Wind Midterm Review

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Environmental Science
Tanzina Mohsin

Lecture 1 Mythological and Cultural Winds  Aeolus (Greek god of Wind), Feng Po Po (Chinese goddess of wind), Haya-ji (Japanese god of wind, whirlwind), Nilch’i (Navajo holy wind)  Japanese culture => gods of Shinto, the god of wind  Hindu and Buddhist religions => nature or state of a god, referred to as “vayu”, “pavan” &“godai”  Wind is one of the five great elements (Islam, Judaism, Christianity) Atmospheric Primer: The history of Wind  The earth’s atmosphere has existed for 4.6 billion years, since the earth was formed  The composition of the atmosphere has not been constant  Variations in atmospheric constituents have occurred due to three main controls: o Geological, biological, anthropogenic Geologic Control  The early earth’s atmosphere had a composition largely the result of volcanic emissions, carbon dioxide (CO2) & methane (CH4)  About 3.8 billion years ago life appeared o Early life => anaerobic (not needing oxygen) in nature flourished in this environment o Called the “Age of Bacteria”  After 2.3 billion years of the methane and carbon dioxide atmosphere, there was an abrupt change in atmospheric conditions Biological Control  2.3 billion years ago oxygen made an appearance in the atmosphere and stabilized at 21% (optimum for aerobic life) o Aerobic life (life needing oxygen) forms appeared and flourished o Atmosphere relatively constant since this time => variations in trace gases such as carbon dioxide  Played key role in determining the thermal conditions of earth  The Gaia hypothesis => James Lovelock (1970s) o Interaction of climate with biology  The basic premise is that life modifies the environment to best suit itself  Mars and Venus are in a static equilibrium with high levels of carbon dioxide and methane o Why switch from anaerobic conditions to aerobic conditions 2.3 billion years ago?  Early sun produced 30% less energy (Archean Era) o Solar output => gradually increasing  The early earth with high levels of methane and carbon dioxide => strong greenhouse effect o Led to conditions warm enough for life  After 2.3 billion year solar output increased => earth => too warm, the switch to aerobic life, reduced the greenhouse gases (CO2, CH4) => cooled the planet o Now => considerably lower levels of CO2 in the atmosphere than the early earth  Gaia hypothesis hypothesizes that atmospheric constituents have been controlled by life to optimize conditions for life Anthropogenic Control  During the Carboniferous Period (360 – 290 million years ago) during the Paleozoic Age life arrived on land  Sun’s energy converted to plant material =>photosynthesis  Not all decaying plants are fully oxidized => source material for coal and oil  Geologic pressure converts the plant material into fossil fuels  These are stored for millions of years => below the earth’s surface o Large reservoir of stored solar energy  Many of today’s air quality problems arise from the rapid release of this stored energy (smog, acid rain, global warming). The Story of Coal  Represents first major anthropogenic modification of the atmosphere o Britain => industrial revolution in the early 19th century => invention of the steam engine o Industrialization of United States in the 19th and 20th centuries o Industrialization of China => Coal was used to forge steel => 11th century=> now: undergoing industrialization at end of the 20th century & beginning of the 21st century  Atmosphere => modern dumping ground  Fossil fuels (coal, oil, and other hydrocarbons) are the major sources of emissions o Acid Rain, Urban Air Quality, Urban Heat Island, Global Warming o Other industrial emissions => Ozone Hole Composition  Matter can appear in three different phases o Solid, liquid and gas  The permanent gases are in dynamic equilibrium and not static equilibrium o 21% oxygen level is the result of both creation and destruction processes that are balanced to leave a constant level  The variable gases include those that are referred to as greenhouse gases What is a greenhouse gas?  A greenhouse gas is an atmospheric constituent that traps outgoing terrestrial radiation  The earth has a temperature => radiates to the atmosphere => outer space o Some gases intercept this radiation and re-radiate the energy, some of it back to the surface  Causes earth’s surface to be warmer than it would otherwise be (on average 33oC warmer) => Natural Greenhouse Effect o Gases in the atmosphere that do this are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3) and chlorofluorocarbons (CFCs) o Anthropogenic emissions of these greenhouse gases traps excess heat =>Enhanced Greenhouse effect=> a.k.a. Global Warming  Is the composition of the atmosphere static? o No, paleo-atmospheres were different (as discussed above with Gaia hypothesis) o Currently atmosphere is undergoing an unprecedented change with substantial increases in greenhouse gases Clouds  The liquid phase in the atmosphere is detectable as clouds  Cloud types are named using five root morphs: strato, cumulo, cirro, nimbo, and alto  Strato – layered