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M. Déqué and J. F. Royer

Abstract

The global T42 version of the French numerical weather prediction model has been used to produce monthly mean forecasts. A study based on 21 cases of 44-day forecasts (for winter months from 1983 to 1990) is presented. Nine forecasts in this database may be directly compared with ECMWF 30-day forecasts. Some skill of 15-day running means exist for both models beyond day 15, and it is better with the ECMWF model. Beyond day 30, the predictive skill does not completely vanish: after systematic error correction, the 50-kPa height anomaly correlation over the Northern Hemisphere is 0.27 for day 15–44 avenges; 3 out of 21 values are negative, and 4 values exceed 0.50. The amplitude of the forecast anomaly explains a small part of this case-to-case skill variability. Similar results are found for the other atmospheric field. However, such a marginal skill could be useful only in association with other predictors in a statistical postprocessing.

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B. Timbal, J-F. Mahfouf, J-F. Royer, U. Cubasch, and J. M. Murphy

Abstract

The production of climate simulations using global coupled ocean–atmosphere models at high resolution is currently limited by computational expense and the long periods of integration that are necessary. A method of increasing the number of experiments that can be performed is the so-called time-slice technique. Using the Arpège-climat atmospheric model three 5-yr integrations of this type were run: a control and two integrations forced with sea surface temperatures derived from coupled model simulations of the transient response to increasing carbon dioxide. These coupled models are the ECHAM1 model of the Max-Planck Institute (Hamburg, Germany) and the U.K. Meteorological Office model of the Hadley Centre. The sensitivity of the response to the oceanic forcing is studied. The results are compared with the 10-yr mean atmospheric response of the coupled models at the time of the doubling of CO2. Global warmings ranging from 1.3 K to 1.9 K are obtained. Special attention is given to the modifications that occur in the hydrological cycle and their sensitivity to the SSTs. Climatic signals related to oceanic forcing, such as the modification of the ITCZ maximum of precipitation, are separated from signals due to the internal feedbacks and physical parameterizations of the models.

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D. B. Stephenson, K. Rupa Kumar, F. J. Doblas-Reyes, J-F. Royer, F. Chauvin, and S. Pezzulli

Abstract

The Indian summer monsoon rainfall is the net result of an ensemble of synoptic disturbances, many of which are extremely intense. Sporadic systems often bring extreme amounts of rain over only a few days, which can have sizable impacts on the estimated seasonal mean rainfall. The statistics of these outlier events are presented both for observed and model-simulated daily rainfall for the summers of 1986 to 1989. The extreme events cause the wet-day probability distribution of daily rainfall to be far from Gaussian, especially along the coastal regions of eastern and northwestern India. The gamma and Weibull distributions provide good fits to the wet-day rainfall distribution, whereas the lognormal distribution is too skewed. The impact of extreme events on estimates of space and time averages can be reduced by nonlinearly transforming the daily rainfall amounts. The square root transformation is shown to improve the predictability of ensemble forecasts of the mean Indian rainfall for June 1986–89.

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V. Masson-Delmotte, S. Hou, A. Ekaykin, J. Jouzel, A. Aristarain, R. T. Bernardo, D. Bromwich, O. Cattani, M. Delmotte, S. Falourd, M. Frezzotti, H. Gallée, L. Genoni, E. Isaksson, A. Landais, M. M. Helsen, G. Hoffmann, J. Lopez, V. Morgan, H. Motoyama, D. Noone, H. Oerter, J. R. Petit, A. Royer, R. Uemura, G. A. Schmidt, E. Schlosser, J. C. Simões, E. J. Steig, B. Stenni, M. Stievenard, M. R. van den Broeke, R. S. W. van de Wal, W. J. van de Berg, F. Vimeux, and J. W. C. White

Abstract

A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

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