Chapter 7 Atmospheric Circulations

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1 Chapter 7 Atmospheric Circulations 1 Convection Cell Warm air is less dense than cool air so it rises The induces low pressure at the surface and high pressure aloft Cold air is more dense so it sinks This creates high pressure at the surface and low pressure aloft The net result is the formation of horizontal pressure gradient forces at the surface and aloft, each pointing in the opposite direction Surface low pressure can create clouds & storms 2

T land > T water during the day T water > T land at night Because the temperature between

2 3 Figure 7-4 p188 Land and Sea Breezes Land has a lower specific heat than water Therefore it heats up much more quickly during the day and cools down faster at night. T land > T water during the day T water > T land at night Because the temperature between land and sea differ, a convection cell develops Direction of circulation dictated by whether land or water is warmer 4

3 5 Figure 7-5a p190 6 Figure 7-5b p190

Precipitation removes nearly all the moisture On the leeward side of the mountains: Dry

4 Chinook Winds Hot Dry winds common in Colorado downwind from the Rocky Mountains On the windward side of the mountains: Moist air cools at moist adiabatic lapse rate Precipitation removes nearly all the moisture On the leeward side of the mountains: Dry air is warmed, due to compressional heating, at the dry adiabatic lapse rate 7 8 Figure 7-11 p193

Global Circulations These are convection cells just like Land/Sea Breezes, only much bigger Three Cells Hadley (Creates NE and SE Trade winds at surface) Ferrell (Creates Midlatitude Westerlies at

5 Global Circulations These are convection cells just like Land/Sea Breezes, only much bigger Three Cells Hadley (Creates NE and SE Trade winds at surface) Ferrell (Creates Midlatitude Westerlies at surface) Polar (Creates Polar Easterlies at surface) In the absence of Earth s rotation (no coriolis force), only the Hadley cell would exist Subtropical & Subpolar Jetstreams Aloft 9 10 Figure 7-24 p202

Copyright 2017 University of Maryland 11 Figure 7-25

Convergence between the Northeastern Tradewinds and

pressure in the tropics Meteorological Equator Moves

6 Copyright 2017 University of Maryland 11 Figure 7-25 p203 Intertropical Convergence Zone (ITCZ) Convergence between the Northeastern Tradewinds and Southeastern Tradewinds creates a band of low pressure in the tropics Meteorological Equator Moves North in June and South in December This movement impacts Monsoon rains 12

7 13 Figure 7-28 p Figure 7-29 p207

Streams are relevant for our region: Subtropical Jet

8 Jet Streams Intense rivers of westerly winds at very high altitude (300 to 250 mb) Induced by global circulation patterns Two Jet Streams are relevant for our region: Subtropical Jet (Hadley-Ferrell boundary) Subpolar Jet (Ferrell-Polar boundary) Figure 7-32 p208

9 17 Figure 7-33 p209 Ocean Currents Dominant Winds and Coriolis Force interact to create major ocean gyres Anticyclonic circulation in midlatiude ocean basins due to subtropical high pressure systems 18

Ekman Transport Ocean Currents travelling equatorward and parallel to continents induce Upwelling Upwelling is a result of coriolis forces driving surface water offshore The surface divergence

11 Ekman Transport Ocean Currents travelling equatorward and parallel to continents induce Upwelling Upwelling is a result of coriolis forces driving surface water offshore The surface divergence encourages deep cold water to rise to the surface The deep cold water is typically rich in nutrients which allows it to support very productive fisheries (eg. Peruvian Anchovies) Figure 7-39 p212

Anchovy fishery Originally considered a local phenomenon, ENSO is now recognized to have global impact La

12 El Niño (ENSO) Coupled interactions between atmosphere and oceans Weakened trade winds in the South Pacific Disrupts upwelling off the coast of Peru Warm nutrient-poor water intrudes into the Peruvian Anchovy fishery Originally considered a local phenomenon, ENSO is now recognized to have global impact La Niña is the opposite phase of the oscillations Features an intensification of normal conditions Figure 7-40a p214

13 25 Figure 7-40b p Figure 7-41 p215

14 27 Figure 7-42a p Figure 7-42b p216

15 Other Oscillations Coupled Ocean-Atmosphere Oscillations are not unique to the South Pacific: North Pacific: Pacific Decadal Oscillation (PDO) North Atlantic: North Atlantic Oscillation (NAO) Difference in pressure between Bermuda High and Icelandic low fluctuates, thereby weakening or strengthening Westerlies over Europe & North America Arctic Oscillation (AO) Pressure differnce also impact the strength of the Westerlies»Westerlies control outbreaks of Arctic air (Polar Vortex) 29 Chapter 8 Air Masses, Fronts, Mid-Latitude Storms 30

