Christophe Genthon

Ground-based measurements of spatial and temporal variability of snow accumulation in East Antarctica

The East Antarctic Ice Sheet is the largest, highest, coldest, driest, and windiest ice sheet on Earth. Understanding of the surface mass balance (SMB) of Antarctica is necessary to determine the present state of the ice sheet, to make predictions of its potential contribution to sea level rise, and to determine its past history for paleoclimatic reconstructions. However, SMB values are poorly known because of logistic constraints in extreme polar environments, and they represent one of the biggest challenges of Antarctic science. Snow accumulation is the most important parameter for the SMB of ice sheets. SMB varies on a number of scales, from small-scale features (sastrugi) to ice-sheet-scale SMB patterns determined mainly by temperature, elevation, distance from the coast, and wind-driven processes. In situ measurements of SMB are performed at single points by stakes, ultrasonic sounders, snow pits, and firn and ice cores and laterally by continuous measurements using ground-penetrating radar. SMB for large regions can only be achieved practically by using remote sensing and/or numerical climate modeling. However, these techniques rely on ground truthing to improve the resolution and accuracy. The separation of spatial and temporal variations of SMB in transient regimes is necessary for accurate interpretation of ice core records. In this review we provide an overview of the various measurement techniques, related difficulties, and limitations of data interpretation; describe spatial characteristics of East Antarctic SMB and issues related to the spatial and temporal representativity of measurements; and provide recommendations on how to perform in situ measurements.

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short‐lived tracers

Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

GCM analysis of local influences on ice core δ signals

A high resolution GCM is used to examine the effect of changes in local surface climate parameters on the ice sheets that can influence the interpretation of the isotopic signal of the ice from deep cores. The model suggests that the 10°C difference between the LGM surface temperature deduced from borehole thermometry and that deduced from the water isotope analysis to a great extent may be due to a modification of the precipitation seasonality in central Greenland. For central East Antarctica, the model tends to suggest a weak opposite bias.

Atmospheric dust under glacial and interglacial conditions

The uplift, transport and deposition of dust have been implemented in the LMDz AGCM. Simulations of atmospheric dust transport have been performed under present day (PD) and Last Glacial Maximum (LGM) conditions. For the PD climate the large-scale atmospheric dust transport as inferred from atmospheric measurements is reproduced. Increased dust amounts are simulated almost everywhere for the LGM climate, and the concentration of dust in Antarctic ice is increased as inferred from ice cores. The model simulates substantially increased dust concentrations in Greenland ice, however they are still lower than values reported from ice cores.

Variability and Trends of the Summer Melt Period of Antarctic Ice Margins since 1980 from Microwave Sensors

Abstract The density and range of observations made by meteorological stations is insufficient to fully characterize decadal climate variability in Antarctica. Satellite-borne instruments, which offer a high spatial and temporal density of information, can contribute complementary data for characterizing Antarctic climate change. Here, partial melting of Antarctic snow, which significantly affects the microwave emissivity of the surface, is identified and counted over 18 yr in the 20-yr period 1980–99. The cumulated product of the surface area affected by melting and the duration of the melting event, called cumulative melting surface (CMS), is one of the three melt indices defined and discussed here. On average over the last 20 yr, the Antarctic CMS has decreased by 1.8% ± 1% yr−1, a result that is consistent with a mean January cooling of the continent recently identified from infrared satellite data. In addition, the interannual signatures of the Antarctic Oscillation, and possibly of the Southern Osci...

Studies of the Antarctic climate with a stretched-grid general circulation model

A stretched-grid general circulation model (GCM), derived from the Laboratoire de Meteorologie Dynamique (LMD) GCM is used for a multiyear high-resolution simulation of the Antarctic climate. The resolution in the Antarctic region reaches 100 km. In order to correctly represent the polar climate, it is necessary to implement several modifications in the model physics. These modifications mostly concern the parameterizations of the atmospheric boundary layer. The simulated Antarctic climate is significantly better in the stretched-grid simulation than in the regular-grid control run. The katabatic wind regime is well captured, although the winds may be somewhat too weak. The annual snow accumulation is generally close to the observed values, although local discrepancies between the simulated annual accumulation and observations remain. The simulated continental mean annual accumulation is 16.2 cm y -1 . Features like the surface temperature and the temperature inversion over large parts of the continent are correctly represented. The model correctly simulates the atmospheric dynamics of the rest of the globe.

