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Ross N. Bannister

1. Introduction a. Sampling error and localization Progress to improve the efficacy of ensemble data assimilation methods like the ensemble Kalman filter (EnKF) has been impeded by problems with sampling error. Sampling error arises when the number of ensemble members ( N ) is much less than the size of the state vector ( n ) (see e.g., Houtekamer and Mitchell 1998 ; Evensen 2003 ; Lorenc 2003 ). The true forecast error covariance matrix, , can be estimated from an N -member ensemble as

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Jeffrey M. Chagnon and Suzanne L. Gray

), diabatic Rossby waves (e.g., Parker and Thorpe 1995 ; Moore and Montgomery 2004 ; Moore et al. 2013 ), and forecast error growth (e.g., Brennan et al. 2008 ; Davies and Didone 2013 ). In the absence of frictional and diabatic processes, PV is materially conserved. Therefore, PV provides a convenient means to characterize the large-scale structure of the atmosphere and its evolution in mid- and high latitudes where the planetary vorticity is nonnegligible. Climatologically, PV increases with height

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David M. Schultz, Bogdan Antonescu, and Alessandro Chiariello

was discovered during the third author’s master’s thesis research ( Chiariello 2006 ). Therefore, the purpose of this article is to present this cold-type occluded front and to demonstrate its consistency with the static-stability rule. 2. Case study of a cold-type occluded front This case was found from looking at occluded fronts over the North Atlantic Ocean using model output from the European Centre for Medium-Range Weather Forecasts at 0.25° × 0.25° latitude–longitude gridded analyses. At

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Sam Hardy, David M. Schultz, and Geraint Vaughan

sensitivity simulations and introduces the QG omega equation diagnostics. In section 3 , a synoptic overview of the cyclone is presented, including analyses of the cyclone’s intensification ahead of the approaching PV anomaly and the development of the slow-moving frontal rainband over the United Kingdom on 24–25 September. Section 4 then introduces the results from a control simulation of the event using the Advanced Research version of the Weather Research and Forecasting (WRF) Model, which

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C. Dearden, G. Vaughan, T. Tsai, and J.-P. Chen

. 2013a , b ; Sulia et al. 2013 ; Milbrandt and Morrison 2013 ). While Forbes and Clark (2003) explored the diabatic role of ice processes in two rapidly deepening winter cyclones, there is a need to expand the literature on this subject, not just to include a consideration of other winter cases, but also in summer too. Summer cyclones often present more of a challenge to forecasters than winter cyclones; recent examples include the low pressure system from 21 September 2012 spawned by Tropical

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Oscar Martínez-Alvarado, Laura H. Baker, Suzanne L. Gray, John Methven, and Robert S. Plant

trajectory analysis (see sections 2c and 2d ) requires a large domain to allow long trajectories to be calculated without the majority of them leaving the domain. c. Trajectory analysis Two trajectory models are used in the paper. The first model is the Reading Offline Trajectory Model (ROTRAJ) as developed by Methven (1997) . Its application to aircraft flights is detailed in Methven et al. (2003) . It calculates trajectories using European Centre for Medium-Range Weather Forecasts (ECMWF) analysis

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Geraint Vaughan, Bogdan Antonescu, David M. Schultz, and Christopher Dearden

because it occurred during the first intensive observation period (IOP 1) of the Tropopause Folding, Stratospheric Intrusions and Deep Convection (TROSIAD) and the Diabatic Influence on Mesoscale Structures in Extratropical Storms (DIAMET; Vaughan et al. 2015 ) projects when extra observations were available from aircraft and ground-based stations. Although a band of showers was forecast by the Met Office up to two days ahead, neither the location nor the tight organization of the convection was well

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Ben Harvey, John Methven, Chloe Eagle, and Humphrey Lean

found to be narrower than can be resolved by current NWP models. Therefore, the modeled frontal widths are typically set artificially by (implicit or explicit) numerical diffusive effects, rather than by resolved physical motions. The degree to which the prediction of associated high-impact weather is affected by this limitation is not understood. Operational local-area forecast models are now approaching convection-permitting grid spacings of O (1) km ( Clark et al. 2016 ). In such models deep

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Jesse Norris, Geraint Vaughan, and David M. Schultz

has formed, is reinitialized with a 4-km nested domain and the different diabatic factors are varied between simulations, subsequent to this time. Thus, this paper demonstrates how precipitation cores along a mature cold front of maritime origin vary, depending on differences in the synoptic and mesoscale flow environment. 2. Method Moist idealized baroclinic-wave simulations were performed with version 3.7.1 of the Advanced Research Weather Research and Forecasting (WRF) Model ( Skamarock et al

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Matthew R. Clark and Douglas J. Parker

, 288 – 296 , doi: 10.1175/1520-0469(1958)015<0288:MOTITO>2.0.CO;2 . Gatzen , C. , 2011 : A 10-year climatology of cold-season narrow cold-frontal rainbands in Germany . Atmos. Res. , 100 , 366 – 370 , doi: 10.1016/j.atmosres.2010.09.018 . Green , A. , 2010 : From observations to forecasts—Part 7: A new meteorological monitoring system for the United Kingdom’s Met Office . Weather , 65 , 272 – 277 , doi: 10.1002/wea.649 . Grumm , R. H. , 2000 : Forecasting the precipitation associated

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