Régions polaires | EPFL Graph Search

thumb|300px|Localisation en vert des régions dites polaires Les régions polaires de la Terre sont les zones du globe entourant les pôles et au-delà des deux cercles polaires que sont le cercle Arctique dans l'hémisphère nord et le cercle Antarctique dans l'hémisphère sud. Ces régions étant centrées sur le pôle Nord et le pôle Sud, elles sont recouvertes par les calottes glaciaires, situées respectivement sur l'océan Arctique et le continent Antarctique. Climat Climat polaire En général dans cette zone, la température ne dépasse pas les 10 degrés Celsius en été, c'est-à-dire que la zone polaire se trouve incluse dans la zone sous la barre isotherme des durant l'été. En hiver, la température peut atteindre des records de froid, en particulier en Antarctique. Depuis 1983, cette région détient le record mondial de la température naturelle la plus basse enregistrée. En effet, le 21 juillet 1983, la base scientifique de Vostok enregistre une température de -89,2°C. Plus récem

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

Wind-packing of snow: How do wind crusts form?

Christian Gabriel Sommer

Wind-packing of snow is the process responsible for the formation of wind slabs and wind crusts. These are hard layers of well-sintered snow often found in the snowpack in mountains as well as in polar regions. Wind-packing affects the local mass balance, the avalanche danger and how the snow cover interacts with the atmosphere. Yet, it is a poorly understood process. Many ideas about what wind-packing actually is have been proposed in the literature but there is almost no quantitative information about the involved physical processes available. A closed-circuit wind tunnel simulating an infinite fetch was specifically designed and built for this purpose. Experiments were performed with natural snow that was collected on a pair of wooden trays. The hardness of the snow and how it changed during experiments was measured with a SnowMicroPen. Meteorological parameters such as wind speed, air temperature, etc. were also measured. In the literature, there is conflicting information about the necessity of drifting snow for wind-packing. The wind tunnel experiments showed that drifting snow is a necessary but not sufficient condition for wind-packing. We observed no hardening during experiments without drifting snow, but not all drifting snow events lead to the formation of a wind crust. The spatial variability of the hardness of wind-influenced snow was high and this appeared to be related to the dynamics of erosion and deposition. To analyze the influence of these processes in more detail and quantitatively, a Microsoft Kinect sensor was added to the wind tunnel. This instrument measures the snow depth in the main test section with a high spatial and temporal resolution. These measurements showed that wind-packing only occurs through the deposition of snow. If drifting snow causes the snow to be eroded, there is no hardening effect on the remaining snow. Furthermore, the Kinect data allowed to explain about 50% of the observed variability of the hardness of wind-packed snow with only two parameters. Most important was the wind exposure. The snow became harder in more wind-exposed positions compared to wind-sheltered areas. We used the parameter Sx to characterize how wind-exposed a certain position is based on the upstream snow surface. A wind-sheltered position has a high Sx value and a wind-exposed position has a negative Sx value. To increase the range of Sx values, some experiments were performed with an artificial obstacle in the main test section, which created highly wind-sheltered positions. Sx alone explained almost 40% of the variability. The second parameter was the deposition rate. We found a negative correlation of -0.4 between the deposition rate and the snow hardness. Wind-packing is more efficient at creating a hard layer if the snow is deposited slowly. The effect of other parameters, such as wind speed, air humidity or initial density of the snow on the hardness was studied. However, there was either no robust trend in our data or the addition to the multilinear regression did not significantly increase the explanatory power of the model. This does not necessarily mean that these parameters are not relevant for wind-packing. The wind tunnel experiments provided quantitative information and new insights about wind-packing of snow and under what conditions it occurs. However, the question about which physical processes are most important could not yet be fully answered.

EPFL2018

A fascinating trip to Antarctica

Armin Sigmund

Researchers from EPFL have been joining the Belgian Antarctic Research Expedition to Princess Elisabeth Antarctica Station every year since 2016. Armin Sigmund, a PhD student in the Laboratory of Cryospheric Sciences, recently participated in the expedition to study snow-atmosphere interactions. After coming back from the extreme and impressive environments of Antarctica, he provided insights into his first trip to the polar regions.

2021