keywords: Antarctic precipitation, regional atmospheric modeling, microphysics, radar data

Precipitation is the major source term of the Antarctic surface mass balance and therefore the main input of water for the ice sheet. While the snow accumulation can be directly retrieved from networks of glaciological stakes the amount of precipitation that actually falls on the Antarctic surface still remains an open question.

Given the sparcity of precipitation measurements over the Antarctic ice sheet, the current  assessment of the Antarctic precipitation relies on reanalysis products or climate model simulations. However, little is known about the ability of models to reliably represent the microphysical processes governing the precipitation. My post-doc project at the Remote -Sensing laboraty (LTE, EPFL, Lausanne, Switzerland) aims to improve the modeling of the precipitation microphysics in atmospheric models by evaluating simulations with remote-sensed and surface measurements. I also aim at gaining insights into the large scale atmospheric circulation patterns that drive the moisture advection and the precipitation towards the Antarctic coast.

keywords: Antarctic margins, Atmospheric modeling, low-level sublimation, radiosoundings, regional and synoptic flow interactions, katabatic winds and jumps

The margins of East Antarctica are a region of great interest in meteorology particularly owing to the fierce katabatic winds that fascinated and severely tested the pioneering scientific expeditions in the far south. From a meteorological and climate perspective, the low-level atmospheric dynamics over the coastal margins of Antarctica plays a key role for the energy , moisture and mass budgets of the atmosphere over the ice sheet. Antarctic margins are also critical places for atmosphere/sea-ice/ocean interactions, in particular where polynias form. Moreover the dry katabatic winds that flows from the Antarctic Plateau sublimate an important part of the precipitation before it reaches the ground surface. One of my post-doc objective is to decipher the complex dynamics and the interactions between scales in this critical region of the Earth climate using in situ observations, atmospheric modeling and reanalyses.

keywords: boundary-layer physics Antarctic climate, Atmospheric modeling, in situ observations

Observation of the Atmospheric Boundary Layers (ABL) above the Antarctic Plateau has revealed the strongest near-surface temperature stratifications on the Earth. A correct parametrization of the very stratified Antarctic ABLs in General Circulation Models (GCM) is critical since they exert a strong control on the continental scale temperature inversion, on the coastal katabatic winds and subsequently on the Southern Hemisphere circulation. The previous Gewex Atmospheric Boundary Layer Studies (GABLS) highlighted that the parametrization of the very stratified, or very stable, ABLs is one of the most critical challenge in the atmospheric modelers community. Indeed, the turbulence and the nature of the other mixing processes are still poorly understood and
the common model’s parametrization generally fail in very stable conditions.

During my PhD, I evaluated and improved the modelling of the ABL over the Antarctic Plateau by the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) GCM, the atmospheric component of the IPSL Earth System Model in preparation for the sixth Coupled Models Intercomparison Project. Before the model evaluation itself, an in-depth study of the dynamics of the atmospheric surface layer and of the stable ABL over the Antarctic Plateau was carried out from in situ measurements at Dome C (performed in the observational program CALVA). The analysis enabled the first estimations of the roughness length and of the surface fluxes during the polar night at this location, the characterization of a summer nocturnal jet as well as the observation of very frequent near-surface moisture supersaturations with respect to ice. Investigation of meteorological measurements along a 45 m tower also revealed two distinct dynamical regimes of the stable ABL at this location. In particular, the relation between the near surface inversion amplitude and the wind speed takes a typical ’reversed S-shape’, suggesting a system obeying with an hysteresis. An analysis with a conceptual model showed that this is a clear illustration of a general and robust feature of the stable ABL systems, corresponding to a ‘critical transition’ between a steady turbulent and a steady ‘radiative’ regime.

2-regime behavior of the near-surface inversion with the wind speed in clear-sky wintertime conditions at Dome C, East Antarctic Plateau (Vignon et al 2017)

LMDZ was then run on 1D simulations during a typical clear-sky summertime diurnal cycle in the framework of the fourth GABLS case. Sensitivity tests to surface parameters, vertical grid and turbulent mixing parametrizations were performed leading to significant improvements of the model and to a new configuration better adapted for Antarctic conditions. 3D simulations were then carried out with the ’zooming capability’ of the horizontal grid and with nudging. These simulations enabled a further evaluation of the model over a full year and extending the analysis beyond Dome C. In particular, the latter analysis sheded light on the importance of the radiative scheme and of the surface layer scheme for the modelling of the ABL during the polar night over the Plateau. Finally, the PhD work extented towards the modelling of the stable ABL over the other continents, assessing how the frequently underestimated subgrid mixing of momentum and heat in stable conditions can be compensated by a transfer of large scale kinetic energy towards turbulent kinetic energy when the flow is slowed down by the drag of the orographic gravity waves.

keywords: stratopause, middle atmosphere, sudden stratospheric warming

While the increasing concentration of greenhouse gases acts to warm the low atmospheric layers, it also acts to cool the stratosphere i.e the layer of the  atmosphere comprised between about 10 and 50 km height. Recent studies show that the stratospheric cooling trend of 1-2K/decade increases with altitude with regional cooling rates reaching 3K/decade near the cap limit of the stratosphere: the stratopause. The stratopause is characterized by a temperature maximum which separates the stratosphere from the overcapping mesosphere. While in the summer hemisphere, the temperature maximum is generated by ozone-photolysis heating, over the winter pole, it is dynamically maintained by the breaking of upward propagating atmospheric gravity waves.

The northern winter stratosphere is often affected by extreme events called sudden stratospheric warmings. These events are characterized by a sharp warming of the polar stratosphere (reaching 20K in 3 day), a strong weakening of the stratospheric polar vortex and a change of its shape. Sudden stratospheric warmings have been classified into two types: the displacement events, when the polar vortex is displaced off the pole, and the splitting events, when the vortex is divided into two smaller ones.

During such extreme events, the polar stratopause drops abruptly. Sometimes it reappears at much higher altitude (~80km) prior to return to its climatological height. These events called ’elevated stratopause’ events basically result from a downward flow in the middle-high mesosphere which affects the downward transport of chemical species. Particularly, they induce intrusions of mesospheric NOx into the stratopshere and lead to an enhancement of the catalytic destruction of ozone.

To better understand the 'elevated stratopause event', it  is therefore crucial to  find out the mechanisms which drive the middle atmosphere dynamics near the stratopause during sudden stratospheric warmings.

My first research work focused on the characterization of the stratopause behaviour during sudden stratopsheric warmings, looking at both satellite and reanalysis data.