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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. These fronts occur on the synoptic scale and may retain their individual identities for many hours or even days. The physical processes that give rise to fronts are controlled by four principal forces: the buoyancy force, the pressure gradient force, the Coriolis force, and the friction force, whose influence is primarily restricted to the lowest 1–1.5 km of the atmo- sphere, the planetary boundary layer. The buoyancy force results from a small imbalance between gravity, which acts downward toward the Earth’s surface, and an upward vertical Figure 1 Conceptual model of a Norwegian cyclone showing (top) pressure gradient force. When the two are equal in magnitude, lower-tropospheric (e.g., 850 mb) geopotential height and fronts, and hydrostatic balance occurs. The direction of the buoyancy force (bottom) lower-tropospheric potential temperature. The stages in the – controls the direction of vertical motions associated with respective cyclone evolutions are separated by approximately 6 24 h and the frontal symbols are conventional. The characteristic scale of the fronts. The horizontal component of the pressure gradient cyclones based on the distance from the geopotential height minimum, force, which is directed toward low pressure, is largely opposed denoted by I, to the outermost geopotential height contour in stage IV by the Coriolis force. The Coriolis force is directly proportional is 1000 km. Adapted with permission from Schultz, D.M., Vaughan, G., to the Earth’s rotation rate and to the magnitude of the hori- 2010. Occluded fronts and the occlusion process: a fresh look at zontal wind vector, directed to the right of the horizontal wind conventional wisdom. Bulletin of the American Meteorological Society in the Northern Hemisphere. These forces tend to be in 92, 443–466.

Encyclopedia of Atmospheric Sciences, Second Edition, 2015, 337–343 Author's personal copy Synoptic Meteorology j Fronts 339 convective clouds. In reality, however, some cold fronts do not characterized by banded convective precipitation (e.g., Novak have coincident wind and temperature changes (e.g., Schultz, et al., 2004). 2004, 2005), are not coincident with clouds and precipitation (e.g., Mass and Schultz, 1993), or may not even be associated Stationary Fronts with temperature drops because cold air pooled in valleys ahead of the front may be colder than the postcold frontal air As the name implies, a stationary front has historically been (e.g., Sanders and Kessler, 1999; Doswell and Haugland, classified as a front that is not moving or moving very slowly. 2007). Thus, the representation of cold fronts should be The stationary front is denoted on the surface map by alter- modified to encompass these cold fronts that do not meet the nating triangles that point away from the cold air side of the traditional criteria. front and semicircles that point away from the warm air side of Different types of cold fronts exist, classified by their the front, indicating the standoff between the advance of the direction of motion (e.g., backdoor cold fronts), location (e.g., cold air and the advance of the warm air. This type of front may southerly buster), or a combination of both (e.g., arctic fronts). reflect either a change in the synoptic-scale circulation pattern For example, a backdoor cold front is one that moves westward that halts the translation of the frontal zone or the heteroge- (against the prevailing westerlies). In the United States, a New neity of the Earth’s surface that provides conditions to fix England backdoor cold front is associated with the westward a frontal transition zone to a preferred location. expansion of a cold surface high-pressure system situated near Schematics of stationary fronts are similar to warm fronts the North Atlantic coast during winter (Bosart et al., 1973; with their gentle slope. Stationary fronts have since been Hakim, 1992). The thermal gradient is reversed in this case, and recognized to have either the slope of warm fronts or cold the frontal zone is characterized by fog and low stratiform fronts. In fact, some stationary fronts in the central United clouds formed from evaporation of moisture from the ocean. States transition rapidly from moving equatorward as cold This cold front moves southward along the eastern slope of the fronts, to becoming stationary, to moving