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Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt. Anthony J. Vega. Part 4. Disturbances. Chapter 10 Mid-latitude Cyclones. Introduction.
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Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt Anthony J. Vega
Part 4. Disturbances Chapter 10 Mid-latitude Cyclones
Introduction • Bjerknes, the founder of the Bergen school of meteorology, developed polar front theory to describe interactions between unlike air masses and related aspects of the mid-latitude cyclone • The Life Cycle of a Mid-latitude Cyclone • Cyclogenesis typically begins along the polar front but may initiate elsewhere, such as in the lee of mountains • Minor perturbations occur along the boundary separating colder polar easterlies from warmer westerlies • A low pressure area forms and due to the counterclockwise flow (N.H.) colder air migrates equatorward behind a developing cold front • Warmer air moves poleward along a developing warm front (east of the system) • Clouds and precipitation occur in association with converging winds of the low pressure center and along the developing fronts
Mature Cyclones • Well-developed fronts circulating about a deep low pressure center characterize a mature mid-latitude cyclone • Chance of precipitation increases toward the storm center • Heavy precipitation stems from cumulus development in association with the cold front • Lighter precipitation is associated with stratus clouds of the warm front • Highly unstable conditions are associated with the warm sector ahead of the cold front • Area may produce heavy precipitation and severe thunderstorms associated with prefrontal waves (squall lines) • The system is capable of creating snow, sleet, freezing rain, and/or hail • Isobars close the low and are typically kinked in relation to the fronts due to steep temperature gradients • Winds, spiraling counterclockwise toward the low, change accordingly as the system, and its associated fronts, moves over particular regions
A mature mid-latitude cyclone, lifting processes, and cloud cover
Two examples of mid-latitude cyclones
Occlusion • When the cold front joins the warm front, closing off the warm sector, surface temperature differences are minimized • The system is in occlusion, the end of the system’s life cycle
Evolution and Movement of Cyclones • A hypothetical mid-latitude cyclone may develop as a weak disturbance east of Japan and travel eastward, guided by upper air trajectories • The system may bring rain to western North America and snow to high elevations • On the lee side of the Rocky Mountains, the system may undergo strengthening, causing blizzard conditions in the central and northeastern U.S. • Occlusion typically occurs in the western North Atlantic • For particular locations, weather conditions may progress from clear to cloudy with cloud cover thickening and lowering • Eventually, light precipitation may begin with warm front advancement • Winds, originally easterly, shift to southeasterly, then southwesterly with warm front advancement • Heavy clouds and precipitation advance with cold front approach • Temperature and humidity plummet with cold front passage
Processes of the Middle and Upper Troposphere • Carl Rossby mathematically expressed relationships between mid-latitude cyclones and the upper air during WWII • Rossby Waves and Vorticity • The rotation of air, or vorticity, may be viewed as either being absolute, the overall rotation, or relative to the Earth’s surface • Air which rotates in the direction of Earth’s rotation is said to exhibit positive vorticity • Air which spins oppositely exhibits negative vorticity • In relation to the upper air, maximum and minimum vorticity occurs in relation to troughs and ridges, respectively • Vorticity changes in the upper atmosphere lead to surface pressure changes • Decreasing vorticity in the zone between a trough and ridge leads to upper air convergence and sinking motions through the atmosphere, which supports surface high pressure areas • Increasing vorticity in the zone between a ridge and trough leads to upper air divergence and rising motions through the atmosphere, which supports surface low pressure areas
Relative vorticity Vorticity around a Rossby wave
Changing vorticity along a Rossby wave Convergence and divergence along a Rossby wave
Values of absolute vorticity on a hypothetical 500 mb map Changes in vorticity through a Rossby wave
The Effect of Fronts on Upper-Level Patterns • Interactions between the upper air and surface and vice versa helps establish and develop mid-latitude cyclones • Thermal differences across cold and warm fronts lead to upper atmospheric pressure differences due to density differences which equate to air temperature as expressed through the hydrostatic equation • Cold Fronts and the Formation of Upper-Level Troughs • Upper air troughs develop behind surface cold fronts with the vertical pressure differences proportional to horizontal temperature and pressure differences • This is due to density considerations associated with the cold air • Such interactions also relate to warm fronts and the upper atmosphere
Temperature variations in the lower atmosphere lead to variation in upper-level pressure
