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Cyclones and Anticyclonesin the Mid-LatitudesVal BenningtonNovember, 2008
Anticyclones High pressure systems Just air masses with temperature andmoisture varying slightly over large area Clear, calm, pretty dry Blob-like, with small pressure gradientsand slower winds
Anticyclone
Anticyclone (High) Which way does thewind blow? Does air diverge orconverge at thesurface? Does air convergeor diverge above thehigh?
Anticyclone (High) Which way does thewind blow?- anti-cyclonic clockwise! Does air diverge orconverge at thesurface?- Diverges! Does air converge ordiverge above the high?-- Converges!
Anticyclones (Highs)
Anticyclones (Highs) Generally boring weather - clear, calmLinger for a while, but can be niceTrap air near surface (sinking motion)Blob-like air massesAir mass stays long can take oncharacteristics of land it is over
Fronts and Cyclones!
Fronts What about when two air masses meet? We get a front - large changes intemperature and moisture over smallarea
What is a Cyclone? A cyclone is simply an area of lowpressure around which the winds flowcounterclockwise in the NorthernHemisphere and clockwise in theSouthern Hemisphere Cyclones form and grow near the front Cyclones (lows) are cloudy, wet, stormy
Cyclones have converging airat surface that rises!
COLD FRONTSCold Front A transition zone where a cold air mass replaces a warm air mass Drawn as a blue line with blue triangles pointing in the direction ofthe front’s movement
Cold FrontsCold Front Cold air is more dense than warm air! As the dense, cold air moves into the warm air region, it forces thewarm air to rapidly rise just ahead of the cold front. This results in deep convective clouds, occasionally producingstrong to severe thunderstorms (depending on how unstable theatmosphere ahead of the cold front is). Often, the precipitation along a cold front is a very narrow line ofthunderstorms
Warm FrontsWarm Front A transition zone where a warm air mass replaces a cold air mass Drawn as a red line with red half-circles pointing in the direction ofthe front’s movement TEMPERATURE CONTRAST ALONG WARM FRONTS ISGENERALLY LESS DISTINCT (SMALLER GRADIENT)
Warm FrontsWarm Front Again, warm air is less dense than cold air. As the warm air moves north, it slides up the gently sloping warmfront. Because warm fronts have a less steep slope than cold fronts, theprecipitation associated with warm fronts is more “stratiform” (lessconvective), but generally covers a greater area.
Occluded FrontsOccluded Front A region where a faster movingcold front has caught up to a slowermoving warm front. Generally occurs near the end ofthe life of a cyclone Drawn with a purple line withalternating semicircles and triangles
Stationary Fronts Front is stalled No movement of thetemperature gradient But, there is still convergenceof winds, and forcing for ascent(and often precipitation) in thevicinity of a stationary front. Drawn as alternating segmentsof red semicircles and bluetriangles, pointing in oppositedirections
Locating FrontsFronts are associated with . . . Strong temperature gradients Positive vorticity (counter-clockwise rotation) Lower pressure Regions of convergence of the winds Often precipitation and clouds (regions of ascent)
Locating FrontsHere, the winds arerapidly changingcounterclockwise acrossthis temperature gradient.The winds are blowingwarm air from the south.This is a warm front.
Locating FrontsIn this case, the winds arealso rapidly changingcounterclockwise acrossthis temperature gradient,indicating positivevorticity.The winds are blowingcold air from thenorthwest.This is a cold front.
Locating FrontsTo find the cyclone: Find the center of cycloniccirculationTo find the fronts: Find large temperature gradients Identify regions of wind shifts Look for specific temperatureadvection (warm/cold) Look for kinks in the isobars(regions of slightly lower pressure)
Locating FrontsTo find the cyclone: Find the center of cycloniccirculationTo find the fronts: Find large temperature gradients Identify regions of wind shifts Look for specific temperatureadvection (warm/cold) Look for kinks in the isobars(regions of slightly lower pressure)
The Life Cycle ofExtra-tropical(Mid-Latitude) Cyclones
The Birth of a Cyclone A mid-latitude cycloneis born in a regionwhere their is a strongtemperature gradientwith forced lifting,perhaps an oldstationary front At the polar front!
Stage Two An instability (kink) forms Warm air pushes to thenortheast Cold air pushes to thesouthwest This will create thefronts!
