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Occluded Fronts ride up over, a slower-moving warm front (in this case, impeded from coming inland by the coastal An occluded front is generally believed to result from mountains of the Scandinavian peninsula). With the merger of a cold front and a warm front during the meeting of the two fronts, the warm air the life of a midlatitude cyclone. The occluded front between them was forced aloft, leaving a boundary can often be identified by a tongue of warm air between two polar air masses that he called the connecting the low-pressure center to the pool of occluded front. Bergeron later determined that the much warmer tropical air in the warm sector of the occluded front formed when the cold front caught cyclone. At the surface, a wind shift and pressure up to the warm front, because the cold front was minimum usually coincide with the front. More rotating around the cyclone center faster than generally, the formation of an occluded front occurs the warm front did. Bergeron’s occlusion process during the occlusion process, a stage in the life cycle was incorporated into the landmark paper by of an extratropical cyclone when the warm sector Jacob Bjerknes and Halvor Solberg in 1922, “Life air is separated from the low center near the surface Cycle of Cyclones and the Polar Front Theory of and the low center becomes wrapped in cold air. Atmospheric Circulation.” The polar front theory [See Cyclones, subentry on Midlatitude Cyclones.] was the first to describe midlatitude cyclones Although controversy has surrounded the occlusion undergoing an evolutionary life cycle rather than process ever since its discovery, a new paradigm for maintaining a static structure. the occlusion process resolves many of these previ- During the formation of an occluded front, ous controversies. the Bergen meteorologists believed that if the The Polar Front Theory of Midlatitude Cyclones. polar air behind the cold front was colder (and The occluded front was first discovered by Tor Bergeron thus denser) than the polar air ahead of the warm while investigating a cyclone on 18 November 1919 off front, then this less dense air and the associated the Norwegian coast. He noticed that the warm sector warm front would ride up off the ground over shrank and separated from the low center as the the cold front as the occluded front formed. This cyclone aged, implying that the static model of extra- type of structure is called a cold-type occlusion tropical cyclones presented by Jacob Bjerknes needed (Figure 1a). In contrast, if the polar air ahead of revision. Bergeron noticed that the faster-moving the warm front was colder than the polar air behind cold front approached, and eventually appeared to the cold front, then the cold front would be lifted

349 350 occluded fronts

ahead of the warm front carry the warm front T T T+dT around the low center, forming a so-called back- T+dT T+2dT T+2dT bent front. [See Cyclones, subentry on Explosive (a) (b) Cyclones.] By generalizing the occlusion process to OCCLUDED FRONTS. Figure 1. Idealized models of occluded one in which the warm sector air is separated from fronts: (a) cold type and (b) warm type. The sketches represent the low center and the low center becomes wrapped frontal surfaces (solid line) and isotherms (dashed line) in in cold air, these other cyclone evolutions can also vertical cross sections normal to occluded fronts, which are be viewed as undergoing the occlusion process. moving from left to right. (From Wallace and Hobbs, 1977, The second criticism is that observations of p. 128. Copyright 1977 by Academic Press.) occluded fronts show that the temperature struc- ture across the occluded front does not determine off the ground by the warm front, thus forming a whether a warm-type or cold-type occlusion forms. warm-type occlusion (Figure 1b). In fact, few, if any, examples of cold-type occlusions Challenges to Polar Front Theory. After this exist, despite the temperature being colder behind theory was proposed, serious criticisms were leveled the cold front than ahead of the warm front. Instead, against the polar front theory of occlusion. These recent research suggests that whether the resultant criticisms can be grouped into four categories. structure is a cold or warm occluded front is con- The first category of criticisms encompasses trolled by the difference in static stability across the objections that the structure of the ideal occluded occluded front, not the difference in temperature. front could be duplicated without the cold front Thus, because warm frontal zones are characterized ever joining the warm front. Indeed, questions by greater static stability than cold frontal zones, arose about how representative the frontal catch- the resulting occluded front is more likely to be a up model was for most midlatitude cyclones, and warm-type occluded front with the cold