High‐frequency paleovariability in climate and CO2 levels from Vostok Ice Core Records

The high resolution of the Vostok records provides a unique look at the causes of paleoclimatic variability during the last complete glacial cycle. The records present strong evidence for the interaction between orbital forcing and internal, physico-chemical mechanisms of variability. This interaction appears to account for the great wealth of spectral features found in the records.

Tropospheric transport of continental tracers towards Antarctica under varying climatic conditions

We present a method to analyse tracer transit time climatologies based on the concept of tracer age.The method consists of introducing idealized, short-lived radioactively decaying tracers in a generalcirculation model of the atmosphere. Tracer age since emission is calculated at any given place in theatmosphere from the ratio of the concentrations of tracers with different lifetimes emitted over thesame source area. An obvious use of this method is the analysis of transport of real tracers with similarlifetimes (such as dust particles) during different climatic periods. Here, this method is applied totransport from southern hemisphere continental source areas towards Antarctica at the present, the lastglacial maximum (21 kyr BP) and the last glacial inception (115 kyr BP). It is found that the variationover time of atmospheric transport efficiency towards Antarctica depends on the tracer source region:changes for Patagonian tracers differ from those for tracers originating over Australia and southernAfrica. Transport towards Antarctica during the last glacial maximum (LGM) is faster for Patagonian, but not for Australian and Southern African tracers. It is shown that for the time of the last glacialinception, tracer transit time towards Antarctica is not significantly different from the present, althoughsigns of a more vigorous atmospheric circulation can be seen in the simulation.

The Concordiasi Project in Antarctica

The Concordiasi project is making innovative observations of the atmosphere above Antarctica. The most important goals of the Concordiasi are as follows: To enhance the accuracy of weather prediction and climate records in Antarctica through the assimilation of in situ and satellite data, with an emphasis on data provided by hyperspectral infrared sounders. The focus is on clouds, precipitation, and the mass budget of the ice sheets. The improvements in dynamical model analyses and forecasts will be used in chemical-transport models that describe the links between the polar vortex dynamics and ozone depletion, and to advance the under understanding of the Earth system by examining the interactions between Antarctica and lower latitudes. To improve our understanding of microphysical and dynamical processes controlling the polar ozone, by providing the first quasi-Lagrangian observations of stratospheric ozone and particles, in addition to an improved characterization of the 3D polar vortex dynamics. Techni...

Radon 222 as a comparative tracer of transport and mixing in two general circulation models of the atmosphere

Radon 222 (222Rn) is used as a tracer to probe and intercompare transport, turbulent mixing, and convective mixing in the Laboratoire de Meteorologie Dynamique (LMD) and Goddard Institute for Space Studies (GISS) atmospheric general circulation models (GCMs). Formulations for tracer transport and mixing and their control on the global distribution and time variability of 222Rn, as well as parameterizations for the continental surface source flux, are directly implemented into the two GCMs and run “in-line.” Tracer formulations are largely inspired by climate variables (heat, moisture, momentum) formulations in the base GCMs. The comparison of model-calculated 222Rn with observations of time (diurnal, seasonal, sporadic), variability, and spatial (horizontal, vertical) distribution shows partial agreement only. Uncertainties of the sources of 222Rn, in particular of the dependence of 222Rn emanation on soil freezing, are substantial, and the significance and reliability of some of the available observations are low. Model intercomparison is not subject to observation limitations, and it clearly indicates that the boundary layer is more homogeneously mixed in the LMD model, whereas deep convection is more efficient at carrying surface-produced quantities to high tropospheric levels in the GISS model. Resolution also makes a large difference. The LMD model has a finer horizontal grid over most of the globe and is almost systematically better than the GISS model at reproducing sharp fluctuations of 222Rn and seasonal cycles. Our results support that 222Rn could provide an unequivocal absolute measure of the GCMs performances if a more comprehensive observational validation was available.