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Bogdan Antonescu, David M. Schultz, Fiona Lomas, and Thilo Kühne

1. Introduction Our current knowledge of the climatology of tornadoes in Europe has been built from historical collections of tornado reports (e.g., Peltier 1840 ; Wegener 1917 ), case studies (e.g., Hepites 1887 ; Lemon et al. 2003 ), and local climatologies (e.g., Snitkovskii 1987 ; Dessens and Snow 1989 ). Unfortunately, these datasets were limited by inconsistencies in observational networks and reporting practices across Europe and have only allowed a simplified and inaccurate

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Clifford Mass and Brigid Dotson

Northwest cyclones are probably the intense, and often rapidly developing, extratropical cyclones of the North Atlantic that move northeastward across the United Kingdom and northern Europe. Cyclones striking both regions develop over the eastern portion of a major ocean and thus exhibit the structural characteristics of oceanic cyclones, as documented by Shapiro and Keyser (1990) . Several of the European events have been described in the literature, including the 15–16 October 1987 storm ( Lorenc et

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Volkmar Wirth, Michael Riemer, Edmund K. M. Chang, and Olivia Martius

central Europe was associated with a long-lived precursor RWP ( Fig. 2c ); that low pressure system brought heavy precipitation in parts of Europe resulting in catastrophic flooding of the river Elbe. The forecast of this event was rather poor as little as a few days ahead of time. Each stage of the RWP life cycle may be subject to forecast errors, and it is important to obtain a better understanding of what stages and which processes contribute most strongly to poor forecasts. In particular, the role

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Julia H. Keller, Christian M. Grams, Michael Riemer, Heather M. Archambault, Lance Bosart, James D. Doyle, Jenni L. Evans, Thomas J. Galarneau Jr., Kyle Griffin, Patrick A. Harr, Naoko Kitabatake, Ron McTaggart-Cowan, Florian Pantillon, Julian F. Quinting, Carolyn A. Reynolds, Elizabeth A. Ritchie, Ryan D. Torn, and Fuqing Zhang

life along the northeast U.S. coast as it underwent ET ( Blake et al. 2013 ; Halverson and Rabenhorst 2013 ). Hurricane Gonzalo (2014), having undergone ET, tracked across Europe and brought flooding and extreme winds to the Balkans ( Brown 2015 ; Feser et al. 2015 ). Extreme precipitation associated with Tropical Storm Etau (2015) during and after its ET over Japan flooded areas north and east of Tokyo ( AIR Worldwide 2015 ; Kitabatake et al. 2017 ). In these examples, the high-impact weather

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Clark Evans, Kimberly M. Wood, Sim D. Aberson, Heather M. Archambault, Shawn M. Milrad, Lance F. Bosart, Kristen L. Corbosiero, Christopher A. Davis, João R. Dias Pinto, James Doyle, Chris Fogarty, Thomas J. Galarneau Jr., Christian M. Grams, Kyle S. Griffin, John Gyakum, Robert E. Hart, Naoko Kitabatake, Hilke S. Lentink, Ron McTaggart-Cowan, William Perrie, Julian F. D. Quinting, Carolyn A. Reynolds, Michael Riemer, Elizabeth A. Ritchie, Yujuan Sun, and Fuqing Zhang

, temperature, and ozone concentration allowed for the creation of new imagery to improve identification of evolving cyclone structures during ET ( Beven 2012a ), first for Europe and Asia and more recently for North America as new geostationary satellites became operational in each region. b. Assessments of forecast skill An operational center’s ET forecast should be assessed within the context of expected numerical model forecast skill, including insight as to when skill is likely to be either anomalously

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Loïk Berre and Gérald Desroziers

and omega balance equations allows for the representation of some effects of the jet dynamics on space and time variations of temperature covariances in particular. Another way to relax the covariance homogeneity assumption is to represent geographical variations of variances while using the homogeneity assumption for correlations. This has been done in the European Centre for Medium-Range Weather Forecasts (ECMWF) variational assimilation system, as described in Derber and Bouttier (1999) , by

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David M. Schultz

indicative of the location of a previous surface-based front having moved aloft ( Fig. 5 ). Stoelinga et al. (2003) showed that the convergence associated with the moving surface pressure field associated with a cold front aloft could initiate and maintain a convective line. In Europe and the eastern United States, structures such as split fronts (e.g., Browning 1990 ; Koch 2001 ; and references therein) may show similar behavior in surface weather patterns, although this has not been demonstrated

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Andrew Staniforth and Jean Côté

acceptable in typical medium-rangeforecast models, an Eulerian spectral control modelwas run at T106 resolution with a X74 diffusion havingthe same coefficient as that used operationally at thisresolution by the European Centre for Medium RangeWeather Forecasts. It was found that this control modellost 3% of its total energy after 20 days, whereas thetwo semi-Lagrangian models at T 126 resolution onlylost 1%. At lower (T63) resolution, the three-time-levelinterpolating semi-Lagrangian model lost 5

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Daniel Keyser and M. A. Shapiro

involved inthe formation of upper-level fronts in idealized twodimensional flows and in three-dimensional barrx:linicwaves. Unresolved issues.and problems awaiting fi~tureresearch are identified and discussed in Section 5, andthe review is concluded in Section 6.2. Observations of upper-level frontal structure The first descriptions of the three-dimensional thermal structure associated with midlatitude baroclinicwaves and cyclones resulted from analyses of observations taken over northern Europe

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Zhiyong Meng and Fuqing Zhang

bias (forecast − observation, dashed) in 24-h forecasts of (a) temperature, (b) meridional wind, (c) geopotential height, and (d) dewpoint temperature from 1 Jan 2005 to 1 Jan 2007. All forecasts are verified against the same set of rawinsonde observations. The black line denotes the European Centre for Medium-Range Weather Forecasts (ECMWF) rawinsonde observation error standard deviation assumed during data assimilation (adapted from Torn and Hakim 2008a ). Most recently, a WRF-based LAM EnKF

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