The atmospheric circulation system

Transcription

1 The atmospheric circulation system Key questions Why does the air move? Are the movements of the winds random across the surface of the Earth, or do they follow regular patterns? What implications do these circulatory systems have for global climate? What other factors govern the geographic and seasonal distributions of temperature and rainfall?

2 The Global Circulatory SubSystems Many components of the Earth are in motion (e.g. the atmosphere, oceans, biological systems), though some move rather slowly (e.g. continents, mantle). Each of these systems play a role in the distribution of energy and thus the climate of the Earth. These systems can be thought of as acting on three different time scales. 1. Very short time scales (days to weeks)- surface winds think storms and weather frontal systems. 2. Short time scales (years to decades)- tropical ocean heat pump think El Niño. 3. Longer time scales (millennial)- deep ocean circulation think Gulf Stream. 4. Longest time scales (10s of millions of years)- plate tectonics think Himalayas.

3 The global imbalance of energy is the reason we have weather on Earth. Some regions receive more energy from the Sun than others. The atmosphere and oceans compensate by redistributing this excess energy by wind and water transport of heat.

4 Thus, both the global wind belts and ocean currents are generated and maintained because the tropics receive more insolation than the poles. That s all there is to it well except for the details

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6 Atmospheric Circulation Air moves (wind) horizontally in response to differences in pressure Air will also move vertically if pushed over something, or because it is more buoyant than surrounding air. Buoyancy is controlled by differences in density, usually the result of differences in temperature and moisture content between air masses. Vertical movement- an air mass will rise if it is less dense than surrounding air. An air mass can become less dense if it is heated, or if it contains more moisture. Effect of heating- air molecules move at higher rates when they heat up, spreading out and reducing density. Effect of moisture- air with water vapor has a lower density because water molecules have a lower molecular weight.

7 Horizontal movement- air flows from higher pressure areas to lower pressure areas. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force." High and low pressure are relative.

8 General Circulation of the Atmosphere Convergence- the merging of air masses that are moving toward a low-pressure region. Intertropical Convergence Zone (ITCZ)- where converging air masses meet in the tropics. Divergence- movement of air outward from a region in the atmosphere.

9 Hadley Circulation- air converges in the tropics at the surface, rises, diverges and then descends 30 north and south of the convergence. This works well everywhere but SE Asia where the Tibetan Plateau interferes by forcing the monsoons. Latent heat resulting from heating of tropical water is the mechanism for driving convection. This is most readily seen in the large convective cells generated in the tropical afternoon.

10 Midlatitude and High-Latitude Circulation The air masses at the poles are dry and cold (particularly in winter) leading to higher pressure, subsidence, and movement toward the equator. Polar front zone- steep temperature gradient (~60 N and 60 S) where the cold polar air meets warmer subtropical air. Cold dense air forces it s way under warm, more buoyant air form the south. Weather occurs at such fronts

11 Building on the earlier diagram, we ve now added the polar front zones.

12 Circulation patterns on a non-rotating earth. In addition to the north-south movements however, we have a strong east-west component to circulation. This east-west component is caused by the Coriolis effect.

13 Global Atmospheric Circulation Model

14 Coriolis Effect- because the speed of rotation is greatest at the equator, any object that moves from one area to another will be deflected to the right in the northern Hemisphere and to the left in the southern Hemisphere. This effect increases with the speed of the object. It is greatest at the poles and zero at the equator.

15 Coriolis Force

16 Distribution of surface winds The mid-latitudes (like Saskatchewan) are characterized by westerly air flow (westerlies), and the tropics by easterly air flow called the northeast and southeast trade winds. Doldrums- region where winds are light and change direction frequently. Now we re getting closer to reality

17 Cyclonic flow- areas of low pressure (~1,000km diameter) pull in air from surrounding areas (curving to the right in the north for counter-clockwise flow). Anticyclonic flow- air flowing out of a high-pressure region (curving to the right in the north for clockwise flow). Extratropical cyclones- low pressure systems that form outside the tropics. These circulation systems mix warm, wet tropical air with colder, drier air from higher latitudes. Warm, wet air rises above the cool, dry air and produces the rain and snow of the mid to high latitudes.

18 Upper level flow- at equal altitudes in the atmosphere, the pressure will be lower to the north thus initiating flow along that pressure gradient to the high latitudes. Because of the Coriolis effect this highaltitude flow is curved to the right in the northern hemisphere. Jet streams- belts of high speed wind that circle the Earth at the position of the greatest temperature/pressure gradient.

