Presentation on theme: "Cyclones and Anticyclones in the Mid-Latitudes"— Presentation transcript:
1 Cyclones and Anticyclones in the Mid-Latitudes Val BenningtonNovember, 2008
2 Air pressure and atmospheric motion Q: What makes the wind blow?A: Air pressure differences. Winds blow from high toward low pressure.
3 Air pressureForce exerted by molecules in atm due to gravity and temperatureJuly 11, 2006NY TimesAs the World WobblesLate last November, as a big low-pressure system built over Europe and Asia and high pressure settled in over the Pacific and Atlantic Oceans, the shifts in the atmosphere caused the earth to jiggle ever so slightly, like a hiker adjusting to a shifting load in a backpack.As a result, the North Pole and its southern counterpart moved about four inches by one measure. (There are several ways to define the poles.)Despite its diaphanous appearance, the atmosphere weighs about 5,000 trillion metric tons, and its mass is unevenly distributed. All those ridges and troughs on a weather map reflect differences of billions of tons of gases.Scientists have long known that as the atmosphere shifts, it influences the earth’s rotation. The recent advent of satellites’ global positioning systems made it possible to confirm even the tiniest movements.There were well-known, regularly occurring wobbles in the earth’s rotation that could shift the poles 30 feet over a year or more. These shifts blocked the detection of subtler, quicker movements caused by day-to-day changes in the atmosphere and the oceans.Now, these small shifts are being measured by institutions devoted to tracking the planet’s behavior, including the earth orientation department of the United States Naval Observatory and the “time, earth rotation and space geodesy section” of the Royal Observatory of Belgium.Experts at the Belgian observatory and the Paris Observatory found the November polar shift and a series of other little loops by looking particularly closely at a period from last November through February, when two of the larger regular wobbles in the axis canceled each other out.They reported their analysis in the July 1 issue of Geophysical Research Letters.
7 Pressure systems Two types: high and low Low: associated with clouds and instability.High: associated with clear conditions and stability
8 Low pressure systems Cyclone Converging rising air at surface Diverging air aloftWinds rotate counterclockwise in NHAreas of “light” atmosphere; air is forced into these locationsUnstable surface conditions
9 High pressure system Anticyclone Converging air aloft Diverging sinking air at surfaceWinds rotate clockwise in NHAreas of “heavy” atmosphere; air is forced out of these locationsStable surface conditions
11 Is the location for these pressure systems the northern or the southern hemisphere?
12 Surface ChartOver surface maps the points with same pressure are connected with curves (Isobars).Isobars characterize the area with lower and higher pressure relative the neighboring points.These areas are named low pressure (or cyclone) and high pressure (or anticyclone) respectively.
13 Surface Chart The solid dark lines are isobars (millibars or hPa). The surface winds tend to blow across the isobars toward regions of lower pressure.
14 Anticyclones High pressure systems Just air masses with temperature and moisture varying slightly over large areaClear, calm, pretty dryBlob-like, with small pressure gradients and slower winds
15 AnticycloneThe large blue H on the map indicate the center of high pressure (anticyclone).Low pressure gradient around the high center.
16 Anticyclone (High) Which way does the wind blow? Does air diverge or converge at the surface?Does air converge or diverge above the high?
17 Anticyclone (High)Which way does the wind blow? > anti-cyclonic = clockwise!Does air diverge or converge at the surface? >Diverges!Does air converge or diverge above the high? -->Converges!
18 Anticyclones (Highs)Due to friction, air is always diverging near surface anticyclones
19 Anticyclones (Highs) Generally boring weather - clear, calm Linger for a while, but can be niceTrap air near surface (sinking motion)Blob-like air massesAir mass stays long can take on characteristics of land it is over
21 What is a Cyclone?A cyclone is simply an area of low pressure around which the winds flow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.Cyclones form and grow near the front.Cyclones (lows) are cloudy, wet, stormy.
22 Cyclones have converging air at surface that rises! Due to friction, air is always converging near surface cyclones
23 CycloneThe large red L on the map indicate the center of low pressure (cyclone).High pressure gradient around the low center.Norwegian meteorologists discovered that extratropical cyclones are associated with fronts.Cyclones: growing from birth as a frontal wave, to maturity as an occluded cyclone, and to death as a cut-off cyclone over the course of several days
24 Locating FrontsFronts are associated with . . .Strong temperature gradientsPositive vorticity (counter-clockwise rotation)Lower pressureRegions of convergence of the windsOften precipitation and clouds (regions of ascent)
25 Locating FrontsHere, the winds are rapidly changing counterclockwise across this temperature gradient.The winds are blowing warm air from the south.This is a warm front.
26 Locating FrontsIn this case, the winds are also rapidly changing counterclockwise across this temperature gradient, indicating positive vorticity.The winds are blowing cold air from the northwest.This is a cold front.