16 Air Masses Large body of air whose temperature and humidity are the same in any horizontal direction Can cover huge areas (hundreds of thousands sq miles) Influenced by the surface over which they form (source region) Air masses can be modified through lifting and heat exchange with the surface 31 Air Masses: Source Regions Two main surface categories Continental Formed over Land Generally dry Maritime Formed over ocean Generally moist Three main location categories Arctic (A) Polar (P) Tropical (T) Formed over Arctic Formed Poleward of 60 latitude Formed 30 S to 30 N Very Cold Cold or cool Hot or warm Characteristics of air masses depend on source regions 32

17 Air Masses: Source Regions 33 Cold Fronts Table 8.2: Essentials of Meteorology 34

forced up over both cold/very cold air masses May

18 Warm Fronts Table 8.3: Essentials of Meteorology 35 Occluded Fronts (cold type occlusion) cp mp Cold front moves faster than warm front, may catch warm front Warm air is forced up over both cold/very cold air masses May have mix of clouds similar to both cold and warm fronts Fig 8.22: Essentials of Meteorology 36

up over both cold/very cold air masses May have mix of clouds

22: Essentials of Meteorology 37 22: Essentials of Meteorology

19 Occluded Fronts (cold type occlusion) cp mp Cold front moves faster than warm front, may catch warm front Warm air is forced up over both cold/very cold air masses May have mix of clouds similar to both cold and warm fronts Fig 8.22: Essentials of Meteorology 37 Occluded Fronts (cold type occlusion) cp mp Cold front moves faster than warm front, may catch warm front Warm air is forced up over both cold/very cold air masses May have mix of clouds similar to both cold and warm fronts Fig 8.22: Essentials of Meteorology 38

Occluded Fronts (warm type occlusion) mp cp Cold front moves faster than warm

of clouds similar to both cold and warm fronts Fig 8.

20 Occluded Fronts (warm type occlusion) mp cp Cold front moves faster than warm front, may catch warm front Cool air is forced up over cold air mass May have mix of clouds similar to both cold and warm fronts Fig 8.22: Essentials of Meteorology 39 Occluded Fronts (warm type occlusion) mp cp Cold front moves faster than warm front, may catch warm front Cool air is forced up over cold air mass May have mix of clouds similar to both cold and warm fronts Fig 8.22: Essentials of Meteorology 40

21 Mid-latitude cyclones Cold, warm, and occluded fronts are often part of a larger system called the mid-latitude or extratropical cyclone Fig 8.24: Essentials of Meteorology 41 Cyclone Model 1920 s: Bjerknes described the evolution of cyclones Begins as a frontal wave along stationary front separating cp air from mt air birth stage Table 10-1 Meteorology: Understanding the Atmosphere 42

Cyclone Model 1920 s: Bjerknes described the evolution of cyclones Open wave develops strong cold and warm fronts, precipitations falls along broad area young adult stage Table 10-1 Meteorology:

22 Cyclone Model 1920 s: Bjerknes described the evolution of cyclones Open wave develops strong cold and warm fronts, precipitations falls along broad area young adult stage Table 10-1 Meteorology: Understanding the Atmosphere 43 Cyclone Model 1920 s: Bjerknes described the evolution of cyclones Occluded front develops, pressure reaches minimum, winds reach maximum, mature stage Table 10-1 Meteorology: Understanding the Atmosphere 44

23 Cyclone Model 1920 s: Bjerknes described the evolution of cyclones Cut-off cyclone develops, pressure rises, clouds and precipitation dissipates death stage Copyright University of Maryland Table 10-1 Meteorology: Understanding the Atmosphere Chapter 9 Weather Forecasting 46

Folklore forecast Red sky at night, sailor s delight Red sky in the morning, sailors take warning If there s a red sunset, it may mean that there s a high pressure system upwind from us High

High pressure is leaving to be followed by low pressure Low pressure = storms 47 Types of Forecasts Folklore: based on traditional proverbs, sometimes accurate.

