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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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R. Uijlenhoet, J.-M. Cohard, and M. Gosset

that would be received in the absence of rainfall, L (km) is the pathlength, k (dB km −1 ) is the specific extinction coefficient, and s (km) is the distance from the transmitter. Equation (1) is based on the assumptions that extinction is solely due to rainfall—that is, that the contributions of fog or clouds, water vapor, and aerosols are negligible, and that multiple scattering does not play a role (e.g., Tam 1980 ; van de Hulst 1981 ; Tam and Zardecki 1982 ; Zardecki and Tam 1982

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

atmospheric rivers (ARs) that concentrate water vapor transport into California’s Sierra Nevada, creating major orographic precipitation (e.g., Pandey et al. 1999 ; Neiman et al. 2002 ; Dettinger et al. 2004 ; Ralph et al. 2005a , 2006 ). When an extratropical storm has strong water vapor transport and stalls, it can produce more than 76.2 cm (30.0 in.) of precipitation in 3 days ( Neiman et al. 2008a ). While Folsom Dam, just upstream of Sacramento, provides significant flood protection, five storms

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Dusanka Zupanski, Sara Q. Zhang, Milija Zupanski, Arthur Y. Hou, and Samson H. Cheung

-Var) algorithm for the assimilation of precipitation-affected microwave radiances. The two-step approach, where satellite radiances are assimilated by the nonlinear 1D-Var step to produce increments of total column water vapor, and then these increments are assimilated by the linear (so-called incremental) 4D-Var step, has proven better in handling nonlinearities than the incremental 4D-Var approach alone. In Vukicevic et al. (2004 , 2006) , a different approach was taken to use a full blown 4D-Var with

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Youcun Qi, Jian Zhang, Brian Kaney, Carrie Langston, and Kenneth Howard

. , Martner B. E. , White A. B. , and Kingsmill D. E. , 2005 : Wintertime nonbrightband rain in California and Oregon during CALJET and PACJET: Geographic, interannual and synoptic variability . Mon. Wea. Rev. , 133 , 1199 – 1223 , doi: 10.1175/MWR2919.1 . Neiman, P. J. , White A. B. , Ralph F. M. , Gottas D. J. , and Gutman S. I. , 2009 : A water vapour flux tool for precipitation forecasting . Water Manage. , 162 , 83 – 94 , doi:10.1680/wama.2009.162.2.83 . Panziera, L. , and

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

the mountainous U.S. west coast. The more intense precipitation events are related to “atmospheric rivers” ( Zhu and Newell 1998 ), narrow plumes of moisture associated with fronts on oceanic cyclones ( Bao et al. 2006 ). These enhanced bands of vertically integrated water vapor typically form as the result of local moisture convergence ( Bao et al. 2006 ). Under a subset of environmental conditions, the moisture can be traced back from the U.S. west coast to the tropics ( Bao et al. 2006 ). These

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Ali Behrangi, Bisher Imam, Kuolin Hsu, Soroosh Sorooshian, Timothy J. Bellerby, and George J. Huffman

; Behrangi et al. 2009a , b ; Capacci and Conway 2005 ) by virtue of improving its GEO based components. While global high temporal and spatial monitoring of cloud-top properties in visible, water vapor, and thermal IR bands are currently available from existing GEO satellites, finer spectral, spatial, and temporal resolution data is forthcoming online through the suite of recent and future geostationary satellites [e.g., Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the Meteosat

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