27 Locating Fronts To find the cyclone: Find the center of cyclonic circulationTo find the fronts:Find large temperature gradientsIdentify regions of wind shiftsLook for specific temperature advection (warm/cold)Look for kinks in the isobars (regions of slightly lower pressure)
28 Locating Fronts To find the cyclone: Find the center of cyclonic circulationTo find the fronts:Find large temperature gradientsIdentify regions of wind shiftsLook for specific temperature advection (warm/cold)Look for kinks in the isobars (regions of slightly lower pressure)Consider the reported phenomenon
30 Pressure…If we have converging air at the surface, must have divergence aloft!Otherwise, air would “fill up” the low and the pressure would rise
31 ReviewWinds converge at a surface low pressure centerWinds diverge from a surface high pressure center(this is because of the frictional force at the surface)This Convergence/Divergence suggests that there must be movement of air in the vertical (can’t lose air parcels)Flow in the upper troposphere is generally in geostrophic balance, so we do not get divergence/convergence high up caused by frictionHow do we get divergence/converge up high?
32 Upper level mapsOver upper level maps, the points with equal elevation above see level are connected with contours.The height contours characterize the areas with higher (anticyclonic circulation) and lower (cyclonic circulation) height relative the neighboring points.
33 The line of constant temperature Upper air chartsThe contour lines are not straight.They bend and turn, indicating ridges (elongated highs) where the air is warmer and indicating depressions, or troughs (elongated lows) where the air is colder.Contours of 500-hPa levelIsothermThe line of constant temperature
34 Upper Tropospheric Flow Typical 500 hPa height patternNotice the troughs (dotted line) and ridgesThe troughs and ridges are successiveIn the northern hemisphere, lower pressure is generally to the north of higher pressure
35 Relative VorticityIf the wind has counterclockwise spin, it has positive vorticity (left)If the wind has clockwise spin, it has negative vorticity (right)Vorticity can be directional (top), or speed shear vorticity (bottom)
36 Vorticity in the Upper Troposphere Vorticity advection: where and what kind?Pinpoint vorticity minima and maxima.Negative vorticity advection (NVA) occurs just “downstream” from a ridge axis (vorticity minimum)Positive vorticity advection (PVA) occurs just “downstream” from a trough axis (vorticity maximum)-+
37 Vorticity Advection and Vertical Motion Positive vorticity advection (PVA) results in divergence at that level* Negative vorticity advection (NVA) results in convergence at that level
38 Vorticity Advection and Vertical Motion Remember that convergence at upper levels is associated with downward vertical motion (subsidence), and divergence at upper levels is associated with upward vertical motion (ascent).Then, we can make the important argument that . . .
39 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 is divergence, resulting in ascent (upward motion).
40 Upper Tropospheric Flow and Convergence/Divergence Downstream of an upper tropospheric ridge axis is a favored location for a surface high pressure.Downstream of an upper tropospheric trough axis is a favored location for a surface low pressure center.
41 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 of the 500 mb flow, the 500 mb flow is also called the steering flow. The cyclone also moves at about half the speed of the 500 mb flow.The surface low pressure center in diagram above will track to the northeast along the upper tropospheric jet (along the surface temperature gradient)
42 Vertical Structure of Cyclones What else do these diagrams tell us?Surface cyclone is downstream from the upper tropospheric (~500 hPa) trough axisMid-latitude cyclones generally tilt westward with height!
43 Vertical Structure of Cyclones 500 hPa positive vorticity advection causes divergence and ascentThis induces a surface cycloneCyclone formation occurs because of this upper-level divergence!
44 Longwaves and Shortwaves The 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 on occasion, and can result in long periods of anomalous weatherShortwaves: Embedded in the longwave pattern are smaller scale areas of high vorticity (lots of curvature). They move quickly east within the longwaves, and generally strengthen when they hit a longwave trough. Often, shortwaves result in huge “cyclogenesis” events such as nor-easters or midwest snowstorms.
45 Longwaves vs. Shortwaves To the left is a North Pole projection of 300 mb heights (contoured) and wind speed (colors)North Pole is at the center, equator is at the edgesNote the prominent longwave troughs and ridges---especially over North AmericaLONGWAVE TROUGH
46 Longwaves vs. Shortwaves Notice two longwave troughs in this 500 mb height (contour) and vorticity (colored) map: One over the NW U.S., and one over eastern Canada.Also, note a very subtle shortwave over Montana/Wyoming (you can see this in the vorticity field as a strip of anomalously large vorticity.LONGWAVE TROUGHSHORTWAVE
47 Vertical Structure of Cyclones 700mbDownstream from troughs are favorable locations for ascent (red/orange)Downstream from ridges are good locations for descent (purple/blue)
48 Cyclone Intensification/Weakening How do we know if the surface cyclone will intensify or weaken?If upper tropospheric divergence > surface convergence, the cyclone will intensify (the low pressure will become lower)If surface convergence > upper tropospheric divergence, the cyclone will weaken, or “fill.”Think of an intensifying cyclone as exporting mass, and a weakening cyclone as importing mass.