poleward as warm Appalachian Mountains, which serve as a barrier to inland fronts (Bosart et al., 2008). Consequently, a modern interpre- penetration. tation of a stationary front would be to continue to analyze The southerly buster or southerly burster is an intense fronts as stationary fronts, but not to apply any given concep- summertime cold front that arrives at the southeastern tip of tual model to it. Australia from the Southern Ocean (Baines, 1980). Arrival of Another example of a stationary front is a coastal front. this front in the afternoon can be accompanied by temperature Coastal fronts are frontal zones separating relatively warm changes of 10–15 C over a period of a few minutes, but oceanic air from colder continental air, such as along the East precipitation is not usually associated with a southerly buster. Coast of the United States in winter. Such environments The front travels equatorward, acquiring a characteristic promote the formation of a stationary front (Bosart et al., 1972; S-shape as its movement is inhibited by the east coast moun- Bosart, 1975). Although these fronts exhibit transition zones tain chain, but movement inland and along coastal waters is that are comparable to those of a cold front, they are of limited less restrained. extent, ranging from 200 to 600 km, and of limited duration, Another subclass of cold fronts associated with the advec- lasting up to a day or less. Coastal fronts often result in bands tion of arctic air equatorward is an arctic front (e.g., Wang et al., of precipitation, parallel to the front, with the maximum 1995). Dramatic examples of these kinds of fronts occur in the precipitation occurring on the cold side, and possibly a transi- central United States, with the cold air traveling as far equa- tion from snow to sleet to freezing rain to rain on the warm torward as Central America (e.g., Schultz et al., 1997, 1998). side. The bitter cold air behind the front is formed from strong radiational cooling in the arctic region (Emanuel, 2008). Occluded Fronts Traditionally, occluded fronts were considered to be the end Warm Fronts products of the evolution of extratropical cyclones, formed by Warm fronts are characterized by the advection of warm air the catch-up of a slower-moving warm front by a faster-moving into cold air. The leading edge of the warm front is delineated cold front. The occluded front is denoted on the surface map by by the semicircles that point in the direction of movement of alternating triangles and semicircles that point in the direction the warm air (Figure 1). Traditionally, a vertical cross section of movement (Figure 1). Two kinds of occluded fronts were through a warm front shows a gently sloping zone over the cold hypothesized to exist: warm-type occlusions and cold-type air. Above the strongly sheared zone, warm air gently rises over occlusions. Warm-type occlusions were hypothesized to form the warm front. At the surface, a warm frontal passage would be if the air ahead of the warm front was colder than the air behind characterized by increasing temperature and dew point, and the cold front, lifting the merging cold front over the warm a cyclonic wind shift. A sequence of clouds would progress front. In contrast, cold-type occlusions were hypothesized to from cirrus to altocumulus to nimbostratus before the frontal form if the air ahead of the warm front was warmer than the air passage, followed by clearing skies after frontal passage. behind the cold front, lifting the merging warm front over the A more modern characterization of warm fronts indicates cold front. that the flow over the warm front may not be characterized by This traditional explanation does not adequately explain gentle ascent. Instead, the flow may consist of convective the structure and evolution of occluded fronts. First, much of elements embedded within the ascent (the so-called escalator– the length of some occluded fronts is formed, not by the catch elevator paradigm of Neiman et al., 1993) or may be up of fronts, but by the lengthening of the front by deformation

