13: Extratropical Cyclones
- Page ID
- 9616
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)A synoptic-scale weather system with low pressure near the surface is called a “cyclone” (Fig. 13.1). Horizontal winds turn cyclonically around it (clockwise/counterclockwise in the Southern/ Northern Hemisphere). Near the surface these turning winds also spiral towards the low center. Ascending air in the cyclone can create clouds and precipitation.
Tropical cyclones such as hurricanes are covered separately in a later chapter. Extratropical cyclones (cyclones outside of the tropics) are covered here, and include transient mid-latitude cyclones and polar cyclones. Other names for extratropical cyclones are lows or low-pressure centers (see Table 13-1). Low-altitude convergence draws together airmasses to form fronts, along which the bad weather is often concentrated. These lows have a short life cycle (a few days to a week) as they are blown from west to east and poleward by the polar jet stream.
- 13.1: Cyclone Characteristics
- This page covers the processes of cyclogenesis and cyclolysis in cyclones, including their formation, evolution, and decay stages. Cyclogenesis is characterized by increasing vorticity and updrafts under specific conditions, while cyclolysis involves weakening due to a lack of jet stream support. The lifecycle is influenced by global circulation patterns and seasonal changes, with variations between hemispheres concerning lifespan and movement.
- 13.2: Midlatitude Cyclone Evolution - a Case Study
- This page covers the dynamics of a severe weather event on April 3-4, 2014, linking upper-level troughs with surface low-pressure systems that caused storms in the Midwest and Mississippi Valley. It discusses air parcel movements and the significance of potential vorticity in weather analysis.
- 13.3: Lee Cyclogenesis
- This page covers Rossby waves and their crucial role in mid-latitude cyclone formation, emphasizing lee cyclogenesis influenced by mountains. It addresses synoptic meteorology concepts such as cyclones and fronts, and examines the relationship between potential vorticity and air movement in mountainous areas.
- 13.4: Spin-up of Cyclonic Rotation
- This page covers cyclogenesis, focusing on the upward motion, decreasing surface pressure, and increasing vorticity critical for analyzing weather systems. It highlights the importance of factors like vorticity advection and wind shear, along with the quasi-geostrophic approximation that aids in predicting geostrophic vorticity changes.
- 13.5: Ascent
- This page covers the estimation of cyclone strength through vertical velocity (ω) linked to pressure changes, introducing equations for analyzing atmospheric ascent. It explores jet stream dynamics, including wind speed changes, ageostrophic winds, and their impact on cyclogenesis. The omega equation is highlighted for diagnosing vertical motion, emphasizing the importance of thermal wind and vorticity.
- 13.6: Tendency of Sea-level Pressure
- This page covers cyclogenesis and cyclolysis, detailing how sea-level pressure changes and geopotential heights relate to air movement. It emphasizes the mass budget concept and how inflow/outflow impacts pressure. The dynamics of pressure changes in the troposphere and effects of latent heating on surface pressure are analyzed, along with the life cycle of extratropical cyclones, which are influenced by temperature gradients and jet stream dynamics.
- 13.7: Cyclone Self Development
- This page explores cyclogenesis, focusing on self-amplifying mechanisms like latent heating that intensify cyclones and lower surface pressure. It addresses the influence of temperature advection on cyclone dynamics and the formation of baroclinic zones affecting isotherm patterns. It also discusses Q-vectors in relation to cyclone movement and cold fronts, and concludes with insights on scientific inquiry and the progression of scientific knowledge.
- 13.8: Review
- This page explains extratropical cyclones, which are large mid-latitude storms with cold cores and counterclockwise rotation. They go through phases of intensification and weakening, causing adverse weather in frontal zones. Their strength is analyzed through vorticity and pressure. Cyclones over the Pacific typically weaken upon reaching North America, showcasing unique cloud structures.
- 13.9: Homework Exercises
- This page presents exercises that enhance understanding of storm dynamics and cyclogenesis through practical engagement with meteorological data and weather maps. It covers critical atmospheric concepts, including calculations of wind direction, pressure changes, and temperature gradients, utilizing methods like the ageostrophic right-hand rule and Q-vector analysis. Key topics include cyclone evolution, the influence of terrain and jet streams, and hypothetical climate scenarios.