Presentation on theme: "Mid-Latitude Cyclones: Vertical Structure"— Presentation transcript:
1Mid-Latitude Cyclones: Vertical Structure Lecture 13CASE STUDY DUE NEXT WEEK!
2ReviewExtratropical cyclones generally first develop along an intersection of two airmasses (like a stationary front)As the cyclone develops, warm and cold fronts form, and the cold front slowly approaches the warm frontOnce an occluded from forms, the cyclone is normally at its most intense, and will begin to weaken afterwardThis is because it is no longer near a region of a horizontal temperature gradient (which is why it developed in the first place)
3Review Continued Finding fronts on weather maps is very useful It is often useful to first find the area of lowest pressure, since fronts typically originate from itIn the case of most fronts (except occluded fronts), there should be a large temperature change across themAll fronts should also have a fairly sharp wind shift from one side to anotherOther factors, like precipitation, cloud cover, and moisture gradients can indicate a front
4Review ContinuedLast week, we discussed the surface structure of mid-latitude cyclones which are crucial in maintaining a temperature equilibrium on our planet.We know that the winds move counter-clockwise and converge around a surface low low 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 (continuity of mass)Also, the flow in the upper troposphere is generally in geostrophic balance, so there is no friction forcing convergence/divergence.
6VorticityVorticity is simply a measure of how much the air rotates on a horizontal surfacePositive vorticity is a counterclockwise (i.e. cyclonic) rotationNegative vorticity is a clockwise (i.e. anticyclonic) rotationTherefore, troughs contain positive vorticity, and ridges contain negative vorticityTroughRidge
9Vorticity Advection and Vertical Motion * Positive vorticity advection (PVA) results in divergence at the level of advection* Negative vorticity advection (NVA) results in convergence at the level of advection
10Vorticity 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 . . .
11Upper 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).
12Upper Tropospheric Flow and Convergence/Divergence Downstream of an upper tropospheric ridge axis is a favored location for a surface high pressure, and of course, downstream of an upper tropospheric trough axis is a favored location for a surface low pressure center.
13Upper Tropospheric Flow and Convergence/Divergence Surface cyclones also move in the direction of the upper tropospheric flow!The surface low pressure center in the diagram above will track to the northeast along the upper level flow
14Vertical Structure of Cyclones What else do these diagrams tell us?Because the surface cyclone is downstream from the upper tropospheric (~500 mb) trough axis, mid-latitude cyclones generally tilt westward with height!
15Vertical Structure of Cyclones To the right is another depiction illustrating the same point:500 mb positive vorticity advection results in divergence and ascent, inducing a surface cyclone.
16Cyclone Growth And Decay Based on what we’ve learned, the position of the surface cyclone in relation to the upper level structure is key to developmentA cyclone will grow if it is below an area of PVA, and weaken if below an area of NVACommonly, a cyclone will intensify until it becomes situated in an unfavorable location in relation to the upper levels
17An Example: Time 1 Above: Upper Level Height and Wind Speed Right: Surface Pressure
18An Example: Time 1 Above: Upper Level Height and Wind Speed Right: Surface Pressure
23Summary of EventAt time 1, the upper levels and lower levels are perfectly set up for the surface cyclone to intensifyAt time 2, the upper trough is almost above the surface cyclone, so the intensification slowsBy time 3, the upper trough is exactly over the surface cyclone, so the intensification has halted
24Cyclone DecayRecall that due to friction, air blows across isobars near the surfaceThis means that the air is always converging at the center of low pressure areasTherefore, unless there is at least enough divergence at upper levels to counteract the convergence at low levels, the surface cyclone will weaken because more mass will be added to the air columnThis will force the surface pressure to rise
25Cyclone 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.