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344 cyclones Explosive Cyclones Mullen, S. L., and P. Baumhefner. “Sensitivity of Numeri- Wash, C. H., et al. “Study of Explosive and Nonexplosive cal Simulations of Explosive Oceanic Cyclogenesis Cyclogenesis during FGGE.” Monthly Weather Review to Changes in Physical Parameterizations.” Monthly 120, no. 1 (1992): 40–51. Weather Review 116, no. 11 (1988): 2289–2329. Whitaker, J. S., I. W. Uccellini, and K. F. Brill. “A Model- Nuss, W. A. “Air-Sea Interaction Influences on the Based Diagnostic Study of the Rapid Development Structure and Intensification of an Idealized Marine Phase of the President’s Day Cyclone.” Monthly Cyclone.” Monthly Weather Review 117, no. 2 (1989): Weather Review 116, no. 11 (1988): 2337–2365. 351–369. Orlanski, I., and J. J. Katzfey. “Sensitivity of Model Simu- Judah Cohen lations for a Coastal Cyclone.” Monthly Weather Review 115, no. 11 (1987): 2792–2821. Petterssen, S., and S. J. Smebye. “On the Development of Midlatitude Cyclones Extratropical Cyclones.” Quarterly Journal of the Royal Midlatitude cyclones (also called extratropical Meteorological Society 97 (1971): 457–482. Roebber, P. J. “Statistical Analysis and Updated Clima- cyclones, or simply cyclones in the rest of this article) tology of Explosive Cyclones.” Monthly Weather are nearly circular regions of reduced surface Review 112, no. 8 (1984): 1577–1589. pressure that generally range in diameter from a Rotunno, R., and M. Fantini. “Notes and Correspon- few hundred to a few thousand kilometers and ’ ‘ ’ dence: Petterssen s Type B Cyclogenesis in Terms occur in association with the jet streams in the ” of Discrete, Neutral Eddy Modes. Journal of Atmo- – – middle-latitude regions of the globe (roughly 30 spheric Sciences 46 (1989): 3599 3604. Sanders, F. “Explosive Cyclogenesis in the West-Central 70 latitude). Cyclones derive their energy from the North Atlantic Ocean, 1981–1984. Part 1: Composite potential energy in the pole-to-equator temperature Structure and Mean Behavior.” Monthly Weather gradient. This temperature gradient can become Review 114, no. 10 (1986): 1781–1794. concentrated within zones called fronts where the “ Sanders, F., and E. P. Auciello. Skill in Prediction temperature changes rapidly and the wind abruptly of Explosive Cyclogenesis over the Western North shifts direction. Winds around a cyclone blow coun- Atlantic Ocean, 1987/88: A Forecast Checklist and NMC Dynamical Models.” Weather and Forecasting terclockwise in the Northern Hemisphere and 4, no. 2 (1989): 157–172. clockwise in the Southern Hemisphere, transporting Sanders, F., and J. R. Gyakum. “Synoptic-Dynamic Cli- warm air poleward and cold air equatorward. Con- matology of the ‘Bomb.’” Monthly Weather Review 108, sequently, cyclones are one means by which heat – no. 10 (1980): 1589 1606. is transported from the tropics to the poles. Because Uccellini, L. W., and P. J. Kocin. “The Interaction of Jet cyclones are the primary source of most winter Streak Circulations during Heavy Snow Events along the East Coast of the United States.” Weather and precipitation in the midlatitudes, understanding Forecasting 2, no. 4 (1987): 289–308. the structure and dynamics of cyclones can lead to Uccellini, L. W., et al. “The President’s Day Cyclone of improved weather forecasts. 18–19 February 1979: A Subsynoptic Overview and History of Research. One of the earliest theories fl Analysis of the Subtropical Jet Streak In uencing of cyclone formation, the thermal or convectional the Pre-Cyclogenetic Period.” Monthly Weather Review theory, was based on James Espy’s work in the 112, no. 1 (1984): 31–55. Uccellini, L. W., et al. “The President’s Day Cyclone of 1840s. Espy argued that, as an organized mass of 18–19 February 1979: Influence of Upstream Trough clouds forms, the release of latent heat of condensa- Amplification and Associated Tropopause Folding on tion in the clouds causes warming, resulting in a Rapid Cyclogenesis.” Monthly Weather Review 113, no. decrease in pressure within the air column. This – 6 (1985): 962 988. decrease in surface pressure leads to increased Uccellini, L. W., et al. “Synergistic Interactions between inflow of warm, moist air in the lower troposphere an Upper Level Jet Streak and Diabatic Processes That Influence the Development of a Low-Level Jet and then to further pressure falls upon condensa- and a Secondary Coastal Cyclone.” Monthly Weather tion. Mounting observational evidence indicated Review 115, no. 10 (1987): 2227–2261. that many cyclones were not warm at mid-levels cyclones Midlatitude Cyclones 345 as the thermal theory predicts, but cold. By the early verifiable representation of midlatitude cyclones, 1900s, the stage was set for one of the most pro- something that had not been developed before. found developments in meteorology—the polar Polar front theory held that the polar front, initi- front theory of cyclones (also called the Norwegian ally a straight (linear) feature, may spontaneously cyclone model). produce small perturbations, or waves (Figure 1). The polar front theory for midlatitude cyclones As the polar front becomes deformed by one of was developed