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Thomas M. Hamill, Jeffrey S. Whitaker, Jeffrey L. Anderson, and Chris Snyder

fact has an RMSE that is only 1.05 times that of the 1000-member ensemble when white noise is removed from the assimilating model. This is consistent with the successful application of EnKFs with 10–100 members, even in large numerical weather prediction models (e.g., Houtekamer et al. 2005 , 2009 ; Whitaker et al. 2008 ). 4. L96 experiments AT09’s EnKF reference was badly degraded by not using covariance localization and/or other methods to stabilize the filter. Much has been learned about the

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Jarvis L. Moyers, William H. Zoller, and Robert A. Duce

large concentrations of ice nuclei are notgenerally observed in polluted air at ground level because the reaction of lead and iodine at the concentrations observed in this work proceeds slowly.1. Introduction There has been considerable interest recently in thepossibility of weather modification by the formation ofice nuclei from air pollution products. Schaefer (1968a,1969) has demonstrated evidence of such ice nuclei atand below inversions near large polluted cities. Schaefer(1966, 1968a, b

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Mark J. Rodwell

even with modification, significant negative PV may be expected to occur over the Arabian Sea. This change to a midlatitude mass source for the inflow is observed periodically during the monsoon season; although it may be more associated with in situ synoptic weather events ( Bannon 1979 ; Cadet and Desbois 1981 ) than to changes in the vertical profile of heating over the tropical Indian Ocean. Figure 2a shows the 887-hPa winds for 11 June 1994 from ECMWF analyses. Here there is a good example

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J. Warner

hailstorms as in ferred from the entraining jet model. J. Atmos. Sci., 27, 1099-1102.Sax, R. I., 1969: The importance of natural glaciation on the modification of tropical maritime cumuli by silver iodide seeding. J. Appl. Meteor., 8, 92-104.Simpson, J., R. H. Simpson, D. A. Andrews and M. A. Eaton,1965: Experimental cumulus dynamics. Rev. Geophys., 3,387-431.Squires, P., and J. S. Turner, 1962: An entraining jet model for cumulonimbus updraughts. Tellus, 14, 422-434.Todd, C. J., W. A

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Alan C. Bemis

ground.Modification of the lapse rate and some of its results.It is obvious that particles falling through a normallapse rate tend to cool the lower layers. In particular,a falling mixture of ice and water is very effective inestablishing an isothermal layer at zero Celsius [SI.The rate at which the isothermal layer extendsdownward is a function of the precipitation rate, andit often reaches the surface in winter, causing thatall-important change of rain to snow [4]. When itdoes not reach the ground

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Clio Michel and Gwendal Rivière

1. Introduction Because of its major role in medium-range weather forecasts, the atmospheric low-frequency extratropical variability has been widely studied in the last decades. One concept of midlatitude low-frequency variability is the weather regime (WR), which corresponds to a recurrent and quasi-stationary state of the large-scale atmospheric circulation persisting over one or several weeks ( Vautard 1990 ). Different methods can be applied to obtain the weather regimes over a given

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Edward N. Lorenz and Kerry A. Emanuel

anticipate that, whatever strategy we may decide upon, the need for modifications, at least of the details, will become apparent as soon as the new observations become a reality. Because of the large number of specific procedures that might be tested, and the considerable number of simulated weather situations to which each must be applied before a definitive choice among them can be made, a full-scale experiment using a reasonably sophisticated model, such as the operational model of a major forecasting

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Simon W. Chang and Harold D. Orville

: Modificationof hurricane rainband clouds and numerical simulation. Presentedat the 3rd Conference on Weather Modification, June 1972, RapidCity, S. D.; Orville, H. D., S. W. Chang and F. J. Kopp, 1972:A numerical experiment on the simulation and modification ofhurricane rainband clouds. Rept. 72-12, Institute of Atmospheric Sciences, South Dakota School of Mines and Technology.16,00014,00012,000I0,0008 000600040002000 0I0070 -60 -50 -4(150200~'50:5004005007O085OI000Fro. 1. Portions of the skew T

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Edwin H. C. Chin

of the aerosol spectrometer. Y. Air Pollution Control Assoc., 10, 378-383.Langer, G., 1965: An acoustic particle counter: Preliminary results. J. Colloid Sci., 20, 602-609. , 1970: Modification of the membrane technique to efficiently detect and study silver iodide ice nuclei. Preprints of Papers, Second Natl. Conference Weather Modification, Santa Barbara, Calif., Amer. Meteor. Soc., 352-356.Middleton, J. R., and A. H. Auer, Jr., 1970: Ice crystals grown within the NCAR ice nucleus

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Kerry H. Cook

events is delivered through the modification of rainfall. A statistically significant precipitation signal due to the imposed warm event emerged at the 95% level and above in only a few regions in the model. The signal over the central Pacific, for example, is realistic and strong. More interestingly, the response in the southern Africa precipitation is nearly as strong as the central Pacific response, with a perturbation that is very similar to the observed response to warm events in that region

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