24 Folklore forecast Red sky at night, sailor s delight Red sky in the morning, sailors take warning If there s a red sunset, it may mean that there s a high pressure system upwind from us High pressure=nice weather If the sunrise is red then high pressure is to the east, downwind from us. High pressure is leaving to be followed by low pressure Low pressure = storms 47 Types of Forecasts Folklore: based on traditional proverbs, sometimes accurate. Uses behavior of animals and other creatures as predictors of future weather. Persistence: assumes that the weather will not exhibit large day to day fluctuations. The weather tomorrow will be like the weather today Climatology: assumes the weather for a day or a season will be close to the average weather for that day or season. Wide brown stripe = mild winter Cows lie down before weather to save warm, dry spot 48

Types of Forecasts Persistence: we know that, at some point, this will be wrong because eventually the weather WILL change Probability/Climatology: we know that, at some point, this will be wrong

25 Types of Forecasts Persistence: we know that, at some point, this will be wrong because eventually the weather WILL change Probability/Climatology: we know that, at some point, this will be wrong because eventually the weather WILL deviate from the average We know that weather changes at a particular spot because weather features move but do the weather features themselves change? 49 Trend Forecast Recognizes that weather causing patterns move but assumes the following remain unchanged: speed intensity size direction 50

Analog Forecast Recognizes that weather causing patterns change but assumes: weather will always behave the same way under a specific set of conditions In other words, weather repeats itself If you

26 Analog Forecast Recognizes that weather causing patterns change but assumes: weather will always behave the same way under a specific set of conditions In other words, weather repeats itself If you find the last time current conditions existed, you can use the historical data to determine how conditions will change 51 Analog Forecast The weather today is analogous to weather at a time in the past Fig 13-4 Meteorology: Understanding the Atmosphere 52

Statistical Forecast Uses Model Output Statistics (MOS) Statistically weighted analog forecast looks at model output that best forecasted past events to make future predictions 53 Types of Forecasts

27 Statistical Forecast Uses Model Output Statistics (MOS) Statistically weighted analog forecast looks at model output that best forecasted past events to make future predictions 53 Types of Forecasts Nowcasting A description of current weather parameters and 0-2 hours description of forecasted weather parameters Very short-range weather forecasting Short-range weather forecasting Medium-range weather forecasting Extended-range weather forecasting Up to 12 hours description of weather parameters Beyond 12 hours and up to 72 hours description of weather parameters Beyond 72 hours and up to 240 hours description of weather parameters Beyond 10 days and up to 30 days description of weather parameters, usually averaged and expressed as a departure from climate values for that period. Long-range forecasting From 30 days up to two years 54

28 Ensemble forecasts: Ensemble Forecasts Run model numerous times for the slightly different initial conditions Perform statistical analysis of all the model runs 55 Forecasting (Surface Map) Fig 9.14: Essentials of Meteorology56

29 Forecasting (500 mb Map) Fig 9.15: Essentials of Meteorology57 Forecasting (Future Surface) Fig 9.16: Essentials of Meteorology58

30 Chapter 10 Thunderstorms and Tornadoes 59 Thunderstorms: Some Key Facts Produced by cumulonimbus clouds and are accompanied by lightning and thunder. Occurs when the atmosphere becomes unstable when a vertically displaced air parcel becomes buoyant and rises on its own. The ideal conditions include warm, moist air near the surface and a large change in temperature with height (large lapse rate) 60

Multi-cell thunderstorms can last for hours and can become very strong.

31 Types of Thunderstorms Air mass thunderstorms usually harmless and short-lived (less than an hour). Multi-cell thunderstorms can last for hours and can become very strong. Examples include: supercell storms and squall lines, MCC s 61 Single Cell Air Mass Thunderstorm 62 Fig. 10-1, p. 265

32 Severe Thunderstorms Can last for hours and produce strong winds, large hail, flash flooding, tornadoes. Have found the secret of longevity Most important types are supercell storms, squall lines, and bow echo storms. 63 What is the secret of the strength and longevity for severe thunderstorms? They all grow in environments with large vertical instability. But they also grow in an environment of large wind shear wind changing significantly with height. What difference does that make? 64

33 Major Thunderstorm Structures updraft Cirrus Anvil, Gust Front, Updraft, Downdraft 65 Supercell Storms One giant updraft that can have upward speeds as high as mph Large size: miles in diameter. The large updraft is often rotating: called a mesocyclone 66

34 67 Fig , p. 291 Squall Lines Long, linear lines of strong thunderstorms Strong narrow convective line, followed by a wide region of stratiform precipitation Mainly in the central and eastern U.S. 68

35 69 Figure p Figure p296

36 Charge Separation in Clouds Charge separation appears to depend on strong updrafts, ice crystals, and supercooled water. Large ice crystals fall rapidly and collect the smaller, slower, supercooled water drops in their path. The drops freeze on the surface of the falling ice crystals, building graupel particles. When graupel particles fall through supercooled water and ice crystals, they acquire one charge, and the water-ice mix acquires the opposite charge. Or so we think! 71 Typical Cloud to Cloud Lightning Stroke (a) Negative charge descends the cloud in a series of steps called a stepped leader 72

37 Typical Cloud to Cloud Lightning Stroke (b) As the stepped leader approaches the surface, positive charges moves upwards to meet it. When the potential gradient (volts per meter) increases to about one million volts per meter, the insulating properties of the air begins to break down 73 Typical Cloud to Cloud Lightning Stroke (negative lightning) (c) With break down, a return stroke begins, with negative charge surging downward in the cloud. 74

38 Origin of rotation in tornadoes Severe thunderstorms associated with mesocyclones (strongest tornadoes) Weaker thunderstorms associated with fronts and shear lines (weaker ones) 75 Why mesocyclones? Why is wind shear important? Origin of rotation in the mesocyclone 76

39 77 Figure p298 Chapter 11 Hurricanes 78

40 Hurricane Anatomy Eye the center of the storm, calm, clear skies Sinking air at edge of storm makes clear skies Eye wall intense thunderstorms surrounding the eye 79 Hurricane Anatomy Vigorous convection of eye wall induces downward air motion in eye due to warming aloft Rain bands in storm structure 80

41 What is required? Hurricane Formation Humid air column good for storm formation Coriolis forces Warm surface ocean waters (80 o F) Trigger to start air convergence (atmospheric wave) No vertical wind shear disperses heat and moisture 81 Hurricane Paths Why do hurricanes typically take this path in the North Atlantic? 82

42 Hurricane Destruction Maximum winds usually occur to the east of the eye wall if the storm is moving due north, why? Where would the maximum winds be of the storm is moving west? Major causes of damage: Storm surge Flooding High winds 83 Chapter 13 The Earth s Changing Climate 84

43 Past Climate Methods to determine past climate Records Tree rings Pollen Ice cores 65 million years ago warmer than today Last ice age 18,000 years ago 0.6 o C warming in 20 th century, about 0.9 o C from preindustrial (about 130 years) 2 o C change over past 10,000 years 85 Past Climate External causes of climate change Solar radiation Atmospheric composition Earth s surface Internal causes of climate change Ocean circulation Atmosphere circulation Feedback mechanisms Positive feedback initial perturbation is reinforced by later events Negative feedback initial perturbation is then weakened by later events 86

Climate Change: Natural Solar Output Changes on an 11 year cycle Volcanoes Spew massive amounts of Sulfur into the Stratosphere corresponds to a global cooling Plate Tectonics Movement of continents

44 Climate Change: Natural Solar Output Changes on an 11 year cycle Volcanoes Spew massive amounts of Sulfur into the Stratosphere corresponds to a global cooling Plate Tectonics Movement of continents over the Earth s surface Distribution of land effects global circulation Milankovitch Cycles Eccentricty shape of Earth s orbit around the sun Precession wobbling of the Earth s axis Obliquity changes in the tilt of Earth s axis 87 Climate Change 88

Climate Change: Human Activity Aerosols and GHGs Sulfate aerosols (anthropogenic and natural) CO 2, N 2 O, CH 4 Aerosols Direct effect: changes in albedo Indirect effect: changes in clouds Land Use

45 Climate Change: Human Activity Aerosols and GHGs Sulfate aerosols (anthropogenic and natural) CO 2, N 2 O, CH 4 Aerosols Direct effect: changes in albedo Indirect effect: changes in clouds Land Use change Forest clearing for farm land and grazing land change the albedo of the area 89 Positive Feedback Ice-albedo feedback Water vapor-greenhouse feedback CH4 permafrost Negative Feedback Land Use Increased plant life Feedback Mechanisms 90

Climate Change: Problems Increasing extreme weather Expansion of Hadley Cells (tropics are growing) Movement of world s deserts Fewer hurricanes but increasing in strength Spread of diseases Sea

46 Climate Change: Problems Increasing extreme weather Expansion of Hadley Cells (tropics are growing) Movement of world s deserts Fewer hurricanes but increasing in strength Spread of diseases Sea level rise due to melting land ice Melting glaciers (though some are growing) Surface warming/stratosphere cooling 91 Climate Change: Future Changing climate most likely due to natural (long term) and anthropogenic (shorter term) influences Projections indicate temperatures rise likely but maybe not quite as fast as some have predicted 92

Wind: Global Systems Chapter 10

Wind: Global Systems Chapter 10 General Circulation of the Atmosphere General circulation of the atmosphere describes average wind patterns and is useful for understanding climate Over the earth, incoming