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General Circulation & Thermal Wind AOS 101 Lecture 11 General Circulation • What is the global picture? • The average flow on the globe... General Circulation: Hadley Cell • Thermally-driven convection: – Warm air rises and cold air sinks, creating circulation General Circulation: 3 Cells • Hadley: Thermally driven circulation confined to tropics • Ferrell: Mid-latitude circulation cell (subtropics to polar front) • Polar: Sinking air at the poles General Circulation: Winds • Trade Winds: Surface easterly winds diverging from subtropical Highs and converging near the Equator • Westerlies: Diverge from subtropical Highs & converge toward polar front • Polar Easterlies: Converge along the polar front General Circulation: Sea Level Pressure • Low Pressure (converging air!) – ITCZ (Intertropical convergence zone), near the equator – Subpolar Lows: along the polar front, near 60° • High Pressure (diverging air!) – Subtropical Highs: near 30° (warm & dry) – Polar High: at the pole (cold & dry) 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!) General Circulation: Jet Streams Pressure • Pressure is the weight of air molecules ABOVE you • Pressure decreases with altitude because there are less air molecules above you as your rise • As a result of pressure changes, Temperature, Density, and Volume change too as you rise http://www.srh.noaa.gov/jetstream//atmos/images/mb_heights.jpg 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) Thickness... • Start with a column of air. • The base of this column is at the surface, so lets say its pressure is about 1000 mb 1000 mb • The top of this column is quite high—let’s say that its pressure is 500 mb 500 mb 1000 mb • This column has some thickness: it is some distance between 1000 mb and 500 mb 500 mb 1000 mb • If we heat the column of air, it will expand, warm air is less dense • The thickness of the column will increase • 500mb is now farther from the ground 500 mb 1000 mb Warmer • If we cool the column of air, it will shrink, cool air is more dense • The thickness of the column will decrease • 500mb is now closer to the ground 500 mb 1000 mb Colder Thickness • In fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layer • It wouldn’t have mattered which pressure we had chosen. They are all higher above the ground when it is warmer…. Thickness • In fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layer • It 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 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 ground • See how “thick” these layers are General Circulation! Let’s think about what thickness means near a polar front, where cold air and warm air are meeting This is a cross section of the atmosphere North COLD South WARM Cold air is coming from the north. This air comes from the polar vortex near the North Pole North COLD South WARM Warm air is coming from the south. This air comes from the subtropical high near 30°N North COLD South WARM These winds meet at the polar front (a strong temperature gradient) POLAR FRONT North COLD South WARM Now, think about what we just learned about how temperature controls the THICKNESS of the atmosphere POLAR FRONT North COLD South WARM On the warm side of the front, pressure levels like 500mb and 400mb are going to be very high above the ground 400mb 500mb POLAR FRONT North COLD South WARM On the cold side of the front, pressure levels like 500mb and 400mb are going to be very low to the ground 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM Above the front, thickness of atmosphere changes rapidly 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM Now, what about the PGF above the front? 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM Let’s draw a line between the cold side of the front and the warm side 400mb 500mb A B 400mb 500mb POLAR FRONT North COLD South WARM What is the pressure at point A? 400mb 500mb A B 400mb 500mb POLAR FRONT North COLD South WARM 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 300mb 400mb 500mb A 300mb B 400mb 500mb POLAR FRONT North COLD South WARM What is the pressure at point B? 400mb 500mb A 300mb B 400mb 500mb POLAR FRONT North COLD South WARM 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 600mb 400mb 500mb A 300mb 600mb B 400mb 500mb POLAR FRONT North COLD South WARM 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 northward 400mb PGF A 300mb 600mb 500mb B 400mb 500mb POLAR FRONT North COLD South WARM • 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? Midlatitude Jet Stream • Suppose we have a “polar front” at the surface • This purple line is the polar front at the surface • As we’ll learn, this is NOT how fronts are correctly drawn, but it will work for now Midlatitude Jet Stream • All along the front, there is a strong pressure gradient force pushing northward Midlatitude Jet Stream • Winds aloft are in geostrophic balance… Midlatitude Jet Stream • So the wind will be accelerated North by the PGF, then turned to the East by the Coriolis effect • The true wind will be a WESTERLY wind, directly above the “polar front” Midlatitude Jet Stream The same diagram from a different angle •Here is the polar front at the surface Midlatitude Jet Stream •Remember, it’s a polar front because it is where warm air from the south meets cold air from the north. Midlatitude Jet Stream •The midlatitude jet stream is found directly above the polar front. Midlatitude Jet Stream • The (Northern Hemisphere) Midlatitude Jet Stream is found directly above the “polar front”, with cold air to the LEFT of the flow • This is because of the changes in thickness associated with the polar front • This same relationship exists near ANY front (temperature gradient): known as the THERMAL WIND RELATIONSHIP Large temperature gradients at the surface correspond to strong winds aloft! Large temperature gradients at the surface correspond to strong winds aloft! Thermal Wind VT VUPPER VLOWER • Upper-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) Thermal Wind • Not an actual wind • Stronger temperature gradients imply Lower Level stronger thermal wind Geostrophic Wind • “Blows” along thickness contours with Upper level geostrophic wind (low thickness) air to the left Thermal Wind VT Upper level geostrophic wind Lower Level Geostrophic Wind Thermal Wind COLD VT 5600 m Upper level geostrophic wind Lower Level Geostrophic Wind 5540 m WARM 5660 m Backing & Veering If winds rotate clockwise from lower level to upper-level veering! Lower level Geostrophic winds Thermal Wind Upper Level Geostrophic wind Backing & Veering If winds rotate clockwise from lower If winds rotate counter-clockwise level to upper-level veering! with height backing! Lower level Geostrophic winds Thermal Wind Upper Level Geostrophic wind Lower Level Geostrophic Wind Upper Level Geostrophic Wind Thermal Wind Backing & Veering If winds rotate clockwise from lower If winds rotate counter-clockwise level to upper-level veering! with height backing! Lower level Geostrophic winds Thermal Wind Upper Level Geostrophic wind Lower Level Geostrophic Wind Upper Level Geostrophic Wind Thermal Wind Backing & Veering If winds rotate clockwise from lower If winds rotate counter-clockwise level to upper-level veering! with height backing! Lower level Geostrophic winds Thermal Wind Upper Level Geostrophic wind Warm Air Advection! Lower Level Geostrophic Wind Upper Level Geostrophic Wind Thermal Wind Cold Air Advection!