frontal zone whether the occluded stage of a midlatitude cyclone being lifted over the warm frontal zone. could be arrived at by other means. In particular, The third criticism is that the occlusion process the cyclones occurring in the lee of the Rocky does not represent the end of development of Mountains sometimes possessed the structure of an a midlatitude cyclone. Indeed, many midlatitude occluded cyclone without undergoing the catch-up cyclones, especially rapidly developing ones, continue of the warm front by the cold front. In other exam- to deepen after forming an occluded front. It is now ples, satellite investigations showed that occlusion- recognized that the process of occlusion is distinct like features could form without catch up of the two from the process of cyclone deepening. In fact, only fronts. In a process called instant occlusion, a polar cyclones that undergo deepening are likely to cloud vortex approaches and merges with a polar undergo occlusion. front cloud band, forming a single comma-shaped The fourth criticism is that the cloud and precipi- cloud mass out of two distinct cloud masses. The tation structure of an occluded front is more com- resulting structure has similarities with the classical plicated than the simple picture of the merger of a occluded cyclone without ever having undergone cold front and warm front suggests. For example, the catch up. A third example comes from rapidly mesoscale observations of occlusions in the 1960s developing cyclones over the oceans, which often and 1970s by Carl Kreitzberg led to the discovery undergo a different frontal evolution from the polar that the structure of occlusions was more compli- front model (the Shapiro–Keyser cyclone model). cated than originally believed. He identified a [See Cyclones, subentry on Midlatitude Cyclones.] prefrontal surge of dry air above the warm front, In these oceanic cyclones, the cold front separates warm tongues of air associated with locally heavy from and then moves perpendicularly to the warm precipitation, and secondary cold fronts behind the front, and hence never catches up. The strong winds primary surface occluded front. Kreitzberg also occluded fronts 351 found that younger occlusions often extended surge. After the passage of the prefrontal surge or vertically throughout the lower troposphere (atmo- the surface occluded front, rapid clearing usually spheric region closest to Earth) and that they split ensues as the dry airstream aloft appears. Tempera- to form features such as squall lines, secondary cold tures at the surface usually remain nearly constant fronts, and rain bands. In contrast, the older occlu- or reach a slight maximum during the passage of sions often dissipated, never reaching the ground. the occluded front. Winds at the surface are usually Thus, the clouds and precipitation associated with from an easterly or southerly direction ahead of occluded fronts are more complicated than the the occluded front and shift to westerly or north- polar front theory. erly after its passage. The dew-point temperature Modern Perspective. Taking into account these (a direct measure of the amount of moisture in criticisms, a modern view of the formation of an the air) usually decreases with the passage of an occluded front can be described as follows. If the occluded front. cyclone becomes intense enough, its structure may On the western coast of the United States, the begin to change. As the cyclone intensifies, it draws polar air behind an occluded front comes from the warm and cold air around its circulation. As the ocean and is warmer and moister than the conti- fronts rotate around the cyclone center, they nental air ahead of the system, and so the tempera- lengthen and approach each other. As cold air ture and dew point will often rise behind an encircles the low and the warm air is lifted from the occluded front coming in off the Pacific Ocean. surface over the warm front, the warm sector narrows. This is one example of the ways local topography Eventually, the two cold air masses meet and lift and geography can affect the kind of weather asso- all the intervening warm air aloft; the remaining ciated with occluded fronts. boundary is the occluded front. The occluded front continues to be lengthened by rotation and defor- [See also Fronts; and Occlusion.] fl mation of the ow around the cyclone. BIBLIOGRAPHY In some cases, the dry airstream from the upper Carlson, T. N. Mid-latitude Weather Systems. Boston: troposphere (also called the dry slot) may descend American Meteorological Society, 1998. over the occluded front. The dry airstream usually Friedman, R. M. Appropriating the Weather: Vilhelm represents the westernmost limit to clouds and Bjerknes and the Construction of a Modern Meteorology. precipitation associated with the cyclone. [See Ithaca, N.Y.: Cornell University Press, 1989. Jewell, R. “Tor Bergeron’s First Year in the Bergen Cyclones, subentry on Midlatitude Cyclones.] School: Towards an Historical Appreciation.” In Weather. Occluded fronts have been described as Weather and Weather Maps: A Volume Dedicated to “back-to-back front,” with the weather ahead of the the Memory of Tor Bergeron, edited by G. H. Liljequist, occluded front being similar to that in advance of a pp. 474–490. Boston: Birkhäuser Verlag, 1981. warm front, and the weather behind it being similar Kreitzberg, C. W., and H. A. Brown. “Mesoscale Weather ” to that behind a cold front. Such an expression Systems within an Occlusion. Journal of Applied Meteorology 9 (1970): 417–432. is not an accurate way to describe the weather Kuo, Y.