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For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial From Schultz, D.M.D., Blumen, W., 2015. Fronts. In: Gerald R. North (editor-in-chief), John Pyle and Fuqing Zhang (editors). Encyclopedia of Atmospheric Sciences, 2nd edition, Vol 5, pp. 337–343. ISBN: 9780123822253 Copyright © 2015 Elsevier Ltd. unless otherwise stated. All rights reserved. Academic Press Author's personal copy Fronts DM (David) Schultz, University of Manchester, Manchester, UK W Blumeny, University of Colorado Boulder, Boulder, CO, USA Ó 2015 Elsevier Ltd. All rights reserved. Synopsis Traditionally, fronts have been defined as boundaries between air masses. Because this definition can be problematic in a modern context, a new definition is proposed. A front is a region characterized by frontogenesis containing both a hori- zontal potential temperature gradient and vorticity maximum. Fronts are classified as surface based or upper level. Surface- based fronts are further classified as those associated with synoptic-scale phenomena (cold, warm, stationary, or occluded) or those associated with mesoscale phenomena (sea breeze, gust, or drainage). Front-like features such as drylines also exist. Introduction Second, although boundaries between air masses may be clear in some cases, what constitutes an air mass or its The traditional definition of a front is the boundary between boundary may not be clear in all cases. As an attempt to address two different air masses, which are large bodies of near-surface this weakness, some investigators have defined a front only if air extending for hundreds or thousands of kilometers with the horizontal temperature gradient or other higher derivatives nearly uniform temperature, moisture content, and static of the temperature gradient exceeds strict thresholds (e.g., stability. Air masses are classified by where they originate from Hewson, 1998; Sanders, 1999; Sanders and Hoffman, 2002). and the surface underlying their origin (i.e., polar vs tropical, Although such approaches automate the analysis and detection continental vs maritime). Being the boundary between two of fronts, they do not help better understand the processes different air masses means that fronts are often regions where affecting fronts and their intensity or why fronts form where changes in temperature, pressure, moisture content, and wind they do. occur over small distances. Fronts can also be associated with Third, the large horizontal scale of air masses implies that precipitation. Thus, for these reasons, identification and fronts – as the boundaries of these air masses – must possess tracking of fronts are crucial for weather forecasting. the same characteristic scale: large mesoscale or synoptic scale This traditional definition of fronts was developed by in the along-front direction (200–2000 km) and small meso- Norwegian meteorologists in the 1910s and 1920s studying the scale in the across-front direction (20–200 km; Keyser, 1986). structure and evolution of extratropical cyclones (Bjerknes, Not all fronts, however, may be this large; some fronts may be 1919; Bjerknes and Solberg, 1922). Using the terminology of small mesoscale in the along-front direction and microscale the recent First World War, fronts were lines on a map drawn (2–20 km) in the across-front direction. Thus, the traditional where the clash of cold and warm air masses occurred. definition may exclude smaller scale features that share the Although abrupt changes in wind and temperature had been same physical processes and deserve to be considered fronts, as previously recognized before the Norwegians (e.g., historical well. reviews by Bergeron, 1959; Kutzbach, 1979, Section 6.7; Finally, and perhaps most significantly, the traditional Davies, 1997; Newton and Rodebush Newton, 1999; Volkert, definition of a front as the boundary between air masses 1999), the Norwegians were the first to embed fronts as the forced meteorologists into an unproductive game of arguing unifying concept within the three-dimensional structure and over where to draw the line on the map representing the front, evolution of extratropical cyclones. rather than focusing on the relevant physical processes creating and maintaining the front and associated sensible weather (e.g., wind shifts, temperature changes, precipita- Toward a New Definition tion). Part of the dilemma with frontal analysis is that the characteristics of fronts used for analysis are not clearly In a modern scientific context, however, this traditional defi- defined, allowing any number of plausible analyses (e.g., nition can be problematic. First, fronts do not always form Uccellini et al., 1992; Sanders and Doswell, 1995; Lackmann, along the edge of air masses, but may form within air masses. 