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

around 80 km. The initial and lateral boundary conditions were given by the Met Office operational analysis valid at 0000 UTC 8 December 2011 and 3-hourly lateral boundary conditions (LBCs) valid from 2100 UTC 7 December 2011 for 72 h. Several previous studies have used resolutions of this order to study this type of storm (e.g., Clark et al. 2005 ; Parton et al. 2009 ; Martínez-Alvarado et al. 2010 ), motivated on the basis that the fastest growing mode of slantwise instability should be

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

with a semi-Lagrangian time-stepping scheme ( Davies et al. 2005 ). Six limited-area simulations are performed with resolutions ranging from 12-km grid spacing with 38 levels to 100-m grid spacing with 140 levels, as described in Table 1 . The 12-km model takes its initial and boundary conditions from a global simulation with 40-km grid spacing, and each subsequent resolution is one-way nested from the previous. The model domains are shown in Fig. 2 . The presence of extreme strong winds and the

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Jeffrey M. Chagnon and Suzanne L. Gray

, precipitation, and latent heating are located. Initial conditions for the 12-km runs are provided by an operational analysis from the Met Office at the same resolution, and lateral boundary conditions are provided by an operational run from the global MetUM with ~40-km resolution and 38 levels. The 4- and 1-km runs receive lateral boundary conditions from the 12- and 4-km runs, respectively, via one-way nesting. Table 1. Summary of MetUM experiments. Diagnosis of accumulated PV sources is performed using a

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David M. Schultz and Joseph M. Sienkiewicz

Research and Forecasting Model (WRF-ARW; Skamarock et al. 2005 ) simulation starting at 1200 UTC 7 December was performed. The model simulation had 12-km horizontal grid spacing. The National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) was used for the initial and lateral boundary conditions. The following physical parameterizations were implemented: the Yonsei University boundary layer scheme ( Hong et al. 2006 ), the Kain–Fritsch convective parameterization ( Kain and

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

.11° (~12 km) in both longitude and latitude on a rotated grid centered around 52.5°N, 2.5°W. The North Atlantic–European domain extends approximately from 30° to 70°N in latitude and from 60°W to 40°E in longitude. The vertical coordinate is discretized in 70 vertical levels with the lid around 80 km. The initial conditions were given by Met Office operational analyses valid at 1200 UTC 17 July 2012 for IOP13 and at 1800 UTC 14 August 2012 for IOP14. The lateral boundary conditions (LBCs) consisted of

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Sam Hardy, David M. Schultz, and Geraint Vaughan

simulations in this study, employing a single domain ( Fig. 2 ) with 60 vertical levels (extending to 50 hPa), horizontal grid spacing of 15 km, and a time step of 75 s. Global Forecast System (GFS) analyses from the National Centers for Environmental Prediction (NCEP) at 0.5° × 0.5° horizontal and 50-hPa vertical grid spacing (25 hPa below 750 hPa) were used as initial and lateral boundary conditions, input every 6 h. The Thompson microphysics scheme was used ( Thompson et al. 2008 ) with the Yonsei

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G. Vaughan, J. Methven, D. Anderson, B. Antonescu, L. Baker, T. P. Baker, S. P. Ballard, K. N. Bower, P. R. A. Brown, J. Chagnon, T. W. Choularton, J. Chylik, P. J. Connolly, P. A. Cook, R. J. Cotton, J. Crosier, C. Dearden, J. R. Dorsey, T. H. A. Frame, M. W. Gallagher, M. Goodliff, S. L. Gray, B. J. Harvey, P. Knippertz, H. W. Lean, D. Li, G. Lloyd, O. Martínez–Alvarado, J. Nicol, J. Norris, E. Öström, J. Owen, D. J. Parker, R. S. Plant, I. A. Renfrew, N. M. Roberts, P. Rosenberg, A. C. Rudd, D. M. Schultz, J. P. Taylor, T. Trzeciak, R. Tubbs, A. K. Vance, P. J. van Leeuwen, A. Wellpott, and A. Woolley

, each ensemble member took its initial and boundary conditions from the corresponding North Atlantic and European regional version of MOGREPS ( Bowler et al. 2008 ). Figure 11 shows a snapshot at 1600 UTC from the first four MOGREPS-UK members, zooming in on Scotland. The color shading is the 850-hPa wind speed, and lines indicate the axes of wind speed maxima. Figure 12 overlays the same lines on the model precipitation rates. The high-wind cores generally lie along the clear slots between the

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

enhances CAPE downstream of it, but has little effect on winds near the lower boundary. Further, Schlemmer et al. (2010) , examining a PV streamer approaching the Alps (a recognized harbinger of heavy convective rainfall), concluded that, although the streamer decreased the static stability throughout the troposphere directly beneath it, “destabilization [was] a second-order effect in the formation of heavy precipitation in the Alps” (p. 2352). Understanding the low-level conditions (moisture and

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Ross N. Bannister

(3) ]. The phases and are chosen with the intention of allowing s to satisfy some imposed lateral boundary conditions consistent with the limited-area model that produced the ensemble. For wind components , , , and , which prohibit flow in or out of the domain. The remaining variables have and , which represent Dirichlet boundary conditions. These conditions, however, are not seen in the correlations implied by (5) because of the normalization mentioned in section 2a(2) . Equation

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Jesse Norris, Geraint Vaughan, and David M. Schultz

domain is 4000 km long, which is equal to the wavelength of the most unstable normal mode of the initial jet ( Plougonven and Snyder 2007 ). The domain has 20-km grid spacing, with 80 vertical levels from the surface up to 16 km. The lower boundary condition is ocean with a roughness length = 0.2 mm and the sea surface temperature (SST) is fixed to the initial temperature of the lowest atmospheric model level. The simulations use the following parameterizations: Thompson et al. (2008

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