWE between the end of October 2019 and end of April 2020. The initial SWE over first-year ice relative to second-year ice increased from 50 % to 90 % by end of the investigation period. Further, we found that the Vaisala Present Weather Detector 22, an optical precipitation sensor, and installed on a railing on the top deck of research vessel Polarstern, was least affected by blowing snow and showed good agreements with SWE retrievals along the transect. On the contrary, the OTT Pluvio2 pluviometer and the OTT Parsivel2 laser disdrometer were largely affected by wind and blowing snow, leading to too high measured precipitation rates. These are largely reduced when eliminating drifting snow periods in the comparison. ERA5 reveals good timing of the snowfall events and good agreement with ground measurements with an overestimation tendency. Retrieved snowfall from the ship-based Ka-band ARM zenith radar shows good agreements with SWE of the snow cover and differences comparable to those of ERA5. Based on the results, we suggest the Ka-band radar-derived snowfall as an upper limit and the present weather detector on RV Polarstern as a lower limit of a cumulative snowfall range. Based on these findings, we suggest a cumulative snowfall of 72 to 107 mm and a precipitation mass loss of the snow cover due to erosion and sublimation as between 47 % and 68 %, for the time period between 31 October 2019 and 26 April 2020. Extending this period beyond available snow cover measurements, we suggest a cumulative snowfall of 98–114 mm.

本研究依托北极气候研究多学科漂流观测站（Multidisciplinary drifting Observatory for the Study of Arctic Climate, MOSAiC）科考队获取的数据，得以在几乎完整的积雪积累季（2019年11月至2020年5月）内，对降雪、积雪积累与侵蚀的时间动态特征开展高分辨率的细致研究。我们基于雪微剖面仪（SnowMicroPen）获取的雪深与雪密度反演结果，以及沿固定样带路径获取的约周尺度雪深观测数据，计算了海冰区的累积雪水当量（snow water equivalent, SWE）。本研究利用反演得到的积雪覆盖区SWE，与MOSAiC科考期间布设的降水传感器开展对比验证，同时还将该数据与对应漂流轨迹的ERA5再分析降雪率进行了对比。研究发现，2019年10月末至2020年4月末期间，累积积雪质量为38 mm SWE。至调查期结束时，一年冰区的初始SWE相对于二年冰区的比例从50%提升至90%。此外，本研究发现，安装在“极星号”（Polarstern）科考船顶层甲板栏杆上的维萨拉当前天气检测仪22（Vaisala Present Weather Detector 22，一款光学降水传感器）受吹雪影响最小，且与样带沿线的SWE反演结果吻合度良好。与之相反，OTT Pluvio2雨量计与OTT Parsivel2激光雨滴谱仪则受大风与吹雪影响显著，导致测得的降水率偏高。在对比分析中剔除吹雪时段后，这类偏差可大幅降低。ERA5再分析资料能够较好地捕捉降雪事件的发生时间，与地面观测结果吻合度较好，但存在一定的高估倾向。船载Ka波段ARM天顶雷达反演得到的降雪量与积雪覆盖区SWE吻合度良好，其偏差幅度与ERA5再分析资料相当。基于上述结果，本研究提出将Ka波段雷达反演降雪量作为累积降雪量范围的上限，将“极星号”科考船搭载的当前天气检测仪观测结果作为该范围的下限。针对2019年10月31日至2020年4月26日的时段，基于上述研究结果，本研究估算累积降雪量为72~107 mm，积雪因侵蚀与升华造成的降水质量损失占比为47%~68%。将该时段延伸至积雪覆盖观测数据的可用范围之外时，本研究估算累积降雪量为98~114 mm。