Encyclopedia of Atmospheric Sciences, Second Edition, 2015, 337–343 Author's personal copy 340 Synoptic Meteorology j Fronts due to differential rotation around the cyclone. Second, many Dryline cyclones continue to deepen after the formation of the The dryline is a zone of strong dew-point temperature occluded front. In fact, the heaviest precipitation and the gradient that forms in the southern United States during the strongest winds in the cyclone typically occur after the occluded warm season, separating moist air originating from the Gulf front has formed. Third, few well-documented examples of of Mexico from drier air originating from the southwest cold-type occlusions exist; yet this is not explained by the United States (Schaefer, 1974, 1986; Hoch and Markowski, traditional model. 2005). Although not a front per se, the dryline is a boundary To address these inadequacies, Schultz and Vaughan (2011) between two synoptic-scale air masses and can sometimes presented a more modern interpretation that ascribes occluded possess frontal characteristics of a temperature gradient and fronts to the wrap up of the cold and warm fronts around an wind shift (Ziegler and Hane, 1993; Buban et al., 2007). Like extratropical cyclone. The extreme length of some occluded fronts, the strength of the dryline (magnitude of the gradient fronts is better characterized as the wrap up of the warm sector of the dew point temperature) can be regulated by defor- and occluded front than by the catch up of one front by mation and convergence (Schultz et al., 2007). Temperature another. In the wrap-up paradigm, the formation of an differences across the dryline can be enhanced by the occluded front no longer determines the end of deepening of differing moisture contents in the air (i.e., moist air cools the cyclone, but is a result of the deepening. The deeper the down more slowly than dry air because of the greenhouse cyclone, the more likely a cyclone will form an occluded front. effect of the water vapor). The wrap-up paradigm also explains why many weak cyclones do not form occluded fronts. Also in the wrap-up paradigm, the two kinds of occluded Upper-Level Fronts fronts are not determined by the relative temperatures across the occluded front, but by their relative static stabilities (Stoe- An upper-level front is a transition zone exhibiting a sharp linga et al., 2002). Warm-type occlusions form if the warm thermal contrast and wind shear that may extend from the frontal zone is more stable than the cold frontal zone, and cold- tropopause down to as low as 2–3 km above the ground. The type occlusions form if the cold frontal zone is more stable than mature upper-level frontal structure displayed in Figure 2 the warm frontal zone. Because warm frontal zones tend to be shows a frontal zone depicted by the concentration of isen- much more stable than warm frontal zones, warm-type tropes (lines of constant potential temperature). Upper-level occluded fronts would be more common, explaining the rela- fronts are approximately 50–200 km wide and hundreds of tive dearth of cold-type occlusions. kilometers long. These fronts are not described as cold or warm Closely related to an occluded front is a back-bent front (e.g., fronts, although cold-air and warm-air advections often occur Bjerknes, 1930; Bergeron, 1937; Shapiro and Keyser, 1990). This along its length (Keyser and Shapiro, 1986; Schultz and front is a warm or occluded front that has been wrapped around Doswell, 1999). Upper-level fronts are not usually associated the cyclone, enclosing a region of relatively warm postcold with the type of weather that characterizes surface fronts. Clear- frontal air. A region of strong winds known as the sting jet is air turbulence and stratospheric–tropospheric exchange of air sometimes found along a back-bent front (Grønås, 1995; are often defining characteristics that distinguish upper-level Browning, 2004; Schultz and Sienkiewicz, 2013). fronts.

Figure 2 Cross section of an upper-level front. The tropopause is denoted by thick solid lines. The thin dashed lines are isentropes (K), and the thin solid lines are isotachs (m s1). J denotes the axis of the jet stream, directed out of the section. Pressure levels (hPa) and standard heights (km) are shown on the abscissa. Adapted with permission from Reed, R.J., 1955. A study of a characteristic type of upper-level frontogenesis. Journal of Meteorology 12, 226–237.

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Figure 3 Schematic illustration of the transverse secondary circulation associated with an upper-level front as shown in Figure 2. Adapted with permission from Danielson, E.F., 1968. Stratospheric–tropospheric exchange based on radioactivity, ozone and potential vorticity. Journal of the Atmospheric Sciences 25, 502–518.