at the Geophysical Institute in these waves, a weak cyclonic circulation causes Bergen, Norway, headed by Vilhelm Bjerknes. In a warm tropical air to move poleward and cold polar series of landmark papers published just after World air to advance equatorward. The cold front rotates War I, Jacob Bjerknes, Halvor Solberg, and Tor around the cyclone more rapidly than the warm Bergeron developed a model for cyclone structure, front, eventually catching up to the warm front based on data collected within numerous cyclones. and forming an occluded front. With the formation Their results built upon the work of Sir William of an occluded front, the cyclone center becomes Napier Shaw, Max Margules, Felix Exner, and other surrounded by cold polar air (also known as the earlier researchers who recognized that cyclones occlusion process). As development of the cyclone possessed discontinuities in wind and temperature is contingent upon the conversion of potential (later called fronts by Bjerknes’s group). Polar front energy in the temperature gradient to kinetic energy theory was an advance over previous models of of the cyclone, the cyclone weakens after occlusion. cyclones for three reasons. First, polar front theory Therefore, the occlusion process, J. Bjerknes and described for the first time the life cycle of cyclones Solberg argued, represents the beginning of the on the polar front, a globe-encircling boundary decay phase of the cyclone. [See Occluded Fronts.] between cold polar air and warm tropical air. The Although polar-front theory was a monumental Bergen meteorologists argued that cyclones are advance, several aspects of the theory were not sup- not unchanging features moving across the Earth; ported by observations of cyclones. First, cyclones, instead, they are born, mature, and die. Second, especially those that deepen rapidly, often continue polar front theory argued that the potential energy to deepen after the occluded front forms. Thus, the in the temperature gradient across the polar front occlusion process is not the end of deepening, as provides the energy for cyclones, not the latent heat the Bergen meteorologists had described. Instead, release due to condensation. Third, polar front the- an explanation for cyclone development would ory represented a simple, elegant, practical, and await further theoretical advances, described below. Cold L (−20) L 2 1 (−10) 500 500 500 millibars millibars millibars Shortwave trough Cold Warm L Cold H L Cold Warm North Warm North North (a) Surface(b) Surface (c) Surface CYCLONES: MIDLATITUDE CYCLONES. Figure 1. The formation of a wave cyclone during self- development. (a) A short-wave trough (heavy dashed line) disturbs the flow aloft, enhancing temperature advection. (b) The trough intensifies and provides the necessary vertical motions for the development of the surface cyclone. (c) The surface cyclone occludes, and a cold pool of air remains above it. (Adapted from Ahrens, 1988, p. 381. Copyright 1988 by West Publishing Company.) 346 cyclones Midlatitude Cyclones Second, the catch up of the cold front by the warm thermal gradient (a frontal zone), cyclonic flow is front does not occur in all cyclones, nor does it induced at the surface. The weak circulation about explain the length of highly spiraled occluded the frontal zone causes deformation of the frontal fronts. Instead, the occlusion process is best viewed zone, resulting in warm air advection ahead of the as the wrap up of the thermal pattern into a spiraled surface cyclone and cold air advection behind. The front, a result of the deformation and rotation in warm advection leads to decreasing surface pres- the flow around the cyclone center. Third, although sure ahead of the cyclone, and hence the surface the Norwegian cyclone model advocates a close asso- cyclone propagates forward. The warming of the ciation between clouds/precipitation and surface air column ahead of the cyclone also builds the fronts, clouds and precipitation are often related to downstream ridge and causes the wave to amplify, processes occurring aloft, not to the surface fronts. thereby increasing the amount of cyclonic vorticity New theories to explain these and other discrepancies advection aloft, leading to further warm advection, between polar front theory and observations of and so on. This “bootstrapping” process is referred cyclones have been proposed and are being evaluated. to as self-development. Eventually, the strength a [See Occluded Fronts.] cyclone can attain through self-development is lim- The next major advance in understanding midlat- ited by the opposing influence of vertical motion, itude cyclones occurred after the discovery of the jet which cools the rising air ahead of the system and stream, a narrow region of high winds in the upper limits the magnitude of the pressure falls. Sutcliffe troposphere. In the late 1930s, the global release and Petterssen also showed that the strength of of instrumented weather balloons, which regularly cyclogenesis depends on the local static stability of measure the temperature, humidity, and winds the atmosphere.

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