ALBERT

Description: Better understanding snowfall microphysics is a key challenge in atmospheric science, crucial for snowfall quantification, remote sensing, and weather forecasting in general. Using meteorological radars, we propose a novel approach to retrieve the microphysical properties of snowfall from dual-frequency Doppler spectral observations, while relaxing assumptions on beam matching and non-turbulent atmosphere. The approach relies on a two-step deep-learning framework inspired from data compression techniques: an encoder maps a high-dimensional signal to a lower-dimensional latent space, while the decoder reconstructs the original signal from this latent space. Here, dual-frequency Doppler spectrograms constitute the high-dimensional input, while the dimensions of the latent space are constrained to represent the snowfall properties of interest. The decoder neural network is first trained to emulate Doppler spectra from a set of microphysical variables, using simulations from the radiative transfer model PAMTRA as training data. In a second step, the encoder network learns the inverse mapping, from measured dual-frequency spectrograms to the microphysical latent space. The method was applied to X- and W-band data from the ICE GENESIS campaign that took place in Switzerland in January 2021. The approach was thoroughly evaluated by comparisons with collocated aircraft in situ measurements collected during three precipitation events, with an overall good agreement. The main contribution of this work is (i) the inversion framework itself, which can be applied to other remote-sensing retrieval applications ; and (ii) the seven retrieved microphysical descriptors providing relevant insights into snowfall processes.

Description: Although crucial for the ice cap surface mass balance, precipitation remains poorly characterized over Antarctica, even less so in complex terrain influencing the local circulation. In this study, one summer month of measurements collected by a transect of three vertically-pointing radars (MRR-PRO) has been used to investigate the spatial variability of precipitation in complex terrain in the vicinity of the Belgian base Princess Elisabeth Antarctica. The occurrence of surface precipitation and virga at the three MRR-PRO locations has been studied, revealing that virgae represent a lower fraction of the total precipitation at higher altitudes. The analysis of the reflectivity profiles suggests that sublimation is predominant below 3.5 km above mean sea level, regardless of the altitude of the considered site. High-resolution simulations from the WRF model (forced by ERA5 data) show lower values of relative humidity with respect to ice in the same section of the profile. The depth of this sub-saturated layer depends on the terrain height, resulting in the enhancement of sublimation in the lowest locations of the transect. The ERA5 reanalysis and WRF simulations also suggest that orographic lifting takes place during precipitation events, increasing the total snowfall above the highest mountain peaks. Finally, the analysis of the succession of virgae and surface precipitation above the three sites indicates that surface precipitation is predominantly associated with a low-pressure system located directly south of PEA, over the Southern Ocean. Low-pressure system locations further to the West or East result in virgae that, respectively, precede and follow surface precipitation.

Description: As the near-surface atmosphere over the Antarctic Plateau is cold and pristine, its physico-chemical conditions resemble to a certain extent those of the high troposphere where cirrus clouds form. In this study, we carry out an observational analysis of two shallow fog clouds forming in situ at cirrus temperatures – that is, temperatures lower than 235 K – at Dome C, Antarctic Plateau. The combination of lidar profiles with temperature and humidity measurements from advanced thermo-hygrometers along a 45 m mast makes it possible to characterise the formation and development of the fog. High supersaturations with respect to ice are observed before the initiation of fog, and the values attained suggest that the nucleation process at play is the homogeneous freezing of solution aerosol droplets. Once nucleation occurs, the relative humidity decreases down to subsaturated values with respect to ice in a few hours, owing to vapour deposition onto ice crystals and turbulent mixing. The development of fog is tightly coupled with the dynamics of the boundary layer which, in the first study case, experiences a summertime weak diurnal cycle, while in the second case, it transits from a very stable to a weakly stable dynamical regime during the polar night. Overall, this work highlights the potential of the site of Dome C for observational studies of very cold cloud microphysical processes in natural conditions and with in situ ground-based instruments.

Description: Antarctic precipitation remains poorly characterised and understood, especially within the boundary layer. This is due in part to a still-limited amount of surface-based remote sensing observations. A suite of cloud and precipitation remote-sensing instruments including a W-band cloud radar and a K-band Micro Rain Radar (MRR) were used to characterise snowfall over Davis (69S, 78E). Surface snowfall events occurred when boundary layer wind speeds were weaker, temperatures were warmer, and relative humidity over ice higher, than when virga were present. The presence of virga is associated with Fohn conditions due to the location of Davis in the lee of an ice ridgeline. Dual wavelength ratio values from the summer indicate particle aggregation at temperatures of -14C to -10C, consistent with observations made elsewhere, including in the Arctic. In-cloud updrafts were stronger in summer than in winter at these temperatures. Larger downward velocities and the presence of super-cooled liquid layers suggest some rimed particles at warm temperatures above -10C during summer. Sublimation of snowfall mass aloft was 50\% between the accumulation peak at 1.2~km and 205~m altitude, which occurs within CloudSat's `blind zone'. An estimated lower bound of blowing snow fraction is 30%. Given the common prevailing winds and numerous ice ridgelines along much of the East Antarctic coastline, these Davis results can be used as a basis to further understand snowfall across the region.

Description: In this era of global warming, the climate of the Antarctic continent is extensively studied with climate models, as even a small loss of its polar cap may have a big impact on global sea level rise. However, the meteorological observations needed to evaluate and calibrate models remain scarce, especially in the atmospheric column, in the winter season and away from any inhabited station.For this purpose, the AWACA (Atmospheric WAter Cycle over Antarctica) project aims to deploy 3 autonomous OPUs (automated Observation Platform Units) sheltering radars and lidars working at various wavelengths along a transect from coastal Terre Adélie to Concordia station, 1100km from the coast to 3200m on the Plateau. These instruments will perform condensed water measurements throughout the lower tropospheric column to characterize clouds and precipitation. Water vapor and snowflakes isotopes measurements will also be made by a spectrometer. The OPUs will operate during 3 years, being visited only once a year. They will be isolated from any station and therefore self-powered.A description of the OPUs’ design, and the challenges associated with their stand-alone deployment in a harsh and remote environment and the various measurements they will perform will be presented.