Superposed on this front and jet-stream system is a trans- Consequently, these fronts may take the form of a density verse or cross-front circulation (Eliassen, 1990; Figure 3). This current. In this section, three types of fronts associated with circulation provides a consistent explanation of how the upper- mesoscale phenomena are discussed: sea-breeze fronts, gust level front is maintained and why stratospheric constituents, fronts associated with convective storms, and drainage fronts. such as ozone and high-level radioactivity from nuclear explosions, can be observed in the lower troposphere. The Sea-Breeze Fronts subsiding branch maintains the most prominent characteristics of the upper-level front. In particular, the descent of strato- Coastlines are prime regions for strong temperature differences spheric air within a narrow pocket can deform the tropopause to develop and take the form of fronts. Like the dryline, into what is called a tropopause fold. temperature gradients across the coast may develop in a diurnal Thermal wind balance provides the explanation for the cycle. Air masses over the water tend to heat up less during the high-speed jet flow above the frontal zone. In this example, the day and cool down less during the night than adjacent air sharp thermal gradient across the front produces a relatively masses over the land. A pressure gradient, which develops in strong jet that flows southward. Two characteristic features of response to this differential heating, drives an onshore flow at an upper-level front are cyclonic shear and the temperature low levels, with a return flow at about 1–2 km above the gradient. These two features are always associated with the surface. The leading edge of the cooler air may develop frontal development and enhancement of a prominent upper-level jet characteristics. The sea-breeze front is characterized by both and frontal system, and with surface-based fronts that extend to a sharp temperature drop of several degrees centigrade or more the upper troposphere. and a marked increase in the humidity that can occur over a horizontal distance of a kilometer or less. Sea-breeze fronts are most prominent in the warm part of the year under rela- Surface Fronts Associated with Mesoscale tively benign synoptic-scale flow. Inland penetration of the sea- Phenomena breeze front by 10 km or more may be opposed by an offshore wind ahead of the front and by turbulent convective mixing Extratropical cyclones are not the only weather systems that over land, which tends to weaken the temperature and organize temperature gradients and wind shifts into fronts; humidity gradient across the frontal zone. mesoscale phenomena can also do so. Small-scale fronts occupy a relatively limited horizontal domain, have relatively Gust Fronts short lifetimes, and are surface-based phenomena. Because they exist only for a few minutes to a few hours, the Coriolis Convective storms are associated with the ascent of warm, force is usually not dominant, but the nonhydrostatic acceler- moist, unstable air. To maintain conservation of mass, descent ation may be important in the dynamics of these fronts. must accompany the storms. The cooler descending air reaches

Encyclopedia of Atmospheric Sciences, Second Edition, 2015, 337–343 Author's personal copy 342 Synoptic Meteorology j Fronts the surface and spreads out. The leading edge of this air is called Bosart, L.F., Vaudo, C.J., Helsdon Jr., J.H., 1972. Coastal frontogenesis. Journal of a gust front and can frequently possess the characteristics of Applied Meteorology 11, 1236–1258. a front. The gust front develops from evaporative cooling Bosart, L.F., Wasula, A.C., Drag, W.H., Meier, K.W., 2008. Strong surface fronts over sloping terrain and coastal plains. Synoptic-Dynamic Meteorology and Weather associated with precipitation below large convective clouds. Its Analysis and Forecasting: A Tribute to Fred Sanders. Meteorologist Monographs, vertical extent is limited by the height of the cloud base, usually No. 55. American Meteorological Society, pp. 35–85. below 2 km. Temperature changes as high as 10 C over a few Browning, K.A., 2004. The sting at the end of the tail: damaging winds associated with tens of meters can occur, and the wind gusts may represent extra-tropical cyclones. Quarterly Journal of the Royal Meteorological Society 130, 375–399. a danger to aircraft that attempt to land on runways where gust Buban, M.S., Ziegler, C.L., Rasmussen, E.N., Richardson, Y.P., 2007. The dryline on fronts are evident. A gust front will last only minutes or tens of 22 May 2002 during IHOP: ground-radar and in situ data analyses of the dryline minutes if the cold air moves away from its source. It may, and boundary layer evolution. Monthly Weather Review 135, 2473–2505. however, persist for a few hours or more if the cold air below Colby Jr., F.P., Seitter, K.L., 1987. A new analysis technique for fronts. Extended the cloud base moves with the convective system. Relatively Abstracts, Third Conf. on Mesoscale Meteorology. 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