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Carl M. Thomas and David M. Schultz

1. Introduction Constructing a spatial climatology of fronts based on a set of manually produced surface analyses can be tedious and time consuming. Indeed, only six studies constructed spatial climatologies of fronts from manual analyses in the 75 years between 1939 and 2014 ( Table 1 ). Since the advent of global reanalysis datasets and powerful computers in the 1990s, the ability to compute global climatologies of fronts has become much easier, proliferating the number of studies. In the 17

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Callum J. Shakespeare

1. Introduction Regions of sharp horizontal density contrast, or fronts, are ubiquitous features near the ocean surface on scales from tens of meters to hundreds of kilometers (e.g., Shcherbina et al. 2013 ; Gula et al. 2014 ; Rosso et al. 2015 ; Capet et al. 2008 ; among others). These fronts are often generated on the periphery of eddies formed during the growth of barotropic and baroclinic instabilities (e.g., Holmes et al. 2014 ; Mahadevan 2006 ; Hoskins and Bretherton 1972 ). As

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Catherine M. Naud, Derek J. Posselt, and Susan C. van den Heever

1. Introduction The midlatitudes, where most of the world’s population resides, are strongly affected by the passage of extratropical cyclones and their warm and cold fronts, and in particular by the amount of precipitation they might produce (e.g., Stewart et al. 1998 ; Kunkel et al. 2012 ). Insufficient precipitation affects crops and water supply, whereas precipitation extremes can result in havoc and severe loss of life and property. In the context of a warming world, it is still unclear

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David M. Schultz, Bogdan Antonescu, and Alessandro Chiariello

1. Introduction The Norwegian cyclone model is a conceptual model describing the structure and evolution of extratropical cyclones and fronts ( Bjerknes 1919 ; Bjerknes and Solberg 1921 , 1922 ). As a part of that evolution and based upon the analyses and insight of Tor Bergeron (e.g., Bergeron 1959 ; chapter 10 in Friedman 1989 ), the Norwegian cyclone model described the process by which a mature cyclone forms an occluded front. The formation of an occluded front was described as a faster

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Dustin J. Swales, George S. Young, Todd D. Sikora, Nathaniel S. Winstead, and Hampton N. Shirer

1. Introduction Warm fronts are often considered to be one of the more benign aspects of baroclinic cyclones. Modern satellite remote sensors, however, tell a different story with warm fronts supporting a variety of interesting mesoscale phenomena ( Young et al. 2005 ). Using ocean surface images from satellite-borne synthetic aperture radar (SAR) and a range of other data sources, Young et al. (2005) examined 22 warm fronts, primarily in the Gulf of Alaska. One of the more striking findings

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Gregory L. West and W. James Steenburgh

1. Introduction The topographically complex western United States is a region where mountains play a major role in frontal evolution (e.g., Braun et al. 1997 ; Colle et al. 1999 ; Steenburgh and Blazek 2001 ; Colle et al. 2002 ; Shafer et al. 2006 ; Shafer and Steenburgh 2008 ; Steenburgh et al. 2009 ). As cold fronts approach the Pacific coast, orographic blocking and friction produce enhanced prefrontal southerly flow, confluent deformation, frontogenesis, frontal deceleration, and, in

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Melissa Payer, Neil F. Laird, Richard J. Maliawco Jr., and Eric G. Hoffman

1. Introduction A small number of studies have examined the frequency of fronts in North America (e.g., Chiang 1961 ; Morgan et al. 1975 ; Cousins 2006 ). Morgan et al. (1975) completed an analysis of frontal frequency across the contiguous United States for the period from January 1961 through December 1970 using the Daily Weather Map Series produced by the National Weather Service (NWS). Additional studies have focused on determining frontal frequencies and associated characteristics for

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Katja Friedrich, David E. Kingsmill, Cyrille Flamant, Hanne V. Murphey, and Roger M. Wakimoto

1. Introduction Boundary layer convergence zones (or boundaries) have long been known to be key factors in convection initiation and evolution (e.g., Byers and Braham 1949 ; Wilson and Schreiber 1986 ; Cohen and Kreitzberg 1997 ; Cohen and Schultz 2005 ). An examination of the literature suggests that boundaries such as gust fronts (e.g., Charba 1974 ; Wakimoto 1982 ; Weckwerth and Wakimoto 1992 ; Friedrich et al. 2005 ), sea-breeze fronts (e.g., Blanchard and Lopez 1985 ; Atkins et

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Nicholas T. Luchetti, Katja Friedrich, Christopher E. Rodell, and Julie K. Lundquist

1. Introduction Forecasting and monitoring the potential for severe wind damage associated with thunderstorm outflow boundaries or gust fronts (GFs) are important for structural and personal safety across a variety of sectors such as aviation, structural engineering, wind energy, the wildland fire community, and the emergency response community. A GF is the leading edge of cold air propagating horizontally away from a thunderstorm ( Simpson 1969 ; Droegemeier and Wilhelmson 1987 ). The pool of

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Nicholas T. Luchetti, Katja Friedrich, and Christopher E. Rodell

1. Introduction Rapid changes in wind direction and speed from passing thunderstorm gust fronts can threaten the safety of wildland firefighters by redirecting fire spread toward locations that were previously considered safe. In the past, studies of gust front characteristics have primarily focused on severe thunderstorms in flatter areas such as the U.S. Great Plains region (e.g., Charba 1974 ; Goff 1976 ; Engerer et al. 2008 ; Bryan and Parker 2010 ). However, little observational work

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