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Jeffrey M. Chagnon and Suzanne L. Gray

with a description of the PV tracer diagnostic. A more detailed description of the PV tracer method is provided in CGM13 . a. MetUM and PV tracers The MetUM is a numerical weather prediction system utilized for weather and climate forecasting by the Met Office. It consists of a dynamical core that approximates solutions to the nonhydrostatic and fully compressible equations of motion on a sphere using a semi-implicit semi-Lagrangian technique, along with a suite of parameterization schemes to

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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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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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Matthew R. Clark and Douglas J. Parker

pressure fields near to NCFRs, and to document the variability in structure between cases. A second aim is to quantify the horizontal convergence and vertical vorticity across the NCFR, and to reflect on how the observed evolution of these parameters may bear on theories of vortex genesis along NCFRs. A third aim is to demonstrate the utility of the 1-min-resolution surface data for the construction of detailed fields of surface parameters. The time-compositing technique is described in section 2

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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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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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G. Lloyd, C. Dearden, T. W. Choularton, J. Crosier, and K. N. Bower

this value for all 2D-S data, increases the possibility that shattered particles could have contaminated the dataset and artificially increased number concentrations, particularly in regions where larger more delicate ice crystals were seen to have developed. This possibility was investigated by examining IAT frequency plots and testing for changes in ice number concentrations when varying the IAT threshold to 1 × 10 −4 s and also by turning the filtering technique off, allowing all particles to

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Oscar Martínez-Alvarado, Suzanne L. Gray, and John Methven

previously described in Martínez-Alvarado and Plant (2014) and Martínez-Alvarado et al. (2014b) , while the diabatic PV tracer technique has been developed in Stoelinga (1996) , Gray (2006) , Chagnon and Gray (2009) , Chagnon et al. (2013) , Chagnon and Gray (2015) , and Saffin et al. (2015) . The diabatic tracers described here are conceptually related to those described by Cavallo and Hakim (2009) and Joos and Wernli (2012) . However, those methods analyze Lagrangian tendencies, whereas

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