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Tammy M. Weckwerth and Ulrike Romatschke

maximum in the central United States. These include, but are not limited to, deep tropospheric gravity waves generated by the Rocky Mountains elevated heat source (e.g., Tripoli and Cotton 1989a , b ; Mapes et al. 2003a , b ; Warner et al. 2003 ), potential vorticity anomalies (e.g., Raymond and Jiang 1990 ; Li and Smith 2010 ), gravity waves generated by convection (e.g., Fovell et al. 2006 ), influence by the low-level jet (LLJ; Trier and Parsons 1993 ; Fritsch and Forbes 2001 ; Keene and

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Rachel L. Miller, Conrad L. Ziegler, and Michael I. Biggerstaff

( Smull and Houze 1987 ). Two of the main objectives of PECAN are to determine whether nocturnal MCSs are elevated or surface based and to document how these systems interact with the stable nocturnal boundary layer (NBL) to sustain themselves long after sunset ( Geerts 2013 ). Due to surface radiative cooling, it has previously been hypothesized that nocturnal convection is typically elevated and would propagate via gravity currents, undular bores, turbulent bores, and solitary waves ( Carbone et al

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David J. Bodine and Kristen L. Rasmussen

estimates were further refined in Corfidi (2003) to incorporate the effects of the cold pool. In some cases, the MCS may propagate in the direction of new convection initiation and merge with the new convection, leading to enhanced forward propagation through discrete propagation (e.g., Zipser 1977 ; Crook and Moncrieff 1988 ; Fovell et al. 2006 ). Fovell et al. (2006) explored discrete propagation using a numerical model and found that new convection was initiated by gravity waves ahead of the

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Dylan W. Reif and Howard B. Bluestein

major forecasting problems ( Davis et al. 2003 ; Trier et al. 2006 ; Surcel et al. 2010 ). One reason is that mesoscale features such as outflow boundaries and horizontal convective rolls (HCRs) are poorly sampled and are poorly resolved in many operational models ( Trier et al. 2014 ). Another complication is that the processes that lead to CI during the day are not as effective at generating convection at night. Features such as the LLJ and gravity waves (including atmospheric bores) are more

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Samuel K. Degelia, Xuguang Wang, and David J. Stensrud

CI events, or if the observation impacts would be as large when the mechanisms are not well captured. As nocturnal convection can be initiated by many other features such as atmospheric bores or internal gravity waves, we plan to conduct a systematic evaluation of the impact of assimilating PECAN field observations on forecasts of nocturnal CI. To facilitate this work, a statistical method is also being developed to systematically quantify timing, location, and orientation errors for CI. By

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