A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.
The behaviour of initially-barotropic vortices in vertically-sheared environmental flows is investigated. The strength and structure of the vortices used are representative of tropical cyclones. The calculations are performed using a primitive-equation numerical model on an f-plane. It is found that the initial response of the vortex to the vertical shear is to tilt in the plane of the shear. As soon as a tilt is established, the upper-and lower-level centres begin to rotate cyclonically about the mid-level centre. This rotation can be understood in terms of upper-and lower-level potential-vorticity anomalies which are displaced in the horizontal relative to one another. The flow associated with the vertical projection of each anomaly advects the other anomaly, leading to the observed cyclonic rotation. The rotation rate decreases with time, so that the direction of tilt becomes constant, but the magnitude of the tilt continues to increase. We argue that the observed rotation acts to oppose the destructive action of the vertical shear on the vortex, even in the absence of diabatic processes.The role of the vertical circulation is considered in detail. It is shown that the vertical circulation develops in a manner which is consistent with the model flow remaining balanced. It is found that the mesoscale nature of the vertical circulation leads to a distortion of the axisymmetric vortex. This results in the inner core having a smaller vertical tilt than the outer region. The vertical circulation does not act on a large enough scale to explain why the vortex is not destroyed by the vertical shear.The behaviour of the vortex is found to depend on various parameters. Results are presented where the vertical shear, the strength and size of the vortex, the Coriolis parameter, and the static stability are varied. With the exception of the vertical shear, altering any of these parameters alters the vertical penetration of a potentialvorticity anomaly. The results show that increasing the penetration depth leads to an increase in the rotation rate of the upper-and lower-level vortex centres about the mid-level centre, and to a reduction in the magnitude of the vertical tilt.
This study highlights the importance of diabatic processes for the complex interaction of weather systems in the North Atlantic-European sector during the week of 7-14 September 2008. A chain of events occurred including the extratropical transition (ET) of hurricane Hanna, a subsequently developing extratropical cyclone, the formation of an upper-level potential vorticity (PV) streamer that protruded towards Europe and triggered intense rainfall, and the genesis of a Mediterranean cyclone. A PV perspective is adopted along with trajectory calculations to elucidate the diabatic modification of the midlatitude flow.Important diabatic PV modifications occurred at upper levels, associated with the cross-isentropic transport of low-PV air within warm conveyor belts (WCBs). These were diagnosed during the ET of Hanna and the development of the extratropical cyclone near Newfoundland. The WCBs contributed to the amplification of ridges downstream of each cyclone and to the subsequent elongation of Hanna's upstream trough into a PV streamer. This streamer eventually triggered the Mediterranean cyclogenesis. The second major effect of the diabatic processes occurred on smaller scales, in the low and middle troposphere. The remnants of Hanna's tropical PV core advected moist air towards the baroclinic zone leading to condensational PV production in the lower troposphere. In contrast, in the case of the extratropical cyclone, diabatic PV production occurred within its WCB at mid levels. These diagnostic analyses corroborate the potential of diabatic processes associated with extratropical flow systems for the modification of both the low-level vortices and the upper-level Rossby wave guide.
The adipocyte-derived hormone leptin is crucial for energy homeostasis in mammals; mice and humans without it suffer from a voracious appetite and extreme obesity. The effect on energy balance of variations in plasma leptin above a minimal threshold is uncertain, however, particularly in humans. Here we examine a group of individuals who are genetically partially deficient in leptin, and show that differences in circulating leptin levels within the range found in normal human populations can directly influence the laying down of fat tissue (adiposity).
ABSTRACT:The interaction of a tropical cyclone undergoing extratropical transition (ET) with the midlatitude synopticscale flow is investigated using full-physics numerical experiments with idealized initial conditions. The emphasis is on the impact on the midlatitude flow downstream of the ET event. The midlatitude flow is represented by a balanced straight jet stream. As the tropical cyclone approaches the jet, a ridge-trough couplet and a distinct jet streak form in the upper-level flow. A midlatitude cyclone develops rapidly downstream of the ET system and the further evolution is characterized by downstream baroclinic development.Based on Hovmöller diagrams, the upper-level development is interpreted as the excitation and subsequent dispersion of a Rossby wave train on the potential vorticity gradient associated with the jet. The characteristics of this wave train are sensitive to the structure of the jet and to moist processes in the midlatitudes. The tropical cyclone undergoing ET acts as a sustained forcing for the wave train and the structure of the ET system impacts the development most significantly one to two wavelengths downstream of ET.Piecewise inversion of potential vorticity, complemented by the partitioning of the flow into its rotational and divergent parts, is applied to assess the impact of the ET system quantitatively. Both the cyclonic circulation and the outflow of the tropical cyclone are important contributors to the formation and amplification of the ridge-trough couplet. The outflow anomaly reduces the eastward motion of the ridge-trough couplet significantly and thus promotes phase-locking between the tropical cyclone and the upper-level pattern.
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