2011, Section 6.1). Some have raised issues with synoptic Because the air near the surface may not be easily classified into analysis (e.g., Mass, 1991; Sanders and Doswell, 1995; large regions of nearly homogenous properties, even within Sanders, 2005), and others have proposed alternative analysis well-defined air masses, gradients of temperature and wind schemes, particularly to deal with fronts associated with may be present and could be considered to be fronts. Thus, we mesoscale phenomena (e.g., Colby and Seitter, 1987; Young seek a definition of a front that could allow for the existence of and Fritsch, 1989). Manual analysis of weather maps is fronts within an air mass. a crucial step to producing a weather forecast, but it is ulti- mately subjective and its overemphasis has probably inhibi- ted a more modern approach to frontal analysis (e.g., y Deceased. Sutcliffe, 1952; Mass, 1991; Schultz, 2008). Encyclopedia of Atmospheric Sciences 2nd Edition, Volume 5 http://dx.doi.org/10.1016/B978-0-12-382225-3.00039-6 337 Encyclopedia of Atmospheric Sciences, Second Edition, 2015, 337–343 Author's personal copy 338 Synoptic Meteorology j Fronts An alternative approach is a mathematical one. Petterssen approximate geostrophic balance above the planetary (1936) defined frontogenesis as the Lagrangian rate of change boundary layer, providing a counterclockwise circulation of the magnitude of the horizontal potential temperature (q) around a low-pressure center. The friction force reduces the gradient due to the horizontal wind (V2 ¼ ui þ vj): wind speed within the planetary boundary layer, and lessens the magnitude of the Coriolis force, which leads to cross- F ¼ d jV qj; isobaric flow toward low pressure. t 2 [1] d The language and notation for synoptic-scale surface fronts where were created by the Norwegian meteorologists (e.g., Jewell, 1981; Friedman, 1989). The strength of the Norwegian cyclone d v v v ¼ þ u þ v ; model was that it created a holistic conceptual model that dt vt vx vy related the structure and evolution of an extratropical cyclone v v (low-pressure system) and anticyclone (high-pressure system) V ¼ þ : 2 i vx j vy with their attendant fronts and sensible weather. In the Norwegian cyclone model, an initial cyclone initiated on This mathematical definition of a front emphasizes that both a broad temperature gradient (Figure 1). As the cyclone the temperature field and the wind field are responsible for deepens, the circulation around the cyclone increases, bringing producing frontogenesis. cold air equatorward to form the cold front and bringing warm For these reasons and to update the traditional definition, air poleward to form the warm front. a modern definition of a front is proposed that combines the physical and mathematical approaches (e.g., Keyser, 1986; Cold Fronts Sanders, 1999; Lackmann, 2011). A front is a region charac- terized by frontogenesis containing both a horizontal potential The passage of a cold front typically is indicated by a drop in temperature gradient and vorticity maximum. Sometimes, the temperature as the advancing cold air replaces warm air (e.g., vorticity is not coincident with the temperature gradient, but Sanders 1955; Schultz 2008; Schultz and Roebber 2008). The often is. The benefit of using frontogenesis in the definition is leading edge of the cold front is delineated by the triangles that that it treats fronts as processes, not rigid objects being advected point in the direction of movement of the cold air (Figure 1). around by the flow, which is an unfortunate result of treating Conceptual models typically depict traditional cold frontal fronts as the boundaries between air masses. If a region passages, not only with a temperature decrease, but also with contains no temperature gradient or vorticity, then it cannot be a cyclonic wind change, pressure minimum, and decrease in a front (Sanders, 1999). If the gradient did not arise through or dew point temperature, all coincident with a line of deep, possess frontogenesis, then it is not a front. This definition also allows for the possibility of fronts forming within air masses and is independent of scale. Finally, this definition alleviates the problem of where to draw the line on the map as regions of frontogenesis are calculated explicitly. Fronts Associated with Synoptic-Scale Phenomena The intensity of a front, measured by changes in the tempera- ture and wind fields that occur across the frontal transition zone, is most pronounced at or near the Earth’s surface and the tropopause.