Today’s Topics Chapter 10 – Extratropical Cyclones and Anticyclones Norwegian cyclone model Cyclogenesis Structure of a mature cyclone Brief look at Anticyclones Chapter 11 – Thunderstorms and Tornadoes Distribution and types Factors for Growth Air Mass Thunderstorm development

Norwegian Model The Polar Front Theory was postulated in the early part of the twentieth century by the Norwegian Jacob Bjerknes to describe the formation, development, and dissipation of mid-latitude cyclones. Mid-latitude cyclones are large systems that travel great distances and often bring precipitation and sometimes severe weather to wide areas. Lasting a week or more and covering large portions of a continent, they are familiar as the systems that bring abrupt changes in wind, temperature, and sky conditions.

As cyclogenesis begins, a “kink” develops along the boundary. Cyclogenesis is the formation of a mid-latitude cyclone. Initially, the polar front separates the cold easterlies and the warmer westerlies. As cyclogenesis begins, a “kink” develops along the boundary. The cold air north of the front begins to push southward behind the cold front, and air behind the warm front advances northward, creating a counterclockwise rotation around a weak low-pressure system.

Occlusion represents the end of the cyclone’s life cycle With further intensification, the low pressure deepens even further and distinct warm and cold fronts emerge from the original polar front. Convergence associated with the low pressure can lead to uplift and cloud formation, while linear bands of deeper cloud cover develop along the frontal boundaries. Occlusion represents the end of the cyclone’s life cycle and takes place as the center of the low pressure pulls back from the warm and cold fronts.

Key factors in Cyclogenesis Surface temperature gradients Particularly if they are tilted with respect to the pressure gradients A strong jet stream aloft Helps to “deepen” the low by providing divergence aloft Mountains or other surface boundaries Causes the air flow to fluctuate, helping to “spin” the air masses.

No temperature advection occurs when the atmosphere is barotropic. Effect of Temperature Gradients (a) A barotropic atmosphere (a) exists where the isotherms (the dashed lines showing the temperature distribution) and height contours (solid lines) are aligned in the same direction. No temperature advection occurs when the atmosphere is barotropic. A baroclinic atmosphere occurs where the isotherms intersect the height contours. Cold air advection (the horizontal movement of cold air) is occurring in (b), while warm air advection is occurring in (c). (b) (c)

Baroclinic Instability The tilted pattern leads to rising and sink air. When the air rises, it can lead to energy release from latent heat.

The figure depicts the typical structure of a mature cyclone and the processes causing uplift. Shaded areas represent the presence of cloud cover. The numbers represent an approximation of the precipitation probability.

Comparison A mature mid-latitude cyclone usually has a distinctive “comma-cloud” shape A tropical cyclone has more of a symmetric spiral. Mid-Latitude Cyclone Tropical Cyclone

The effect of differing vertical pressure gradients on either side of warm and cold fronts leads to upper-tropospheric troughs and ridges.

Divergence in the upper atmosphere, draws air upward from the surface and provides a lifting mechanism for the intervening column of air. This can initiate and maintain low-pressure systems at the surface. Conversely, convergence aloft and the sinking of air, creates high pressure at the surface. Surface low-pressure systems resulting from upper-tropospheric motions are called dynamic lows and are distinct from thermal lows caused by localized heating of air from below.

Speed divergence and convergence occur when air moving in a constant direction either speeds up or slows down. Speed divergence occurs where contour lines come closer together in the downwind direction. In the top figure, the wind speed, indicated by the length of the arrows, increases in the direction of flow and causes speed divergence. Speed convergence occurs when faster-moving air approaches the slower-moving air ahead (bottom).

Diffluence and confluence occur when air stretches out or converges horizontally due to variations in wind direction. In the top figure, a certain amount of air is contained in the shaded area between points 1 and 3. As it passes to the region between points 2 and 4, the same amount of air occupies a greater horizontal area. This is diffluence, a pattern that commonly appears when divergence occurs. Confluence is shown in the bottom.

Structure in 3D Birth Young Adult Mature Upper low is in the trough Divergence is downwind from the upper low Minimal development of isotherms Upper low is upwind of the surface low Young Adult Upper low has closed the isotherms Begins moving out of the trough, upper level winds pull and steer the system towards the northeast Mature Upper low is out of the trough and is directly overhead of the surface low System is now occluded with heavy clouds over the occluded region

In the figure, the height contours exhibit a zonal pattern with a minimum of north–south displacement. Because they have no pronounced vorticity changes, zonal patterns hamper the development of intense cyclones and anticyclones. They are therefore more often associated with a large-scale pattern of light winds, calm conditions, and no areas of widespread precipitation.

while others are calm and dry. The pattern above shows a strong meridional component, which can lead to the formation of major cyclones and anticyclones. Some areas will experience cloudy and wet conditions while others are calm and dry.

The figure shows a center of low surface pressure gradually moving northeastward relative to the wave. In doing so, it moves away from the region of maximum divergence aloft and evolves as it travels. It goes through the various stages of its life cycle, typically occluding and dissipating as it approaches the upper-level ridge.

Anticyclones Don’t draw as much attention as cyclones. Usually have weak pressure gradients; consequently they usually generate only mild winds. Cyclones “live fast and die young” usually lasting no more than a week. Since Anticyclones have calm winds, they often persist for weeks.

Summer Time Blues About the only time anticyclones cause dangerous conditions is in the summer when if they are cut off from the steering for the jet stream. In this case, an anticyclone may linger in a location for days or weeks – resulting in “heat waves”, droughts, or air quality emergencies (e.g. pollution builds up). Also as we saw last week, -- Santa Ana Winds

Chapter 11- Thunderstorms

Basic Types Ordinary Single Cell / Air Mass Thunderstorms Usually the result of ground heating and convection. These are short lived (an hour or so) and are rarely severe. Entrainment of air is a important process to initiate collision-coalescence of the cloud droplets Multicell Thunderstorms Supercell Thunderstorms

Factors affecting Growth Need a lifting Mechanism (chapter 4) , the dryline (chapter 9) can cause lifting. Unstable Atmosphere Lifted index is one method for gauging the amount of instablity Vertical wind shear The greater the shear the more tilt. Tilted storms can begin to rotate leading to severe weather

Factors Affecting Growth (2) Low Level Jet Stream Transports warm moist air into a developing storm. This mechanism is largely responsible for Thunderstorms that form at night Capping Inversions Can lead to concentration of energy and “explosive” development

Air mass thunderstorms are the most common and least destructive usually lasting for less than an hour. The cumulus stage begins when unstable air begins to rise and cool adiabatically to form fair weather cumulus clouds. The mature stage begins when precipitation starts to fall dragging air toward the surface as downdrafts form in the areas of intense precipitation. As the cloud yields heavy precipitation, downdrafts occupy an increasing portion of the cloud base, the supply of additional water vapor is cut off, and the storm enters its dissipative stage.