19 Geostrophic wind- air flow generated by a balance between the pressure gradient force and the Coriolis effect. Air flow is thus to the right of the pressure gradient force why? Wind flowing perpendicular to the pressure gradient is forced to the right by the Coriolis effect. The faster it flows, the harder it gets pushed to the right. Eventually, a balance is reached between the flow perpendicular to the pressuregradient and the coriolis effect. At this point the winds will flow parallel to the isobars.

20 Isobars- lines connecting places of equal pressure on a map. Rossby waves- ridges and troughs in upper atmospheric air flow that dictate the movement of high and low pressure systems. Northern Hemisphere mean January geopotential height of the 300mb surface.

21 The Jet Stream and Rossby Waves

22 Seasonal Variability Obliquity- the tilt of the Earth s axis relative to the plane of the planet s orbit around the Sun. The Earth is tilted at 23.5 from the perpendicular. Obliquity is the reason for seasons.

23 Earth-Sun Relations

24 The ITCZ is always chasing the Sun, but never really catching it. Nonetheless, it moves north and south in response to the seasonal changes in insolation. This gives the tropics and subtropics distinctive rainy seasons.

25 Global Distribution of Temperature and Rainfall Moisture transport and ultimately precipitation are strongly controlled by the same temperature contrasts that drive circulation. This moisture transport (via latent heat) also moves very significant amounts of energy toward the poles. Land-Ocean Contrasts Thermal Conductivity- the rate at which heat is transferred through a substance. Water can transfer heat by convection and conduction. Land can give up heat by convection but only transfer it downward by conduction. Heat Capacity- the energy required to raise the temperature of a unit mass of a substance by 1 K or 1 C without changing its volume. Water has an heat capacity 3-4 times higher than land.

26 Sea Breeze as an illustration of the differences in heat capacity and thermal conductivity between land and water.

27 Continentality- seasonal variability in climate. Land surfaces are much colder in the winter and hotter in the summer than the oceans.

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29 Seasonal Pressure and Precipitation Patterns

30 Monsoon- seasonal reversal in wind that pulls warm wet air into the continents during the summer months and pushes it away in the winter.

31 Seasonal variation in average sea-level pressure patterns.

32 Seasonality of temperature

33 The Global Hydrologic Cycle Water is the most important chemical compound in the Earth System. It cover 70% of the Earths surface as a liquid and even more if we include ice cover. In the atmosphere, clouds cover about 50% of the Earth at any given time. Water vapor in the atmosphere varies from near zero at the poles to about 7% in the tropics. Latent heat of vaporization- the energy required to convert liquid water to water vapor. Latent heat of fusion- the energy required to convert ice to water.

34 Latent heat and water

35 97% of the Earths water is in the oceans 3% is on land (75% of that is ice; most of the rest is groundwater; only 1% of the water on land is found in lakes and rivers, and soil) About two-thirds of the liquid water on the land surface is in lakes, and one third is in the soil. Rivers have very little water in them. The Hydrologic Cycle

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37 Precipitation and Saturation Vapor Pressure The transfer of water between the land/ocean surface and the atmosphere takes place through evapo-transpiration and precipitation. Atmospheric pressure is the sum of the partial pressures exerted by all of the gases in the atmosphere. Vapor pressure is the pressure exerted by water vapor in the atmosphere. This is very temperature sensitive. If the air temperature rises over a body of water, more water molecules become excited enough to leave a body of water and evaporation proceeds until the air becomes saturated and condensation equals evaporation. This is known as the saturation vapor pressure.

38 In order to estimate when we might have rain, snow, or dew we need to know the relative humidity. Ex. Beartooth mts. Relative humidity- the ratio of the vapor pressure to the saturation vapor pressure at that temperature. Generally, when the air reaches 100% relative humidity we have saturation and condensation. Clean air can go above 100% because it lacks CCNs (condensation cloud nuclei).

39 For condensation to occur (including precipitation), the air mass almost always has to get colder. This can happen in a number of different ways. Convective Lifting or Convection- ascending air caused by strong heating at the Earth's surface. Orographic Lifting- air that is forced upward by mountainous terrain.

40 Convergence- forced upward by convergence of air near the ground. A trough line, depicted on a weather map as a dashed line, identifies a line of convergence at the surface. Frontal wedging- air is forced upward by another airmass. This occurs near cold fronts, where cold air displaces warm air and forces the warm air upward. Near warm fronts, warm air ascends over cold surface air that moves relatively little.

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43 Precipitation belts are determined by the location of moist air masses and a means to cool them off.

44 Seasonal differences in precipitation are related to the change in the energy balance due to obliquity.

45 Deserts result for a couple different reasons. High pressure cells Rainshadows Cold water offshore

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