Presentation on theme: "General Circulation & Thermal Wind"— Presentation transcript:
1 General Circulation & Thermal Wind AOS 101Lecture 11
2 General Circulation What is the global picture? The average flow on the globe...
3 General Circulation: Hadley Cell Thermally-driven convection:Warm air rises and cold air sinks, creating circulation
4 General Circulation: 3 Cells Hadley: Thermally driven circulation confined to tropicsFerrell: Mid-latitude circulation cell (subtropics to polar front)Polar: Sinking air at the poles
5 General Circulation: Winds Trade Winds: Surface easterly winds diverging from subtropical Highs and converging near the EquatorWesterlies: Diverge from subtropical Highs & converge toward polar frontPolar Easterlies: Converge along the polar front
6 General Circulation: Sea Level Pressure Low Pressure (converging air!)ITCZ (Intertropical convergence zone), near the equatorSubpolar Lows: along the polar front, near 60°High Pressure (diverging air!)Subtropical Highs: near 30° (warm & dry)Polar High: at the pole (cold & dry)
7 General Circulation: Climate Deserts at subtropical highs (High = sinking air!)Rainforests near ITCZ (Low = rising air & clouds!)Polar regions are deserts and receive very little precipitation each year (High = sinking air!)
9 Pressure Pressure is the weight of air molecules ABOVE you Pressure decreases with altitude because there are less air molecules above you as your riseAs a result of pressure changes, Temperature, Density, and Volume change too as you rise
10 Upper Tropospheric Pressure Surfaces The height of a pressure surface above ground is analogous to the pressure.As an example, a low height of the 500 mb surface is analogous to lower pressure. This will be very important when we analyze upper tropospheric data.Figure: A 3-dimensional representation of the height of the 500 mb surface (in meters)
13 The base of this column is at the surface, so lets say its pressure is about 1000 mb
14 The top of this column is quite high—let’s say that its pressure is 500 mb
15 This column has some thickness: it is some distance between 1000 mb and 500 mb
16 500 mbIf we heat the column of air, it will expand, warm air is less denseThe thickness of the column will increase500mb is now farther from the ground1000 mbWarmer
17 If we cool the column of air, it will shrink, cool air is more dense The thickness of the column will decrease500mb is now closer to the ground500 mb1000 mbColder
18 ThicknessIn fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layerIt wouldn’t have mattered which pressure we had chosen. They are all higher above the ground when it is warmer….
19 ThicknessIn fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layerIt wouldn’t have mattered which pressure we had chosen. They are all higher above the ground when it is warmer….…which is what this figure is trying to show
20 Thickness At the poles, 700 mb is quite low to the ground These layers are not very “thick”In the tropics, 700mb is much higher above the groundSee how “thick” these layers are
21 General Circulation!Let’s think about what thickness means near a polar front, where cold air and warm air are meeting
22 This is a cross section of the atmosphere NorthCOLDSouthWARM
23 Cold air is coming from the north Cold air is coming from the north. This air comes from the polar vortex near the North PoleNorthCOLDSouthWARM
24 Warm air is coming from the south Warm air is coming from the south. This air comes from the subtropical high near 30°NNorthCOLDSouthWARM
25 These winds meet at the polar front (a strong temperature gradient) NorthCOLDSouthWARM
26 Now, think about what we just learned about how temperature controls the THICKNESS of the atmosphere POLAR FRONTNorthCOLDSouthWARM
27 On the warm side of the front, pressure levels like 500mb and 400mb are going to be very high above the ground400mb500mbPOLAR FRONTNorthCOLDSouthWARM
28 On the cold side of the front, pressure levels like 500mb and 400mb are going to be very low to the ground400mb500mb400mb500mbPOLAR FRONTNorthCOLDSouthWARM
29 Above the front, thickness of atmosphere changes rapidly 400mb500mb400mb500mbPOLAR FRONTNorthCOLDSouthWARM
30 Now, what about the PGF above the front? 400mb500mb400mb500mbPOLAR FRONTNorthCOLDSouthWARM
31 Let’s draw a line between the cold side of the front and the warm side 400mb500mbAB400mb500mbPOLAR FRONTNorthCOLDSouthWARM
32 What is the pressure at point A? 400mb500mbAB400mb500mbPOLAR FRONTNorthCOLDSouthWARM
33 The pressure at point A is less than 400mb, since it is higher than the 400mb isobar on this plot. Let’s estimate the pressure as 300mb400mb500mbA300mbB400mb500mbPOLAR FRONTNorthCOLDSouthWARM
34 What is the pressure at point B? 400mb500mbA300mbB400mb500mbPOLAR FRONTNorthCOLDSouthWARM
35 The pressure at point B is more than 500mb, since it is lower than the 500mb isobar on this plot. Let’s estimate the pressure as 600mb400mb500mbA300mb600mbB400mb500mbPOLAR FRONTNorthCOLDSouthWARM
36 The pressure gradient force between point B & A is HUGE Therefore, all along the polar front, there will be a strong pressure gradient force aloft, pushing northward400mbPGF500mbA300mb600mbB400mb500mbPOLAR FRONTNorthCOLDSouthWARM
37 Strong PGF is:Aloft (above the surface)Above the Polar Front (strong temperature gradient!)PGF pushes to the north (in the Northern Hemisphere)How does this cause the midlatitude jet stream?
38 Midlatitude Jet Stream Suppose we have a “polar front” at the surfaceThis purple line is the polar front at the surfaceAs we’ll learn, this is NOT how fronts are correctly drawn, but it will work for now
39 Midlatitude Jet Stream All along the front, there is a strong pressure gradient force pushing northward
40 Midlatitude Jet Stream Winds aloft are in geostrophic balance…
41 Midlatitude Jet Stream So the wind will be accelerated North by the PGF, then turned to the East by the Coriolis effectThe true wind will be a WESTERLY wind, directly above the “polar front”
42 Midlatitude Jet Stream The same diagram from a different angleHere is the polar front at the surface
43 Midlatitude Jet Stream Remember, it’s a polar front because it is where warm air from the south meets cold air from the north.
44 Midlatitude Jet Stream The midlatitude jet stream is found directly above the polar front.
45 Midlatitude Jet Stream The (Northern Hemisphere) Midlatitude Jet Stream is found directly above the “polar front”, with cold air to the LEFT of the flowThis is because of the changes in thickness associated with the polar frontThis same relationship exists near ANY front (temperature gradient): known as the THERMAL WIND RELATIONSHIP
46 Large temperature gradients at the surface correspond to strong winds aloft!
47 Large temperature gradients at the surface correspond to strong winds aloft!
48 Thermal WindUpper-level winds will be much stronger than low-level winds (i.e. thermal wind will be very close to upper-level wind)Equal to the SHEAR of the geostrophic wind (i.e. change of geostrophic wind with height)Not an actual windStronger temperature gradients imply stronger thermal wind“Blows” along thickness contours with (low thickness) air to the leftThermal WindLower Level Geostrophic WindUpper level geostrophic wind