Laboratoire de teledetection environnementale

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Snowfall and snow accumulation during the MOSAiC winter and spring seasons

Michael Lehning, Marc Oggier, Marc Schneebeli Zeugin, David Nicholas Wagner

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 ERAS 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 Pluvio 2 pluviometer and the OTT Parsivel 2 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. ERAS reveals good timing of the snowfall events and good agreement with ground measurements with an overestimation tendency. Retrieved snowfall from the ship-based K-a-band ARM zenith radar shows good agreements with SWE of the snow cover and differences comparable to those of ERAS. Based on the results, we suggest the K-a-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.

COPERNICUS GESELLSCHAFT MBH2022

Radar processing techniques to study summer snowfall in the Sør Rondane Mountains, Antarctica

Alfonso Ferrone

Snowfall is one of the main positive contributors to the surface mass balance of the Antarctic ice sheet. There are several techniques to monitor solid precipitation on the continent. Among them, meteorological radars provide the means to characterize the phenomenon on a local scale, with an unparalleled view of the processes in the lower levels of the atmosphere. However, due to the harsh weather and the logistic difficulties associated with their deployment in Antarctica, their usage is often limited to the immediate surroundings of the few scientific bases scattered on the continent.In this thesis, meteorological radar observations are used to investigate the effect of complex orography on precipitation in the vicinity of the Belgian research base Princess Elisabeth Antarctica (PEA). Two algorithms for the processing and calibration of the radar measurements collected had to be developed and evaluated. One of them has been applied to the data from three K-band Doppler profilers (MRR-PRO), deployed in a transect across the Sør Rondane Mountains, south of PEA. These datasets have been used together with atmospheric model simulations, reanalysis, and complementary observations to examine the role of sublimation and other local phenomena on precipitation at the three sites. The possible connections with the synoptic conditions on a larger scale have also been investigated.The first part of the thesis is devoted to the development of two algorithms. First, we introduce a differential reflectivity calibration method for dual-polarization weather radars with vertical scanning capability. Even though the technique functions with solid and liquid precipitation, it is particularly suited for radars deployed in polar regions with no access to rainfall. The offset derived from each vertical scan undergoes an Ordinary Kriging interpolation, which determines the temporal variability of the calibration. A processing algorithm for the MRR-PRO measurements has also been developed. The method improves the sensitivity of the radar products and the consistency of the Doppler velocity field. At the same time, it addresses some artifacts in the data caused by interference and noise within the instrument.The second part of the thesis revolves around the analysis of the datasets collected at PEA. The MRR-PRO measurements are used to investigate the presence of virga and surface precipitation at the three sites. Moreover, a series of high-resolution simulations reveal similarities in the wind speed and relative humidity profiles in the lower troposphere. The varying depth of a dry layer in the lowest levels causes differences in sublimation across the transect. Additionally, analysis of the wind direction indicates that orographic lifting enhances precipitation near the mountain peaks. Finally, our observations, consistent with previous studies, suggest that virga and surface precipitation represent different stages of the same large-scale precipitation systems.Altogether, this thesis proposes a series of tools for studying snowfall through radar observations. Moreover, the analysis of the interaction between orography and precipitation at PEA covers locations that have not yet been the subject of scientific studies. It demonstrates the existence of a significant degree of variability on relatively small scales, highlighting the effect of complex terrain and its interaction with the local and large-scale circulation on snowfall in the region.

EPFL2022