Understanding Weather and Climate 3rd Edition Edward 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

Cyclogenesis

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

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 patternMeridional 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

Typical winter mid-latitude cyclone paths

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 10 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt