Cyclogenesis is overarching term for the development or strengthening of a closed, low pressure, circular motion of the atmosphere driven by inward spiraling winds rotating in the same direction as the planet. Extratropical cyclones form along weather fronts (baroclinic zones) and mature into cold core cyclones. This is distinct from tropical cylcones which are driven by the latent heat associated with intense convection and are warm, central core systems. The notion that extratropical cyclones evolve through a predictable sequence as they pass along the frontal boundary has led to the development of a variety of classification and diagnostic schemes. One such scheme, due to Petterssen and Smebye (1950's), adopts two broad developmental paths, Type A (directed by low level processes) and Type B (directed by upper level processes). These differ as to the forcing terms responsible for upward vertical motion and development of the kinetic energy in the systems. Despite the widespread use of this classification, details of the dynamical processes in extratropical cyclones remains unclear. This project, analyzing a number of cyclogenesis events observed during FASTEX (the Fronts and Atlantic Storms Experiment, 1997) and along with other selected data sets, seeks to employ two different methodologies to better diagnose and interpret the two category types. These methodologies are respectively conventional quasigesotrophic analyses based on Q vector analysis, and an adjoint model derived sensitivity analysis, both identifying atmospheric parameters (e.g. shearwise and transverse vertical motions) that relate to mid latitude cyclone development.

Broader impacts of these studies include the furtherance of undergraduate and graduate education of University of Wisconsin students, development of community resources and the investigation of new methodologies in weather predictability research on the topic of extratropical cyclone development. The economic impact of extratropical cyclones such as the 2009-2010 US winter storms were significant examples thereof.

Project Report

Our work on the superpopsition of polar and subtropical jet streaks has demonstrated that the Pacific jet stream, often portrayed by a variety of different types of averages in climate and large-scale dynamical studies, is much more complicated than those portrayals would suggest. We have found that it is nearly always a hybrid structure, a superposition of subtropical and polar jets. It is the only such standing hybrid jet structure in the Northern Hemisphere and is strongly influenced by both tropical convection and polar vortices. This discovery will have a dramatic impact on any climate or large-scale dynamical study that seeks to better understand aspects of the general circulation in an altered climate Despite the substantial influence that the wintertime Pacific jet has on both sensible? weather and large-scale circulation in the hemisphere, current understanding of? its intraseasonal variability is far from complete. In prior work, we identified the two primary modes of this ?variability; an extension/retraction mode wherein the eastern end of the jet can be located?anywhere from 160°E to 120°W, and a meridional shift mode wherein the latitude of the? jet axis (in the vicinity of its eastern end) can vary by ~20°. Though prior work has described the?composite structure and evolution of rapid jet retractions in the Pacific (e.g. transitions that occur on time scales of ~10 days), nothing is currently known about ?other preferred transitions of the jet between its leading modes of variability. We are working toward developing better understanding of wintertime Pacific jet variability and its consequences on the winter weasther over North America. Contributions to the severe weather environment that characterized the second day of the 1–3 May 2010 flood in Nashville, Tennessee were examined from the perspective of polar and subtropical jet superposition and its influence on the secondary ageostrophic circulation. The analysis revealed that the poleward moisture flux increased nearly 120% prior to the second day of the event in response to the superposed jet’s ageostrophic circulation, helping to fuel the production of heavy rainfall. Since superposed jets develop on the poleward edge of tropical or subtropical air, it is suggested that the mutually reinforcing interaction between the dynamic and latent heat release portions of the circulation may routinely characterize the involvement of superposed jet structures in high-impact weather events. By making a simple partition of the total geostrophic vorticity into its shear and curvature components, and taking advantage of the previously established fact that the majority of QG omega is forced by vorticity advection by the thermal wind, we have examined some basic questions regarding mid-latitude cyclone life cycles. An example is our recent examination of the so-called Shapiro effect (the role of geostrophic cold air advection in cyclonic shear) in assisting the rapid development of upper level fronts from the perspective of shear and curvature vorticity advections by the thermal wind. Upper fronts generally produce the upper tropospheric waves that, in turn, spawn surface cyclones and anticyclones – the basic elements of our mid-latitude weather. Traditionally, the Shapiro effect has been thought to only involve rearrangement of vertical circulations distributed in couplets straddling the vertical shear (known as transverse circulations). Upon partitioning the vorticity into its shear and curvature components, it is clear that the most substantial component of the subsidence necessary to tilt isentropes and vertical shear into the horizontal is provided by negative shear vorticity advection by the thermal wind. The resulting subsidence is part of a vertical circulation that is distributed in couplets along the shear (i.e. a shearwise circulation). This new insight will substantially change the view of upper frontogenesis and the energetic feedback that it has on the larger scale environment while leading us to new insights regarding the types of lower tropospheric cyclogenesis that can develop in the middle latitudes. Finally, high impact mid-latitude weather events and regimes are often associated with increases in the amplitude of planetary waves, as such increases are dynamically linked to robust cyclogenesis and anticyclogenesis events as well as the development of blocked flows. In spite of this well-known relationship, no widely accepted metric exists for quantifying the waviness of the circulation. Our most recent research appropriates a measure common in geomorphology – sinuosity – to measure the waviness of the mid-tropospheric flow using 500 hPa geopotential height contours. A seasonality in the sinuosity of the flow is demonstrated, with a maximum in summer and minimum in winter. The 500 hPa zonal wind is found to weaken when sinuosity increases and a strong negative correlation is found between the observed daily sinuosity and the daily Arctic Oscillation (AO) index in all seasons. Further, the DJF average sinuosity is shown to be highly correlated with the seasonal average AO, suggesting that physical mechanisms, such as Arctic amplification, that encourage weaker mid-latitude westerlies, may also encourage increased waviness.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
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Peter J. Milne
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University of Wisconsin Madison
United States
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