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. above the surface, made it possible to analyze the Yet another approach to understanding cyclogen- structure and motions within the jet stream. Dis- esis was pioneered by Jule Charney in 1947 and Eric turbances in the jet stream, called jet streaks and Eady in 1949. This theoretical approach states that shortwave troughs (Rossby waves), are associated cyclones are the result of an instability in the with convergence and divergence. Regions of diverg- jet stream called baroclinic instability. Baroclinic ing air at the level of the jet stream are favorable instability theory links the observational approach locations for surface cyclones to form owing to to understanding cyclones from polar front theory evacuation of air in the column. Therefore, for a and the practical approach of Sutcliffe and Petters- surface cyclone to deepen, the divergence of air sen. Baroclinic instability theory states that if the aloft must be greater than the convergence of air temperature gradient is large enough (or equiva- into the low-pressure center near the surface. Diver- lently, if the vertical shear of the horizontal wind gence aloft tends to occur on the east side of a is large enough), then the jet stream will spontane- trough, making that region favorable for surface ously break down into Rossby waves, resulting in the cyclone development. [See Jet Stream.] formation of cyclones. In most observed cases, dis- As the vertical structure of cyclones and their turbances in the jet stream appear to be linked with relationship to the jet stream became better under- surface cyclogenesis, suggesting the validity of bar- stood, practical means were explored for determin- oclinic instability as an explanation for cyclogenesis ing whether a cyclone would intensify or weaken. in the atmosphere. In addition, baroclinic instability The most significant contribution during the 1940s theory is often used for theoretical studies of cyclo- and early 1950s came from two European meteorol- genesis, providing further support for its utility in ogists, Reginald Sutcliffe and Sverre Petterssen. explaining observations of cyclogenesis. [See Baro- They found that as a trough in the jet stream clinic Instability.] (a region of cyclonic vorticity advection aloft) and Another way of viewing the structure of cyclones its associated cyclonic flow move over a low-level is to depict the different airstreams that flow cyclones Midlatitude Cyclones 347 through the cyclone. This view, pioneered by Jerome Prerequisites for cyclone development include a Namias in the late 1930s, became popular in the mid lower-tropospheric frontal zone and an upstream to late 1960s. Instead of looking at discontinuities in upper-tropospheric disturbance, usually a jet streak temperature (fronts), a more holistic view examines or a shortwave trough in the jet stream. The upper- the different source regions of the air flowing level disturbance generally moves faster than the through the cyclone. This airstream model yields surface frontal zone, so the upper-level disturbance three main airflows in midlatitude cyclones: the will move over the frontal zone. Cyclonic flow asso- warm conveyor belt, the cold conveyor belt, and the ciated with the upper-level disturbance will deform dry airstream. the surface frontal zone, forming a weak surface The warm conveyor belt originates in the tropical low-pressure system. The cyclonic flow induced air in the warm sector and rises up over the warm from the upper-level disturbance will cause warm front into the jet stream. The warm conveyor belt is air to the south of the frontal zone to be advected responsible for most of the clouds and precipitation northward, east of the low center. The movement of associated with cyclones. The cold conveyor belt warm air replacing cold air forms a warm front. originates in the lower troposphere in the cooler Likewise, on the west side of the low center, cold air ahead of the cyclone, travels westward under- air to the north will be advected southward, repla- neath the warm conveyor belt, and then turns cing the warm air and forming a cold front. [See cyclonically around the low center. The dry air- Fronts.] If the cyclone is strong enough, the move- stream originates in the middle and upper tropo- ment of air around the cyclone eventually stretches sphere west of the cyclone and then descends the cold front and warm front, bringing them closer behind the cyclone. The dry airstream provides the together, just as ribbons of milk lengthen and merge westward limit to most of the clouds and precipita- when stirred into coffee. As the air in the cold tion in a midlatitude cyclone. conveyor belt wraps around the low center and the Finally, a recent way of viewing the atmosphere is air in the warm conveyor belt is lifted over the warm to examine the structure of the potential vorticity front, the amount of warm air near the cyclone field. Sutcliffe–Petterssen self-development or baro- center is reduced and the surface cyclone becomes clinic instability theory can be viewed in the frame- wrapped in cold air. Around this time or shortly work of potential vorticity as a region of locally high after, the upper-level disturbance catches up to the potential vorticity (a depression of the tropopause), surface cyclone, and the three-dimensional struc- which approaches another region of high potential ture of the low-center becomes vertically stacked. vorticity (a lower tropospheric area of warm air). The cloud pattern of a midlatitude cyclone is The induced cyclonic circulation associated with typically in the shape of a comma. The head of the the tropopause depression causes the deformation comma is nearly coincident with the low-pressure of the warm pool near the surface, in turn strength- center at the surface. Warm rising air in the warm ening the tropopause depression. The cyclone there- conveyor belt is responsible for most of the clouds fore develops by mutual amplification of potential and precipitation in the comma head. Steady precip- vorticity anomalies on the tropopause and near itation, often with embedded regions of heavier pre- the surface. When moisture is present, a third cipitation, falls out of the clouds ahead of the warm potential vorticity anomaly may form beneath re- front. As the warm front approaches, surface tem- gions of condensation. The formation of this anom- peratures rise. In the warm air, skies may be clear or aly and its associated cyclonic flow can enhance the partly cloudy, or they may have scattered showers intensity of the surface cyclone. [See Potential and thunderstorms. The tail of the comma is often Vorticity.] associated with convection that forms along a line Life Cycle. The current view of the life cycle extending equatorward from the low center. This of a midlatitude cyclone is illustrated in Figure 1. line may sometimes be associated with the passage 348 cyclones Midlatitude Cyclones

of the cold front or occluded front, producing heavy to intensify, the back-bent front encircles the rela- precipitation. Following the cold frontal passage, tively warmer air behind the cold front, resulting in skies clear and surface temperatures fall as the a pool of warm air over the low center, known as the winds shift from the south to the west and north. warm seclusion. This cyclone evolution is called the Geographical Variability. Midlatitude cyclones Shapiro–Keyser cyclone model.[See Cyclones, suben- occur in many midlatitude locations around the try on Explosive Cyclones.] world, but they tend to move along preferential Modeling results suggest that the roughness of routes called storm tracks. In the Northern Hemi- the Earth’s surface may affect the types of frontal sphere, two primary storm tracks lie across the structures that arise. For instance, when surface North Atlantic Ocean and the North Pacific Ocean. friction is high, as it is over land, cyclones tend to In contrast, cyclones in the Southern Hemisphere undergo an evolution more consistent with the most commonly travel within a single storm track polar front cyclone model. When the surface friction around the Southern Ocean, best defined over the is lower, as it is over the ocean, the cyclone tends to southern Indian Ocean and least well defined over develop features more akin to the Shapiro–Keyser the South Pacific Ocean. In the Northern Hemi- cyclone model. sphere, a large number of cyclones generally inten- Research suggests that the shape of the jet stream sify at the entrance region (western end) of the over the surface cyclone also affects the resulting storm tracks off the east coasts of North America frontal structure. In cases where the jet stream is and Asia, travel across the oceans, and weaken at diffluent, warm fronts are short and weak while cold the end of the storm tracks over the eastern ocean fronts are long and strong. These cyclones tend to basins. Although most cyclones follow these storm resemble the polar front cyclone model. In conflu- tracks and look like cyclones in the polar front ent flow, the warm fronts are long and strong and theory, individual cyclones may differ substantially the cold fronts are short and weak. These cyclones from this conceptual model. A few examples of tend to have structures like the Shapiro–Keyser these differences are discussed next. cyclone model. Since 1980, the meteorological community has Because of the Rocky Mountains, a developing low- placed particular emphasis on understanding rap- pressure center in central North America may be idly developing ocean cyclones, which have been inhibited from developing in the same manner as named bombs, and are often poorly forecast. In the an ideal cyclone. Such cyclones develop most often late 1980s, several field projects began discovering in Colorado or Alberta, where the slope of the Rockies unusual frontal structures in these cyclones. Cyclo- is steepest. The cyclones that develop here are likely genesis appears to be initiated much as described to exhibit certain structures (Figure 3). South of the above, but instead of the cold front rotating into the low center, a lee trough separates warm, moist, warm front to form an occluded front, the cold front southerly air to the east from warm, dry air that has breaks (or fractures) from the warm front and be- recently descended off the mountains. A lee trough gins to move perpendicularly to the warm front, so has a structure very similar to that of a warm front. that it never catches up (Figure 2). The rapid move- Depending on the amount of moisture ahead of the ment of the surface low center also results in the lee trough, the lee trough may also resemble a dryline, warm front being left behind the cyclone in the form a type of air-mass boundary in the south-central of a back-bent front. A region of strong localized United States that is often a locus of severe weather. surface winds can sometimes occur in association Southwest of the cyclone, a cold front separates the with the back-bent front and is called the sting jet. subsided air off the mountains from moist Pacific When they occur, sting jets can cause extensive Ocean air. Northwest of the low center, a cold front wind damage, especially in the United Kingdom occurs at the leading edge of southward-moving and continental Europe. As the cyclone continues arctic air trapped against the Rockies. North of the cyclones Midlatitude Cyclones 349

IV

III

L

II L

I L

L

Warm

CYCLONES: MIDLATITUDE CYCLONES. Figure 2. An alternative model of frontal-cyclone evolution: Incipient broad-baroclinic phase (I), frontal fracture (II), bent-back front and frontal T-bone (III), and warm-core frontal seclusion (IV). Upper: sea-level pressure (solid), fronts (bold), and cloud pattern (shaded). Lower: temperature (solid), and cold and warm air currents (solid and dashed arrows, respectively). (From Shapiro and Keyser, 1990, p. 188. Copyright 1990 by the American Meteorological Society.) low center, an inverted trough separates easterlies In desert areas during the summer, intense solar over the midwestern states from the northerly arctic heating and the lack of moisture available for evapo- air against the Rockies. Often a quasi-stationary or ration can lead to very high surface temperatures warm front south of the easterlies separates the (higher than about 35C). As the air warms, it warm moist southerly air from the Gulf of Mexico. expands, and compensating circulations arise that Finally, a squall line in the warm southerly air is often remove mass from the column of air. As a result, the associated with an upper-level frontal zone advanc- pressure falls. These low-pressure centers are not ing above; the term cold front aloft has sometimes associated with the polar front and jet stream and been applied to this feature. These cyclones differ are usually not migratory. They are called thermal substantially from the polar front and Shapiro– lows or heat lows to indicate their method of forma- Keyser cyclone models presented earlier. tion, and they are distinct from midlatitude cyclones. 350 cyclones Midlatitude Cyclones

Fresh polar/artic H air H H 50 50 Inverted Inverted trough trough H Jet axis

Older modified Fresh polar Polar/arctic Older air Surge air modified line polar air L Jet axis

Pacific air Cyclone track Subsided Gulf Pacific (mixed) air Subsided cold air Developing 30 front 30 pacific Dry warm front air Squall line Gulf Polar/arctic Pacific line -110 air -90 -110 front cold front -90 (a) (b) CYCLONES: MIDLATITUDE CYCLONES. Figure 3. Schematic of cyclogenesis east of the Rockies when an inverted trough is present for (a) the initial time and (b) some later (about 24 hours) time. Solid lines denote mean sea-level isobars. Principal frontal boundaries are denoted by conventional symbols with the inverted trough appearing as a dashed line. Approximate position of the jet stream is shown by the dotted line in (b). The squall line is indicated by a dash–dot line. Estimated surface air trajectories within the labeled air masses are denoted by the hatched arrows.

As their name suggests, midlatitude cyclones important, about the causes of these structural (whose energy is derived from the pole-to-equator and developmental differences. temperature gradient) are typically distinct from cyclones in the tropics (whose energy is derived [See also Storms.]

from the release of latent heat of condensation). BIBLIOGRAPHY Sometimes, however, tropical cyclones may transi- Ahrens, C. D. Meteorology Today: An Introduction to tion into midlatitude cyclones as they move pole- Weather, Climate, and the Environment. 3d ed. ward. [See Cyclones, subentry on Tropical Cyclones.]. St. Paul, Minn.: West, 1988. Because of the variety of topography and geogra- Bluestein, H. B. Synoptic-Dynamic Meteorology in Midlat- phy on the Earth, midlatitude cyclones across the itudes. New York: Oxford University Press, 1993. Carlson, T. N. Mid-latitude Weather Systems. Boston: world possess a great variety of frontal structure American Meteorological Society, 1998. and evolutions. For example, the Gulf Stream and Davies, H. C. “Emergence of the Mainstream Cyclogene- Kuroshio ocean currents are an important source of sis Theories.” Meteorologische Zeitschrift 6, no. 6 the low-level temperature gradients and low static (1997): 261–274. stability needed for rapid cyclone development. Friedman, R. M. Appropriating the Weather: Vilhelm In another example, off the south coast of Australia Bjerknes and the Construction of a Modern Meteorol- ogy. Ithaca, N.Y.: Cornell University Press, 1989. and in the center of the Pacific Ocean, cyclones Keshishian, L. G., L. F. Bosart, and W. Bracken. “Inverted usually develop without strong warm fronts. Much Troughs and Cyclogenesis over Interior North America: remains to be learned about how midlatitude A Limited Regional Climatology and Case Studies.” cyclones vary around the world, and more Monthly Weather Review 122, no. 4 (1994): 565–607. cyclones Subtropical Cyclones 351

Kocin, P. J., and L. W. Uccellini. Northeast Snowstorms 420 kilometers (300 miles) from the center. Unlike (Volume I: Overview, Volume II: The Cases). Boston: tropical cyclones, subtropical cyclones often American Meteorological Society, 2004. have large centers, as much as 140 kilometers Kutzbach, G. The Thermal Theory of Cyclones: A History (100 miles) in diameter. Within this zone, precipita- of Meteorological Thought in the Nineteenth Century. Boston: American Meteorological Society, 1979. tion is light and pressure gradients are weak. Neiman, P. J., and M. A. Shapiro. “The Life Cycle of an While tropical cyclones depend on latent and Extratropical Marine Cyclone. Part 1: Frontal-Cyclone sensible heat as driving mechanisms, subtropical Evolution and Thermodynamic Air–Sea Interaction.” storms develop from cold upper-level polar troughs – Monthly Weather Review 121, no. 8 (1993): 2153 2176. (as do extratropical storms). Occasionally the south- Newton, C. W., and E. O. Holopainen, eds. Extratropical ern portion of an upper-level trough “cuts off ” and Cyclones: The Erik Palmén Memorial Volume. Boston: American Meteorological Society, 1990. develops an upper-level cold-core low. If this circu- Palmén, E., and C. W. Newton. Atmospheric Circulation lation extends to the surface, the development of a Systems: Their Structure and Physical Interpretation. subtropical storm is initiated. Although the original New York: Academic Press, 1969. polar trough from which a subtropical storm devel- “ Sanders, F., and J. R. Gyakum. Synoptic-Dynamic Cli- ops has most of its precipitation east of its axis, matology of the ‘Bomb’.” Monthly Weather Review 108, subtropical storms themselves are marked by a no. 10 (1980): 1589–1606. Schultz, D. M., D. Keyser, and L. F. Bosart. “The Effect of high degree of symmetry. Large-Scale Flow on Low-Level Frontal Structure and Once formed, these storms are noted for their high Evolution in Midlatitude Cyclones.” Monthly Weather level of persistence, a result of their being well devel- Review 126, no. 7 (1998): 1767–1791. oped at upper levels (for example, a closed cyclonic “ Schultz, D. M., and G. Vaughan. Occluded Fronts and circulation at 500 millibars) while becoming progres- the Occlusion Process: A Fresh Look at Conventional sively weaker toward the surface. Thus, the effect of Wisdom.” Bulletin of the American Meteorological Society. friction is small; in tropical cyclones, by contrast, Shapiro, M. A., and S. Grnäs, eds. The Life Cycles of friction plays a major role in dissipation over land. Extratropical Cyclones. Boston: American Meteorolog- Rather than dissipating, subtropical storms are often ical Society, 1999. absorbed into advancing polar troughs. “ Shapiro, M. A., and D. Keyser. Fronts, Jet Streams, and In some regions, subtropical storms are an integral the Tropopause.” In Extratropical Cyclones: The Erik part of the hydrological cycle. For example, in Hawaii Palmén Memorial Volume, edited by C. W. Newton and E. O. Holopainen, pp. 167–191. Boston: American the subtropical storm known locally as the Kona Meteorological Society, 1990. storm provides a large portion of the winter rainfall. Steenburgh, W. J., and C. F. Mass. “The Structure and Most subtropical storms form from upper-level Evolution of a Simulated Rocky Mountain Lee Trough.” cold-core lows, but there are also other modes of – Monthly Weather Review 122, no. 12 (1994): 2740 2761. formation. For example, a hurricane that moves David M. Schultz inland can change into a subtropical storm as part of its decay process. This often produces more pro- longed and intense rainfall than would a dissipating, Subtropical Cyclones purely tropical system. A subtropical storm can also Subtropical cyclones have characteristics similar to become converted to a tropical system when warm, those of extratropical and tropical cyclones, but moist air flows closely around the center. Rainfall, unlike true tropical storms, subtropical storms can which had been heavy on the storm’s periphery, slack- occur at any time of the year. Because they are ens as a new enhanced area of rainfall develops hybrid storms, it is difficult to define consistent close to the center. Fluxes of latent heat now increase physical characteristics for them. near the center. The net result is an increase in tem- Most subtropical storms have their maximum perature in the center and conversion to a warm-core intensity of rain and wind approximately tropical system. For this reason, subtropical