49 Pressure…If we have converging air at the surface, must have divergence aloft!Otherwise, air would “fill up” the low and the pressure would rise
50 Example of Cyclone Development Forced by Upper Flow Red line: TROUGH AXISExample 300 mb flow which resulted in a massive cyclone development over the midwest.
51 Example of Cyclone Development Forced by Upper Flow Surface cyclone (over NW Oklahoma) is positioned just downstream of the trough axis in the previous image.Same time as the previous image.
52 Example of Cyclone Development Forced by Upper Flow 12 hours later, the jet speed maximum has shifted downstream with the trough, and there appear to be two trough axes.The trough is “negatively tilted,” (NW-SE in orientation) often a sign of very strong PVA and forced ascent.TROUGH AXIS
53 Example of Cyclone Development Forced by Upper Flow Now, the surface cyclone has deepened to a very low 977 mb.In general, it is still located downstream of the trough axis, but the trough axis appears to be catching up to the surface cyclone.
54 Example of Cyclone Development Forced by Upper Flow 12 hours later:300 mb upper tropospheric low hasn’t moved too muchUpper low is situated over eastern Lake Superior.TROUGH AXIS
55 Example of Cyclone Development Forced by Upper Flow SFC at same time:Surface cyclone is also over eastern Lake Superior!This means that the surface cyclone is no longer in a favorable position for PVA (or upper divergence and ascent)At this point, the surface cyclone will weaken!Cyclone is “vertically stacked.”
57 Midlatitude cyclonesStrong, “deep” interaction between surface and upper levelsMay travel large distances around the globeMidlatitude cyclone
58 High and low pressure systems Occur on a variety of spatial and temporal scalesSome pressure systems may be stationary for a long period of time, others may migrate rapidly around the planetSome pressure systems are closed, others are more belt-like and open
63 ATMS 316- Background Conservation of potential vorticity conserved for adiabatic frictionless motionRatio of absolute vorticity and depth of vortexIsentropic Potential Vorticity(Holton 2004, p. 96)
64 ATMS 316- Background Conservation of potential vorticity for a homogeneous incompressible fluidz evaluated at constant heightPotential Vorticity(Holton 2004, p. 96)
65 ATMS 316- Background Conservation of potential vorticity When the depth of the vortex changes following motion, its absolute vorticity must change to maintain conservation of potential vorticity(Holton 2004, p. 98)
66 ATMS 316- Background Conservation of potential vorticity For westerly flow impinging on an infinitely long mountain range…(a) upstream, zonal flow is uniform (du/dy = 0, v=0), z = 0(b) deflection of upper q surface upstream of barrier increases h absolute vorticity must increase air column turns cyclonically(Holton 2004, p. 98)
67 ATMS 316- Background Conservation of potential vorticity For westerly flow impinging on an infinitely long mountain range…poleward drift in (b) also causes increase in f(c) as column crosses mountain, h decreases absolute vorticity must decrease z becomes negative air column drifts equatorward(Holton 2004, p. 98)
68 ATMS 316- Background Conservation of potential vorticity For westerly flow impinging on an infinitely long mountain range…equatorward drift in (c) also causes decrease in f(d) as column crosses mountain, h increases absolute vorticity must increase z becomes positive air column drifts poleward
69 ATMS 316- Background Conservation of potential vorticity For westerly flow impinging on an infinitely long mountain range…(e) alternating series of ridges and troughs downstream of mountain rangecyclonic flow pattern immediately to the east of the mountains (lee side trough)
70 ATMS 316- BackgroundLee cyclogenesisStrong cross-mountain flow alone, however, is not a sufficient cause of cyclogenesisLee cyclones function in a similar manner to ordinary cyclones, deriving their kinetic energy from the APE of the baroclinic atmosphereLee cyclogenesis is favored where there is a strong cross-mountain flow in the jet stream(a)(b)(c)(d)(e)- Also must have low static stability in the lee of the mountain range
71 ATMS 316- Background Lee cyclogenesis Preferred regions of cyclogenesisRocky Mountains(Ahrens 2005, p. 222)
72 ATMS 316- Background Lee cyclogenesis Preferred regions of cyclogenesisAlpsNarrow mountain rangeTheory that applies to Alps lee cyclogenesis is slightly different from that used to describe lee cyclogenesis of the RockiesAgeostrophic effects dominate and the modification of baroclinic instability by the Alps is more difficult to analyze
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