Mature Stage Takes 12-24 hours todevelop Warm front moves NE Cold front moves SE Region between frontscalled warm sector Low pressure lowers(deepens) Wide-spread precip aheadof warm front Narrow band of precip atcold front Wind speeds increase
Cyclone Movement Cyclone moveseastward (or to NE) Starts to occlude(cold front catchingup) Storm most intense Triple point is wherecold, warm, andoccluded frontsmeet
Final Stage Warm sector shrinks Occlusion grows All energy fromtemperaturecontrast has beenused up Warm air has beenlifted Cold air has sunk STABLE
Weather with the Cyclone of late fall or early spring
Precipitation Around a Cyclone and its FrontsTo the right is a major cyclonethat affected the central U.S. onNovember 10, 1998.Around the cold front, theprecipitation is more intense, butthere is less areal coverage.North of the warm front, theprecipitation distribution is more“stratiform”: Widespread andless intense.http://weather.unisys.com
Precipitation Around a Cyclone and its FrontsAgain, in this radar and surfacepressure distribution fromDecember 1, 2006, theprecipitation along the cold frontis much more compact andstronger.North of the warm front, theprecipitation is much morestratiform.Also note the kink in the isobarsalong the cold front!
Locating a Cyclone1. Find the region oflowest sea levelpressureL2. Find the center of thecyclonic (counterclockwise) circulation
What about VerticalStructure?
Pressure If we have converging air at the surface, must havedivergence aloft! Otherwise, air would “fill up” the low and the pressurewould rise
Review Winds converge at a surface low pressure center Winds diverge from a surface high pressure center(this is because of the frictional force at the surface) This Convergence/Divergence suggests that there must bemovement of air in the vertical (can’t lose air parcels) Flow in the upper troposphere is generally in geostrophicbalance, so we do not get divergence/convergence high up causedby friction How do we get divergence/converge up high?
Upper Tropospheric FlowTypical 500 mb height pattern Notice the troughs (dotted line) and ridges The troughs and ridges are successive In the northern hemisphere, lower pressure is generally to thenorth of higher pressure
Relative VorticityIf the wind hascounterclockwise spin, it haspositive vorticity (left)If the wind has clockwisespin, it has negative vorticity(right)Vorticity can be directional(top), or speed shear vorticity(bottom)
Vorticity in the Upper TroposphereWhere is there vorticityadvection?Pinpoint vorticity minimaand maxima.Negative vorticity advection(NVA) occurs just“downstream” from a ridgeaxis (vorticity minimum)Positive vorticity advection(PVA) occurs just“downstream” from a troughaxis (vorticity maximum)
Vorticity Advection and Vertical Motion* Positive vorticity advection (PVA) results in divergence at thatlevel* Negative vorticity advection (NVA) results in convergence atthat level
Vorticity Advection and Vertical MotionRemember that convergence at upper levels is associated withdownward vertical motion (subsidence), and divergence at upperlevels is associated with upward vertical motion (ascent).Then, we can make the important argument that . . .
Upper Tropospheric Flow and Convergence/Divergence Downstream of an upper tropospheric ridge, there is convergence,resulting in subsidence (downward motion). Likewise, downstream of an upper tropospheric trough, there isdivergence, resulting in ascent (upward motion).
Upper Tropospheric Flow and Convergence/Divergence Downstream of an upper tropospheric ridge axis is a favoredlocation for a surface high pressure. Downstream of an upper tropospheric trough axis is a favoredlocation for a surface low pressure center.
Upper Tropospheric Flow and Convergence/Divergence Surface cyclones move in the direction of the upper tropospheric flow! The storm speed and direction can also be identified on the 500 mb map. Cyclones move in the direction ofthe 500 mb flow, the 500 mb flow is also called the steering flow. The cyclone also moves at about half thespeed of the 500 mb flow. The surface low pressure center in diagram above will track to thenortheast along the upper tropospheric jet(along the surface temperature gradient)
Vertical Structure of CyclonesWhat else do these diagrams tellus? Surface cyclone is downstreamfrom the upper tropospheric( 500 mb) trough axis Mid-latitude cyclones generallytilt westward with height!
Vertical Structure of Cyclones 500 mb positive vorticityadvection causes divergence andascent This induces a surface cyclone Cyclone formation occursbecause of this upper-leveldivergence!
Longwaves and ShortwavesThe flow in the upper troposphere is characterized as having . . . Longwaves: There are typically 4-6 of these around the planet.The longwave pattern can last for as long as 2-3 weeks onoccasion, and can result in long periods of anomalous weather Shortwaves: Embedded in the longwave pattern are smallerscale areas of high vorticity (lots of curvature). They movequickly east within the longwaves, and generally strengthen whenthey hit a longwave trough. Often, shortwaves result in huge“cyclogenesis” events such as nor-easters or midwest snowstorms.