Interaction of Surface and Upper-Level Patterns • The upper atmosphere and the surface are inherently connected and linked • Divergence and convergence relate to surface pressure differences in cyclones and anticyclones, respectively • Surface temperatures influence vertical pressures and upper atmospheric winds • Upper level flow patterns explain why mid-latitude cyclones exist in addition to aspects of their life cycles • An example is the typical position of mid-latitude cyclones downwind of trough axes in the area of decreasing vorticity and upper-level divergence Relationships between a mid-latitude cyclone and a trough and ridge
An Example of a Mid-Latitude Cyclone • April 15 - A mid-latitude cyclone is centered over the upper midwestern U.S. • Heavy rains, high winds, and overcast skies dominate the regions near the central low pressure center • Recording of wind trajectories at stations throughout the central U.S. depict the counterclockwise rotation of the system • The 500 mb chart shows the storm upstream of the trough axis in the region of decreasing vorticity and upper-level divergence
April 16 - The northeasterly movement of the storm system is seen through a comparison of weather maps over a 24-hour period • Occlusion occurs as the low moves over the northern Great Lakes • In the upper air, the trough has increased in amplitude and strength and become oriented northwest to southeast • Isobars have closed about the low, initiating a cutoff low
April 17 - Continual movement towards the northeast is apparent, although system movement has lessened • The occlusion is now sweeping northeastward of the low, bringing snowfall to regions to the east • In the upper air, continued deepening is occurring in association with the more robust cutoff low
April 18 -The system has moved over the northwestern Atlantic Ocean, but evidence persists on the continent in the form of widespread precipitation • The upper atmosphere also shows evidence of the system, with an elongated trough pattern
Flow Patterns and Large-Scale Weather • Zonal height patterns obstruct development of surface pressure systems as vorticity remains constant • Zonal conditions are indicative of rather benign atmospheric conditions at the surface, although small scale disturbances may occur • Meridional conditions actively support surface cyclone development as vorticity changes appreciably between troughs and ridges • Large-scale flow conditions in the upper atmosphere may persevere for long periods, locking in particular weather situations to affected regions Zonal flow pattern Meridional flow pattern
The Steering of Mid-Latitude Cyclones • The movement of surface systems can be predicted by the 500 mb pattern • The surface systems move in about the same direction as the 500 mb flow, at about 1/2 the speed • Must predict changes in the 500 mb flow patterns in order to correctly predict surface system movement • Upper-level winds are about twice as strong in winter than summer • During winter, net radiation decreases rapidly with increasing latitude, which creates a stronger latitudinal thermal gradient • This results in stronger pressure gradients (and winds), resulting in stronger and more rapidly moving surface cyclones • Winter mid-latitude cyclones may be grouped by common paths across North America • Alberta Clippers are associated with zonal flow and usually produce light precipitation • Colorado Lows are usually stronger storms which produce more precipitation • East Coast storms typically have strong uplift and high water vapor content
Migration of Surface Cyclones Relative to Rossby Waves • Upper-air divergence must be present for a mid-latitude cyclone to form • If divergence aloft exceeds surface convergence, the surface low will deepen and a cyclone forms • If convergence at the surface exceeds divergence aloft, the low “fills” • Surface cyclones are pushed along the upper air wind trajectory • They generally move in the same direction as the 700 mb winds and at about 1/2 the speed • The surface low generally moves southwest to northeast relative to the Rossby wave configuration • As such, it moves away from the region of maximum divergence aloft, eventually dissipating as it approaches the upper-level ridge
The Modern View - Mid-latitude Cyclones and Conveyor Belts • The conveyor belt cyclone model helps describe conditions associated with mid-latitude cyclones through the entire profile of the atmosphere • The warm conveyor belt originates in the lower atmosphere of the warm sector • Air flowing toward the storm center is displaced aloft until it overrides the warm front where it turns to the right (N.H.), becoming part of the westerly flow aloft • The cold conveyor belt lies north of the warm front • It streams westward near the surface toward the surface low, where it ascends and turns clockwise (N.H.) to become part of the westerly upper air flow • The dry conveyor belt originates in the upper troposphere as part of the normal westerly flow • Air sinks into the trough only to rise over the region of the surface low before continuing along its eastward path • Integral to maintaining separate cloud bands which give the system its characteristic comma shape
End of Chapter 10Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt