Transcript Chapter 7
Today in Weather History • • • 1909- Tornado outbreak in Arkansas, The town of Brinkley AR was struck by a tornado which killed 49 persons and caused 600,000 dollars damage. The tornado, which was two-thirds of a mile in width, destroyed 860 buildings. Entire families were killed as houses were completely swept away by the tornado. Tornadoes killed 64 persons and injured 671 others in Dallas and Monroe counties during the Arkansas tornado outbreak. 1989 - While arctic cold gripped the northeastern U.S., unseasonably warm weather prevailed across the southwestern states. Albany NY reported a record low of 2 degrees below zero. Tucson AZ reported a record high of 90 degrees. 1990 – Long track supercell produced severe weather in east central Iowa and west central Illinois. Supercell spawned a tornado south of Augusta IL which traveled 42 miles to Marbleton. Golf ball size hail was reported at Peoria IL and near Vermont IL. • Launch of new GOES-P geostationary satellite • Satellite also carries an X-ray imager to sense xray emissions from the sun • Joins GOES 11, 12, 13 Global Circulation Chapter 7 ATMO 1300 Fall 2009 General Circulation of the Atmosphere • Represents the average wind flow around the globe… • Winds at any one place may vary substantially from this average • Underlying cause of the general wind pattern around the globe is the unequal heating of the Earth’s surface • Remember: Energy surplus at the equator compared to the poles! • Circulation patterns develop in an attempt to achieve balance General Circulation of the Atmosphere • Flow of air across the globe is quite complex • Can be viewed in the form of conceptual models • Simplified examples of the flow of air Fig. 7-11, p. 197 Single-Cell Model • Assumes Earth’s surface is uniformly covered by water • Assumes the Sun is always directly over the equator • Assumes the Earth is not rotating • So… the only force we have to consider is pressure gradient (no rotation = no Coriolis) Single-Cell Model Single-Cell Model • Single-cell model referred to as Hadley cell • Where might this model be applied? Hadley Cell • Thermally driven convective cell. • Energy surplus at the equator creates and area of rising motion and thus low pressure at the surface near the equator • At the poles, the energy deficit creates sinking motion and high pressure near the surface • Pressure gradient force is directed southward (northern hemisphere) from the pole toward the equator (remember no Coriolis) Hadley Cell • At the upper-levels the flow of air is reversed • Air flow from the equator toward the poles • Through this manner, some of the excess of energy in the tropics is transported as sensible and latent heat to the regions where there is an energy deficit (poles) • Closed circuit of air • Does not happen on Earth, Earth is rotating • So what does the Earth’s rotation do??? Three-Cell Model • Allow the Earth to rotate now… • The simplified convective cell is now broken down into 3 cells. • Remember: Tropics still have an energy surplus • Broad trough still located at the equator • Broad ridge located near the poles Fig. 7-6, p. 193 Three-Cell Model • Between 0˚ and 30˚ of latitude the convective cells resemble our Hadley Cell (Single Cell model) • Over equatorial oceans, air is warm and pressure gradient is relatively weak. Therefore surface winds are generally light. Referred to as the Doldrums • Often warm air rises into deep towering cumulus clouds, can be referred to as “Hot Towers” due to the enormous amount of Latent Heat that is released Three-Cell Model • The latent heat released within tropical convection aids in strengthening the Hadley Cell due to enhanced rising motion over the tropics • As the rising air reaches the top of the troposphere it spreads out poleward • Coriolis now deflects the flow to the right (northern hemisphere) or left (southern hemisphere) • The Coriolis effect now produces westerly winds aloft in both hemispheres Three-Cell Model • As air moves poleward it cools and converge as it approaches the middle latitudes • The convergence leads to a piling up of air aloft • This increases the pressure at the surface (more stuff now above) • Creates sub-tropical highs • The converging air aloft leads to sinking motion beneath the sub-tropical highs. The subsiding air warms and produces generally clear skies and warm surface temperatures • This is what produces the worlds deserts • Over the oceans, the center of the subtropical highs produce weak winds, referred to as “Horse Latitudes” Three-Cell Model • From the Horse Latitudes some of the surface air flows back toward the equator • Coriolis deflects this air creating our low-level easterlies or trade winds • Air moves from the northeast in the northern hemisphere and the southeast in the southern hemisphere • These are fairly steady surface winds and provided sailing ships with an ocean route to the Americas. Hence the title “Trade Winds” • Near the equator the trade winds in each hemisphere collide creating a large area of surface convergence, Inter-Tropical Convergence Zone (ITCZ) Typical Locations of the ITCZ ITCZ varies little by the season, due to the required energy to change the temperature of water Fig. 7-20, p. 203 Three-Cell Model • Back to 30˚ Latitude… not all the surface air moves toward the equator, some move towards the poles • This air is deflected by Coriolis creating middle latitude westerly winds • This flow is not consistent as migrating areas of surface high and low pressure break up the flow pattern • As the air travels poleward it encounters cold air moving equatorward from the poles. The two airmasses (more later) do not mix readily (think density differences) • The boundary between these airmasses is referred to as the Polar Front • The upper-level Jet Stream resides in this area D:\Media\Active_Figures\56_General_Circulation\G_56.html Fig. 7-13, p. 199 Fig. 7-14, p. 199 Fig. 7-1, p. 189 General Circulation of the Atmosphere • What happens in the “Real World”? • A few area of semi-permanent highs and lows • In the eastern Atlantic we see the BermudaAzores high, drives tracks of tropical cyclones • Counterpart in the Pacific, Pacific High • Islandic Low, Aleutian Low occur where we expect the polar front to lie. • Siberian High Fig. 7-12a, p. 197 Fig. 7-12a (1), p. 197 Fig. 7-12a (2), p. 197 General Circulation of the Atmosphere • Thermal lows lead to monsoon circulations • Leads to moist flow off the oceans onto the adjacent land surfaces. • Often responsible for large percentage of precipitation in these regions Fig. 7-21, p. 205 General Circulation and Precipitation Patterns • On the global scale… we expect abundant rainfall where we see general rising motion. • Thus over the tropics, there are areas of high precipitation due to the ITCZ • Also high precipitation between 40˚ and 55˚ latitude where mid-latitude storms and the polar front force air upward • Areas of low precipitation are found near 30˚ latitude in the vicinity of sub-tropical highs Fig. 7-4a, p. 191 Fig. 7-4b, p. 191 Average Wind Flow and Pressure Patterns Aloft Average Wind Flow and Pressure Patterns Aloft • In both hemispheres, the air is warmer over the low latitudes and colder over the high latitudes • This creates a temperature gradient which leads to a pressure gradient which causes our generally westerly flow in the mid-latitudes • Temperature gradients are steeper in winter than summer. Consequently the winds aloft are stronger during the winter than summer • In the mid and high latitudes wind speeds will continue to increase above 500 mb (no slowing due to surface friction) Average Wind Flow and Pressure Patterns Aloft • Geostrophic wind is related to the pressure gradient but also inversely proportional to density • Density decreases with altitude in the atmosphere, therefore a given pressure gradient will result in stronger winds with altitude. • We see concentrated bands of strong winds near the top of the troposphere – The Jet Stream Jet Stream • High speed “rivers” of wind • Several thousand km long but only a few km deep • Wind speeds can often exceed 100 kts • Polar Jet (near the polar front) • Sub-tropical Jet (over the subtropical high) Polar Jet • In the winter the Polar Jet is typically stronger and located further south • In the summer the jet is weaker and retreats well into the higher latitudes (less temperature gradient, less pressure gradient) Sub-Tropical Jet • Forms along the northern side of the Hadley Cell circulation • Warm air carried poleward by the Hadley Cell produces sharp temperature contrasts • Sharp temperature contrasts produce sharp pressure gradients and therefore stronger winds Jet Streams • Another mechanism can produce strong westerly flow aloft (other than temperature gradients) • Conservation of Angular Momentum • Air on the earth moves in a circular pattern due to the Earth’s rotation and therefore has angular momentum • Angular momentum is dependant on: Mass, Speed, and distance (radius) between the blob of air and the axis about which it rotates. Angular Momentum = mVr So if there are no external forces at work on the rotating system… angular momentum is conserved or does not change So if we decrease distance or radius… Speed must increase to compensate. Fig. 7-8, p. 194 Conservation of Angular Momentum and Jet Streams • Consider heated parcels or blobs of air rising from the equator… • The air parcels rise and reach the tropopause, they spread out and head poleward • The air as it moves poleward gets closer to the axis of rotation of the earth (e.g. radius of the earth gets smaller with increasing latitude • Because angular momentum is conserved, the speed of the parcels must increase (mass of the air is unchanged) Fig. 7-7, p. 194 Fig. 7-9, p. 195 General Circulation of the Atmosphere • We know that the winds in the upperatmosphere (troposphere) flow in a wave-like pattern with troughs and ridges • These features move cold air equatorward and warm air poleward • The northern hemisphere is typically encircled by several of these waves at any given time • These waves are called long-waves or Rossby Waves (named for Carl Gustav Rossby) General Circulation of the Atmosphere • Just like electromagnetic waves, waves in the atmosphere have a wavelength, amplitude and period • Describing the movement of these waves is a key component in weather forecasting (remember the vertical motions associated with troughs and ridges) • Small amplitude waves result in a nearly zonal flow (west to east flow pattern). The flow is nearly parallel to lines of latitude • In this regime cold air tends to remain poleward General Circulation of the Atmosphere • Meridional flow pattern means highly amplified troughs and ridges • In this pattern, cold air flows toward the equator and warm air flows poleward General Circulation of the Atmosphere • Superimposed on the long-waves or Rossby waves are smaller features called short-waves • These features travel quickly through the Rossby waves • Difficult to observe and track, adds to uncertainty in weather forecasts Fig. 7-18a, p. 202 Fig. 7-18b, p. 202 Other Types of Jet Streams • Tropical Easterly Jet • Stratospheric polar night jet • Low-level Jet Low-Level Jet • Common over the Great Plains of the US • Can reach 50-60 kts only a few hundred meters above the surface • Surface winds remain light • Atmosphere is effectively “decoupled” during the night from surface friction effects • Sloping terrain east of the Rockies also plays a role Atmosphere – Ocean Interactions • As wind blows over ocean surfaces it imparts a force on the water below • The surface water begins to move along with the prevailing wind • The moving water gradually piles up creating pressure differences, just like we see in the atmosphere • This leads to motions within the ocean several hundred meters deep Atmosphere – Ocean Interactions • The general low-level flow around the globe starts the major ocean currents • Due to larger frictional drag, ocean currents move slower than the wind above • Typical speeds range from a few km/day to several km/hr Atmosphere – Ocean Interactions • Ocean currents don’t quite follow the wind flow perfectly • They typically show a more spiral-like appearance, called gyres • Water is deflected by Coriolis just like the moving air • Surface water typically moves at an angle between 20˚ and 45˚to the direction of the wind • Water will move in a circular pattern as wind moves outward from a high pressure center • Gulfstream along the US east coast • Transports warm water northward Loop Current – Gulf of Mexico California Current • Southward flowing current along the coast of California • Brings cool water along the coast • Responsible for moderating temperatures along the west coast Upwelling