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F. M. Ralph, T. Coleman, P. J. Neiman, R. J. Zamora, and M. D. Dettinger

1. Introduction Past studies have shown that atmospheric rivers (ARs), which are regions of the lower atmosphere characterized by strong winds and large water vapor contents (usually associated with a surface cold front in the midlatitudes), are key features of the global water cycle (e.g., Zhu and Newell 1998 ), are detectable in satellite observations (see example in Fig. 1a ) ( Ralph et al. 2004 ; Neiman et al. 2008a ), and are associated with heavy rain and flooding on the U.S. West

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Timothy J. Lang, Steven A. Rutledge, and Robert Cifelli

particles unobservable by the S-Pol. Topographic data were obtained from a digital elevation model (DEM) that was matched to the radar grids, exactly as in Lang et al. (2007a) and Rowe et al. (2008) . Terrain bands were broken down as follows: Gulf of California (over water; 12 726 grid points in the DEM), coastal plain (0–500 m MSL; 6713 grid points), 500–1500 m MSL (2934 grid points), and >1500 m MSL (4852 grid points). The land terrain bands roughly corresponded to the low, middle, and high

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F. M. Ralph, E. Sukovich, D. Reynolds, M. Dettinger, S. Weagle, W. Clark, and P. J. Neiman

state-of-the-art mesoscale numerical model forecasts. Acknowledgments Thanks to Wes Junker and Mike Ekern of the NWS for providing key data and useful guidance. Without the efforts of the overall HMT team, especially David Kingsmill and Tim Schneider, this work would not have been possible. The authors also wish to thank Lynn Johnson and Ed Tollerud for providing feedback and helpful suggestions. REFERENCES Anthes, R. A. , 1983 : Regional models of the atmosphere in middle latitudes. Mon. Wea

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Sandra E. Yuter, David A. Stark, Justin A. Crouch, M. Jordan Payne, and Brian A. Colle

-barrier moisture flux and potential instability also contribute to the high precipitation intensities associated with stronger cross-barrier wind speed. Fig . 9. Horizontal spatial patterns of radar-derived variables at 2-km altitude associated with <25th vs >75th percentile conditions of (left) , (middle) U , and (right) 0°C level height (m); Z ≥ 13 dB Z exceedance frequency (%) for (a)–(c) >75th percentile of variable and (d)–(f) <25th percentile of variable. Z25/Z13 ratio (relative precipitation

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