[1] Six levels of meteorological sensors have been deployed along a 45 m tower at the French-Italian Concordia station, Dome C, Antarctic. We present measurements of vertical profiles, the diurnal cycle, and interdiurnal variability of temperature, humidity, and wind speed and direction for 3 weeks during the southern summer of 2008. These measurements are compared to 6-hourly European Center for Medium-Range Forecasts (ECMWF) analyses and daily radiosoundings. The ECMWF analyses show a 3-4°C warm bias relative to the tower observations. They reproduce the diurnal cycle of temperature with slightly weaker amplitude and weaker vertical gradients. The amplitude of the diurnal cycle of relative humidity is overestimated by ECMWF because the amplitude of the absolute humidity diurnal cycle is too small. The nighttime surface-based wind shear and Ekman spiral is also not reproduced in the ECMWF analyses. Radiosonde temperatures are biased low relative to the tower observations in the lowest 30 m but approach agreement at the top of the tower. Prior to bias correction for age-related contamination, radiosonde relative humidities are biased low relative to the tower observations in the lowest 10 m but agree with tower observations above this height. After correction for the age-related bias, the radiosonde relative humidity agrees with tower observations below 10 m but is biased high above this height. Tower temperature observations may also be biased by solar heating, despite radiation shielding and natural ventilation.

International audienceThe Concordiasi project is making innovative observations of the atmosphere above Antarctica. The most important goals of the Concordiasi are as follows: 1. 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 understanding of the Earth system by examining the interactions between Antarctica and lower latitudes. 2. 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. Techniques for assimilating these Lagrangian observations are being developed. A major Concordiasi component is a field experiment during the austral springs of 2008-10. The field activities in 2010 are based on a constellation of up to 18 long-duration stratospheric super-pressure balloons (SPBs) deployed from the McMurdo station. Six of these balloons will carry GPS receivers and in situ instruments measuring temperature, pressure, ozone, and particles. Twelve of the balloons will release drop-sondes on demand for measuring atmospheric parameters. Lastly, radiosounding measurements are collected at various sites, including the Concordia station

[1] A Rayleigh fractionation model is developed to investigate postdepositional modification of stable isotopes of water in the near-surface snow of East Antarctica. The processes of forced ventilation, pore-space diffusion, and intra-ice-grain diffusion are parameterized through characteristic time constants. Routine meteorological observations, their derived products, and general glaciological conditions from the South Pole are used to simulate the d 18 O of near-surface snow as it evolves in time. The sensitivity of d 18 O of near-surface snow to wind speed, surface temperature, and accumulation rate is tested through model experiments. A steady wind of 5-10 m s À1 results in annual mean d 18 O enrichment of 3-7%. Enrichment of the heavy isotope occurs predominantly in buried winter snow layers during summer. However, in many simulations, there is a slight depletion of d 18 O summer layers that occurs as atmospheric water vapor is deposited within the snow during winter. Postdepositional modification of water stable isotopes in near-surface snow also depends on the snow accumulation rate; high accumulation rates quickly advect snow layers away from the influence of the atmosphere, preventing significant modification. Low accumulation rates (e.g., at Vostok and Dome C) allow significant postdepositional modification because the near-surface snow is exposed to forced ventilation for several annual cycles. Postdepositional modification during the Last Glacial Maximum (LGM) at Summit, Greenland is estimated to be greater than the present-day postdepositional modification. Therefore the temperature difference for LGM may be larger than that previously inferred from the d 18 O record. However, because of the compensating effects of lower temperatures and smaller accumulation during the LGM in Antarctica, the postdepositional enrichment in East Antarctica may be approximately the same for LGM as for the modern climate.Citation: Town, M. S., S. G. Warren, V. P. Walden, and E. D. Waddington (2008), Effect of atmospheric water vapor on modification of stable isotopes in near-surface snow on ice sheets,

Estimates of cloud cover over the South Pole are presented from five different data sources: routine visual observations ; C vis ), surface-based spectral infrared (IR) data (2001; C PAERI ), surface-based broadband IR data (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003); C pyr ), the Extended Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder (APP-x) dataset (1994-99; C APP-x ), and the International Satellite Cloud Climatology Project (ISCCP) dataset (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003); C ISCCP ). The seasonal cycle of cloud cover is found to range from 45%-50% during the short summer to a relatively constant 55%-65% during the winter. Relationships between C pyr and 2-m temperature, 10-m wind speed and direction, and longwave radiation are investigated. It is shown that clouds warm the surface in all seasons, 0.5-1 K during summer and 3-4 K during winter. The annual longwave cloud radiative forcing is 18 W m Ϫ2 for downwelling radiation and 10 W m Ϫ2 for net radiation. The cloud cover datasets are intercompared during the time periods in which they overlap. The nighttime bias of C vis is worse than previously suspected, by approximately Ϫ20%; C ISCCP shows some skill during the polar day, while C APP-x shows some skill at night. The polar cloud masks for the satellite data reviewed here are not yet accurate enough to reliably derive surface or cloud properties over the East Antarctic Plateau. The best surface-based source of cloud cover in terms of the combination of accuracy and length of record is determined to be C pyr . The use of the C pyr dataset for further tests of satellite retrievals and for tests of polar models is recommended.

Annual cycles of downwelling broadband infrared radiative flux and spectral downwelling infrared flux were determined using data collected at the South Pole during 2001. Clear-sky conditions are identified by comparing radiance ratios of observed and simulated spectra. Clear-sky fluxes are in the range of 110-125 W m Ϫ2 during summer (December-January) and 60-80 W m Ϫ2 during winter (April-September). The variability is due to day-to-day variations in temperature, strength of the surface-based temperature inversion, atmospheric humidity, and the presence of "diamond dust" (near-surface ice crystals). The persistent presence of diamond dust under clear skies during the winter is evident in monthly averages of clear-sky radiance.About two-thirds of the clear-sky flux is due to water vapor, and one-third is due to CO 2 , both in summer and winter. The seasonal constancy of this approximately 2:1 ratio is investigated through radiative transfer modeling. Precipitable water vapor (PWV) amounts were calculated to investigate the H 2 O/CO 2 flux ratio. Monthly mean PWV during 2001 varied from 1.6 mm during summer to 0.4 mm during winter. Earlier published estimates of PWV at the South Pole are similar for winter, but are 50% lower for summer. Possible reasons for low earlier estimates of summertime PWV are that they are based either on inaccurate hygristor technology or on an invalid assumption that the humidity was limited by saturation with respect to ice.The average fractional cloud cover derived from the spectral infrared data is consistent with visual observations in summer. However, the wintertime average is 0.3-0.5 greater than that obtained from visual observations. The annual mean of longwave downwelling cloud radiative forcing (LDCRF) for 2001 is about 23 W m Ϫ2 with no apparent seasonal cycle. This is about half that of the global mean LDCRF; the low value is attributed to the small optical depths and low temperatures of Antarctic clouds.