-H., R. J. Reed, and S. Low-Nam. “Thermal Struc- associated with occluded fronts, however. A few ture and Airflow in a Model Simulation of an hundred kilometers ahead of the occluded front, Occluded Marine Cyclone.” Monthly Weather Review the highest cirrus clouds are first visible. As the 120 (1992): 2280–2297. occluded front approaches, the height of the cloud Newton, C. W., and E. O. Holopainen, eds. Extratropical base lowers and the clouds thicken and become Cyclones: The Erik Palmén Memorial Volume. Boston: American Meteorological Society, 1990. darker. Persistent rain, freezing rain, sleet, and/or Reed, R. J., Y.-H. Kuo, and S. Low-Nam. “An Adiabatic snow may fall from nimbostratus clouds, and heavier Simulation of the ERICA IOP 4 Storm: An Example of rain and thunderstorms may be embedded along Quasi-Ideal Frontal Cyclone Development.” Monthly the elevated warm front and at the prefrontal Weather Review 122 (2004): 2688–2708. 352 occluded fronts

Schultz, D. M., and C. F. Mass. “The Occlusion Process in a Mid-latitude Cyclone over Land.” Monthly Weather Warm Review 121 (1993): 918–940. Schultz, D. M., and G. Vaughan. “Occluded Fronts and Cold Cool the Occlusion Process: A Fresh Look at Conventional Wisdom.” Bulletin of the American Meteorological Society, submitted. Low Stoelinga, M. T., J. D. Locatelli, and P. V. Hobbs. “Warm Occlusions, Cold Occlusions, and Forward-Tilting Cold Fronts.” Bulletin of the American Meteorological 1000 Cool Society 83 (2002): 709–721. Cold Wallace, J. M., and P. V. Hobbs. Atmospheric Science: An 1004 Introductory Survey. Burlington, Mass.: Academic Press, 1977. 1008 Warm David M. Schultz (a) Occlusion fi The Norwegian cyclone model was the rst to Warm describe the evolution of midlatitude cyclones. Cool In this conceptual model, cyclones are disturbances Cold that develop along fronts. In a developing storm there are two fronts: a warm front preceded by (b) warm-air advection, and a cold front followed by OCCLUSION. Figure 1. (a) A cross-sectional illustration of cold-air advection. The warm front is ahead of the an occluded front. First-order discontinuity in the pressure cold front; however, owing to stronger steering winds field is represented by the isobars in this cold-type occlusion. associated with the cold front, the cold front travels (b) A cross-sectional illustration of a warm-front type occlusion. faster than the warm front. In mature cyclones, the (Adapted from Cole, 1980.) part of the cold front nearest the storm catches up to the warm front, forming what is known as an reducing the storm’s forward speed. When an occlu- occluded front; the process by which an occluded sion occurs, the warm air near the surface storm front forms is referred to as occlusion (see Figure 1). center is forced aloft and is replaced by colder air. The intersection of the occluded front, cold front, According to the Norwegian model, when the warm and warm front is known as the triple point. This air is forced aloft this stabilizes the atmosphere, classical theory of how an occluded front forms has and steady precipitation is cut off and replaced by been controversial since it was first published in drizzle. Unlike the case around cold fronts and warm 1922, and alternative theories have since been pro- fronts, no strong thermal gradients exist adjacent to posed (e.g., Davies, 1997) [see Occluded Fronts]. an occluded front. The air behind an occluded front It is believed that an occlusion usually signals that can be either colder (referred to as a cold occlusion) the storm has finished developing and is beginning or warmer (a warm occlusion) than the air ahead. to decay, though the data seem to show that storms However more recent studies question the assump- continue to deepen even after the occlusion. Some- tions of the Norwegian model. For example heavy times a secondary storm will form at the triple precipitation bands do occur even after the occlu- point. Occluded cyclones also tend to be vertically sion process, occluded fronts can have strong ther- stacked or aligned (that is, the surface cyclone lies mal gradients (e.g., Shultz and Vaughn 2010) [see directly beneath the upper-troposphere storm), Cyclogenesis; Fronts; and Occluded Fronts].