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AIRMASSES • An airmass is a huge volume of air (several thousand sq. km) that is “relatively uniform” horizontally in temperature and water vapor content. • Airmasses take on the character of their source region- that is, the region of the planet’s surface within which they are in contact for several days to weeks. For example: warm ocean, snowcovered prairie, etc. • For our purposes we can distinguish air masses as either: • 1) warm or cold (tropical or polar), and as • 2) moist or dry (maritime or continental) Typical configuration of air masses Note BOTH Arctic front and Polar Front CASE OF NO ROTATION This movement of air in the upper atmosphere would tend to converge near the poles causing subsidence (sinking) in polar regions This cooled air would then return equatorward near the surface. In this simple conceptual model, the circulation occurs as hemispheric HADLEY CELLS For our purposes we can distinguish air masses as either: 1) warm or cold (tropical or polar), and as 2) moist or dry (maritime or continental) giving us continental tropical cT (dry and hot), continental polar cP (dry and cold) maritime tropical mT (moist and warm), and maritime polar mP (moist and cool). We will also add arctic cA (very dry and very cold) The Planetary Circulation Recall from our earlier discussion that “warm air rises” and “cold air sinks”. • While this is an over-simplification it is a useful starting point. Consider an idealized non-rotating planet covered by ocean: • The intense insolation near the equator would likely create regions of intense convection (thunderstorms). • Since thunderstorms have strong updrafts => move air upward, lots of air. This air near the tropopause would tend to move poleward, cooling as it does so. Idealized planetary circulations • If we now add rotation, the flows at the surface would feel the coriolis force. • In the N. Hemisphere, surface flow would turn towards the right (west) against the sense of Earth’s rotation, creating a drag on the planet. • HOWEVER, with rotation our simple model breaks down anyway and the real solution is one where the flow tends to occur in three belts of latitude. Thus, we have a big convective current. 1 Hadley Cell let’s add some rotation to our simple model The Conceptual picture from before still works, but the subsidence occurs at about 30° N and 30° S in “symmetric” belts. 30 N EQ 30 S This gives us: The Inter-Tropical Convergence Zone (ITCZ), the band of convection near the equator where air rises (surface low pressure) This “subtropical Hadley Cell Model” also gives us: • The “Horse Latitudes” near 30° N and S where air sinks (surface high pressure) • As surface air flows south in N.H. it is deflected by coriolis force to right (west) giving northeasterly TRADE WINDS. • Subsidence in Horse Latitudes is responsible for most of world’s great deserts. (why?) HIGH LATITUDE WINDS • The poles, again in agreement with the simple conceptual model are typically regions of subsidence and high surface pressure, especially during the winter season. • Again, (cold) surface air flows equatorward, in N.H. is deflected to right, again giving us surface north easterly winds. • This southward flow of cold air helps nature “mix out” the pole-to-equator temp gradient. Midlatitude Winds More on Midlatitude Winds • It is in the mid-latitudes that our conceptual model really breaks down. • Some of the descending subtropical air (Horse Latitudes) moves north rather than completing the Hadley Cell. • This air is again deflected to right by coriolis and gives the surface westerlies we are so familiar with between 25-35° and 55-65°, the midlatitudes. • In the midlatitudes, winds are typically westerly through-out the depth of the troposphere. In other words, no “Hadley Cell” type of phenomena typically occurs. • Note that where the midlatitude westerlies (which on average have a slight southerly component) meet the cold polar northeasterlies, convergence occurs. • This has 2 consequences 2) strong temp. gradients – 1) forced upward motion 2 The previous results give us convenient bands of latitude to refer to when talking about global circulation and global climate Of course these are idealizations of how climate works, Actually the boundaries btwn bands are not constant but change with time The “cells” and surface wind regimes Figure 11.2a The world’s major regions of cloudiness occur in areas of surface convergence Figure 11.2b Figure 11.5 REALITY BITES! • So much for conceptual models, both rotating and non-rotating... What about the Real Earth!... • Well, reality always rears it’s ugly head. • The main effects of land masses are: – 1) thermal inertia. Continents are warmer in summer and cooler in winter than oceans – 2) Orography (mountains.) Mountains provide a substantial “aerodynamic drag” on the winds and also act as a barrier to air mass motion, especially near the surface. Semipermanent features in N. HemisphereWinter 3 N hemisphere summer 4