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J. Bader, W. A. Stahel, and W. Schmid

Abstract

The data obtained in Grossversuch IV about hail prevention triggered the hypothesis that only the first rocket launched into a potential hail cell decreases hail kinetic energy in an effect-time interval around 10 min after launching time. Several variations of a randomization test were applied to substantiate this hypothesis. All showed a tendency to confirm the hypothesis, the most significant result being a p value of 0.2%. However, since the test was applied to the data that had been explored to generate the hypothesis, the significant result should be interpreted as a strong hint rather than as a statistical proof.

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David C. Bader, Thomas B. McKee, and Gregory J. Tripoli

Abstract

The continuous development of a meso-β-scale boundary layer over sloping terrain upwind of a high mountain barrier was simulated through a complete diurnal cycle using a nonhydrostatic boundary-layer model. The simulation detailed the evolution of a 500–800-m deep nocturnal boundary layer containing 1–3 m s−1 thermal circulations in the region upwind of a high ridge. Shear between the 5 m s−1 gradient level winds above the boundary layer and the mesoscale thermal circulations maintained the turbulent mixing of cold air upward against the stable stratification. The nocturnal boundary layer is replaced the following morning by a growing convective boundary layer containing 3–5 m s−1 warm thermal flows under its base. A multiple layer structure appears during the morning transition with the coexistence of the synoptic, nocturnal and developing daytime wind systems. As the morning progresses, the downwind edge of the stable layer slowly retreats back toward lower elevations while the convective layer grows under its base. By 5 h after sunrise, the morning transition is complete. Comparisons of the model simulation with field data show that the model accurately simulates the diurnal development of the mesoscale boundary layer.

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G. Bala, R. B. Rood, A. Mirin, J. McClean, Krishna Achutarao, D. Bader, P. Gleckler, R. Neale, and P. Rasch

Abstract

A simulation of the present-day climate by the Community Climate System Model version 3 (CCSM3) that uses a Finite Volume (FV) numerical method for solving the equations governing the atmospheric dynamics is presented. The simulation is compared to observations and to the well-documented simulation by the standard CCSM3, which uses the Eulerian spectral method for the atmospheric dynamics. The atmospheric component in the simulation herein uses a 1° latitude × 1.25° longitude grid, which is a slightly finer resolution than the T85-grid used in the spectral transform. As in the T85 simulation, the ocean and ice models use a nominal 1-degree grid. Although the physical parameterizations are the same and the resolution is comparable to the standard model, substantial testing and slight retuning were required to obtain an acceptable control simulation. There are significant improvements in the simulation of the surface wind stress and sea surface temperature. Improvements are also seen in the simulations of the total variance in the tropical Pacific, the spatial pattern of ice thickness distribution in the Arctic, and the vertically integrated ocean circulation in the Antarctic Circumpolar Current. The results herein demonstrate that the FV version of the CCSM coupled model is a state-of-the-art climate model whose simulation capabilities are in the class of those used for Intergovernmental Panel on Climate Change (IPCC) assessments. The simulated climate is very similar to that of the T85 version in terms of its biases, and more like the T85 model than the other IPCC models.

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B. Federer, A. Waldvogel, W. Schmid, H. H. Schiesser, F. Hampel, Marianne Schweingruber, W. Stahel, J. Bader, J. F. Mezeix, Nadie Doras, G. D'Aubigny, G. DerMegreditchian, and D. Vento

Abstract

The main results of a randomized hail suppression experiment, Grossversuch IV, are presented in this paper. Grossversuch IV tested the “Soviet” hail prevention method during five years (1977–81). The field experiment took place in central Switzerland with the participation of research groups from France, Italy and Switzerland.

A very dense hailpad network (330 hailpads over 1300 km2) and a carefully calibrated 10-cm radar were used to measure in two independent ways the hail kinetic energy of seeded and unseeded hail cells. The total sample included 216 cells. The main result of the confirmatory as well as most of the exploratory analyses is that there is no statistically significant difference between seeded and unseeded hail cells. A detailed discussion of the reliability of the measurements, tests and methods is given together with a discussion about possibilities of future evaluations of the Grossversuch IV data and other cloud seeding experiments.

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James W. Hurrell, M. M. Holland, P. R. Gent, S. Ghan, Jennifer E. Kay, P. J. Kushner, J.-F. Lamarque, W. G. Large, D. Lawrence, K. Lindsay, W. H. Lipscomb, M. C. Long, N. Mahowald, D. R. Marsh, R. B. Neale, P. Rasch, S. Vavrus, M. Vertenstein, D. Bader, W. D. Collins, J. J. Hack, J. Kiehl, and S. Marshall

The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad community of scientists. Additionally, the model source code and associated documentation are freely available to the scientific community to use for Earth system studies, making it a true community tool. This article describes this Earth system model and its various possible configurations, and highlights a number of its scientific capabilities.

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Belen Rodríguez-Fonseca, Elsa Mohino, Carlos R. Mechoso, Cyril Caminade, Michela Biasutti, Marco Gaetani, J. Garcia-Serrano, Edward K. Vizy, Kerry Cook, Yongkang Xue, Irene Polo, Teresa Losada, Leonard Druyan, Bernard Fontaine, Juergen Bader, Francisco J. Doblas-Reyes, Lisa Goddard, Serge Janicot, Alberto Arribas, William Lau, Andrew Colman, M. Vellinga, David P. Rowell, Fred Kucharski, and Aurore Voldoire

Abstract

The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decadal forecasting. The agreement among future projections has improved from CMIP3 to CMIP5, with a general tendency for slightly wetter conditions over the central part of the Sahel, drier conditions over the western part, and a delay in the monsoon onset. The role of the Indian Ocean, the stationarity of teleconnections, the determination of the leader ocean basin in driving decadal variability, the anthropogenic role, the reduction of the model rainfall spread, and the improvement of some model components are among the most important remaining questions that continue to be the focus of current international projects.

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