Longwaves vs. ShortwavesTo the left is a NorthPole projection of300 mb heights(contoured) and windspeed (colors) North Pole is at thecenter, equator is atthe edges Note the prominentlongwave troughsand ridges--especially over NorthAmerica
Longwaves vs. ShortwavesNotice two longwavetroughs in this 500 mbheight (contour) andvorticity (colored) map:One over the NW U.S.,and one over easternCanada.Also, note a very subtleshortwave overMontana/Wyoming (youcan see this in thevorticity field as a stripof anomalously largevorticity.SHORTWAVELONGWAVE TROUGH
Vertical Structure of Cyclones700mb Downstream from troughs are favorable locations for ascent (red/orange) Downstream from ridges are good locations for descent (purple/blue)
Cyclone Intensification/WeakeningHow do we know if the surface cyclone will intensify or weaken? If upper tropospheric divergence surface convergence, thecyclone will intensify (the low pressure will become lower) If surface convergence upper tropospheric divergence, thecyclone will weaken, or “fill.” Think of an intensifying cyclone as exporting mass, and aweakening cyclone as importing mass.
Pressure If we have converging air at the surface, must havedivergence aloft! Otherwise, air would “fill up” the low and the pressurewould rise
Example of Cyclone Development Forced by Upper FlowExample 300 mbflow whichresulted in amassive cyclonedevelopment overthe midwest.TROUGH AXIShttp://weather.unisys.com
Example of Cyclone Development Forced by Upper FlowSurface cyclone(over NWOklahoma) ispositioned justdownstream of thetrough axis in theprevious image.Same time as theprevious image.
Example of Cyclone Development Forced by Upper Flow12 hours later, the jetspeed maximum hasshifted downstreamwith the trough, andthere appear to be twotrough axes.The trough is“negatively tilted,”(NW-SE in orientation)often a sign of verystrong PVA and forcedascent.TROUGH AXIS
Example of Cyclone Development Forced by Upper FlowNow, the surfacecyclone hasdeepened to a verylow 977 mb.In general, it is stilllocateddownstream of thetrough axis, but thetrough axis appearsto be catching up tothe surfacecyclone.
Example of Cyclone Development Forced by Upper Flow12 hours later: 300 mb uppertropospheric lowhasn’t moved toomuch Upper low issituated overeastern LakeSuperior.TROUGH AXIS
Example of Cyclone Development Forced by Upper FlowSFC at same time: Surface cyclone isalso over eastern LakeSuperior! This means that thesurface cyclone is nolonger in a favorableposition for PVA (orupper divergence andascent) At this point, thesurface cyclone willweaken! Cyclone is “verticallystacked.”
Temperature AdvectionConsider a longwave over a stationary front, seen in (a). The height lines and theisotherms are parallel to each other, we can say the atmosphere is barotropic.At time (b) a shortwave moves into the longwave trough and intensifies. The shortwavecaused the isotherms to cross the height lines, thus the atmosphere is baroclinicWest of the height trough, a region of (CAA). Here, the cold air is more dense and willcause sinking motions.East of the trough, a region of (WAA). Here, the warm air will produce rising motions.
Jet Streaks and ShortwavesIf a shortwave trough is intensified in a longwave trough, height lines areforced together: large PGF in the base of a shortwave trough We have seen before that large PGF corresponds to large wind speeds.
Jet Streaks and Shortwaves Largest wind speeds where height lines are the closest together on anupper level map. Wind speed decreases outward from this point. Therefore we have a convergence of wind to the left/west of a trough andthe divergence of wind to the east/right of a trough.
Jet Streaks and ShortwavesIf we look at the full vertical structure we will see that thedivergence and convergence associated with a jet streakare directly above the low and high pressures at thesurface.
The full picture
Creating a CycloneIf an upper level shortwave intensifies in alongwave : Jet streak creates upper levelconvergence and divergence Surface convergence occurs directlybelow upper level divergence Cyclone begins to develop
Cyclone Intensifies and Fades Cyclone goes through its life cycle,intensifies, its low pressure decreases, warmfront and cold front move Upper level low to west of surface low(westward tilt with height) Upper level trough begins to catch up tosurface low (tilt decreases) When cyclone is vertically stacked (no tilt),cyclone begins to die(no divergence above the surface convergence)
Cold Fronts Cold Front Cold air is more dense than warm air! As the dense, cold air moves into the warm air region, it forces the warm air to rapidly rise just ahead of the cold front.