clouds. Greater horizontal extent than vertical extent  Cumulo – puffy clouds. Similar horizontal and vertical extent  Nimbo – rain clouds  Alto – mid level clouds, between 2 km and 7 km  Cirro – high level clouds (above 7 km)  Stratus – low level, layer cloud  Altostratus – mid level layer cloud  Cirrostratus – high level layer cloud  Nimbostratus – layer cloud which produces rain. Darker than stratus and greater vertical extent  Cumulus – low level puffy cloud  Stratocumulus – a layer of puffy clouds  Cumulonimbus – a towering cloud extending through the troposphere (0-11 km) which produces intense precipitation including hail  Altocumulus – mid level puffy clouds  Cirrocumulus – high level puffy clouds  Cirrus – wispy high level clouds Atmospheric Layers  All of these clouds occur in the lowest layer of the atmosphere, the troposphere  Polar stratospheric clouds appear over the poles in the stratosphere o Involved in the creation of the ozone hole  Divided into four vertical levels: o Defined by vertical temperature structure 1) Troposphere a. Extends to approximately 11 km b. Layer => well mixed vertically c. Virtually all weather occurs in this part of the atmosphere d. 75% of the mass of the atmosphere  The tropopause => isothermal region separating troposphere & stratosphere  Temperature decreases in the troposphere 2) Stratosphere a. Extends from approximately 11 km to 50 km b. Temperature increases with height in this layer  Warming due to conversion of incoming solar radiation into kinetic energy of motion via a layer of ozone  Process => relates to the ozone hole issue  Layer => very stable (very little vertical mixing) 3) Mesosphere a. Extends from 50 to 85 km b. Temperature decreases with height in this layer c. Well mixed layer & less stable than stratosphere 4) Thermosphere a. Extends above 85 km b. Temperature increases with height c. Incoming solar radiation => absorbed by molecular oxygen & energy is converted to the kinetic energy of motion  Although temperature => very warm =>not feel warm at that level of the atmosphere because air has very low density at this height => a low transference of heat =>feel warm What is wind?  Wind cannot be seen directly o Movement of objects (trees, grass) o Causing objects to become airborne (dust)  Wind can be felt & heard  Wind results from the differences in air pressure, either vertical or horizontal  Differences in pressure produce a pressure gradient  Air tends to move from high pressure to low pressure => pressure gradient force What do we measure?  In general the following are measured: temperature, pressure, humidity, wind speed and direction, precipitation, sky condition & Thermometer  Invented => late 16th century  Liquid-in glass thermometer  Temperature changes measured => expansion of a fluid (mercury, alcohol)  Another type => electrical thermometer o Electrical resistance is a function of temperature => exploited to form a thermometer  Another type => bimetallic thermometer (two metals welded together) o Two metals have different thermal expansions => bend in different directions depending if the temperature is warm or cool Barometer  Measures pressure using mercury  Torricelli; 1643.  Pressure changes are measured using the movement of mercury in a column  Another barometer => aneroid barometer o Pressure measured by changes in volume of a partially evacuated box o Commonly used in aircraft and referred to as an altimeter Humidity  Humidity: Water vapour content of the atmosphere  Hygrometer & psychrometer  Hygrometer => 1780 o Linked expansion & contraction of material due to changes in humidity o Hair used in this instrument  Psychrometer => two temperatures the dry bulb temperature and the wet bulb temperature o The dry bulb temperature is the ambient temperature => measured by a thermometer o The wet bulb temperature => lowest temperature attainable due to the cooling of air due to evaporation of water until saturation occurs Wind speed  Measured using anemometer o Series of small cups => which catch the wind and rotate o The faster the rotation, the faster is the wind o Weather vane indicates wind direction Precipitation  Measured using rain and snow gauges Lecture 2  Coriolis force => “fictitious” force due to rotation of the earth (Gaspard Gustave de Coriolis: 1835)  Northern Hemisphere: deflect the wind to the right  Southern Hemisphere deflect the wind to the left  Above the surface of the earth (1 km or more) there is a balance between pressure gradient force and the Coriolis force forming the geostrophic wind o Only way to achieve a balance between these forces is for the wind to flow perpendicular to the pressure gradient  At and near the surface => friction plays role forming a three way balance of forces o Component of the wind that flows towards the center of a low and away from the high-pressure center o This surface convergence of winds at the center of a low is critical for the formation of midlatitude cyclones Global circulation  The basic physics behind the formation of winds is pressure differences  Air has a natural inclination to move from areas of high pressure to those of low pressure o Earth’s rotation creates the Coriolis effect, which causes the wind to deflect (to the right in the northern hemisphere and to the left in the southern hemisphere) o Earth’s surface contributes friction which slows down and redirects (funnels) the wind o Land/sea contrast leading to temperature differences between land and water surface also influences wind o Winds vary with season  On the global scale there are three main circulation cells per hemisphere: 1) Hadley cell a. Air rising in the equatorial region =>driven by warm surface conditions b. Upper troposphere =>air starts to move horizontally towards the poles c. At 30.N and 30.S =>air descends to the surface d. At surface=> air returns to the equatorial region to complete the circulation e. Occurs in both the northern and southern hemispheres 2) Ferrel (sometimes spelled: Ferrell) cell a. Shares descending branch of air at 30.N and S with Hadley cell b. At surface =>air moves poleward until about 60.N and S =>ascends c. Circulation completed by a return flow equatorward in the upper troposphere 3) Polar cell a. Shares ascending branch with the Ferrel cell at 60.N and S b. Upper troposphere =>air from this cell flows poleward =>descends at the pole c. Complete circulation =>air flows equatorward at the surface to 60.N and S  Good model for the winds at the surface of the earth if world didn’t rotate: o Earth does rotate => causes deflection of winds due to the Coriolis force  As a result => surface distribution of winds that are different than this three-cell model o Between the equator and 30.N the wind should flow to the south  However, with rotational deflection the winds actually flow to the southwest; these are called the Trade Winds  Steadiest winds in the world  Winds from 30. to 60.N should flow north o The rotational deflection causes these winds to move in a northeast direction  These are called the Westerlies, named for the direction from which they come (not the direction they are heading)  Prevailing wind in the midlatitudes  Predominant wind in Toronto  North of the Westerlies => get winds flowing south from the pole to 60.N. o With deflection => winds head to the southwest => polar easterlies indicating the direction from which they come from  The horse latitudes occur near 30.N and 30.S o Region of little wind between Westerlies and Trade Winds and corresponds to the descending branch of Hadley/Ferrel cells o European sailors named it; ships in this region did not make much progress because there is no strong prevailing wind  Ditch some of their cargo, which included horses (the livestock would die off as water and food ran out)  Corresponds to the ascending branch of two Hadley cells, also called the Intertropical  Convergence Zone (ITCZ) =>surface convergence of air from either side of the equator  The Polar Front is a very active region between the Ferrel and Polar cells, where cold air and warm air collide o Source of most intense large scale storms in the midlatitudes o Roams north and south considerably with season Surface Winds  Three-cell theory predicts surface winds that are modified by the rotation of the earth, causing the winds to deflect to the right in the northern hemisphere and to the left in the southern hemisphere o Reflected in the Trades, Westerlies and polar easterlies  Also modifying wind structure is land/sea contrast and seasonality o Land/sea contrast arises =>the land surface has a lower heat capacity than water =>heats more rapidly & cools more rapidly  Warmer surface =>lower surface pressure & cooler surface =>higher surface pressure o Northern hemisphere winter high pressures form over land and lows over the oceans (Opposite occurs in the summer) Upper level winds  Not affected by friction from the surface, so winds at this level are generally geostrophic  Much less affected by the land/sea contrast, so high & low pressures are distributed more or less evenly along lines of latitude Jet streams  At the upper level (5 to 10 km above the surface) => current that is especially noteworthy  Jet streams, regions of intensified flow =>occur  Air flows => west to east  Two of these occur roughly above Hadley and Ferrel cells (the subtropical jet) and the Ferrel and Polar cells (the polar front jet)  Key role in determining the nature of the surface conditions =>guide to surface storms & aviation Air masses  Air mass is a large body of air spanning up to several thousands of kilometers in the lower troposphere  An air mass is characterized by horizontally uniform temperature and moisture content (humidity) at any given altitude  Form in a source region: o Area of large horizontal extent with relatively uniform surface properties such as an ocean/ continent  Air mass theory views the weather as a result the presence and interaction of these large air masses o Relatively stable high pressures with low pressure systems (storms or midlatitude cyclones) forming at the boundary between these air masses  North America=> four main types of air masses: o Air masses are classified using a two letter nomenclature o First letter (written in lower case) =>whether the air mass formed over land (continent) or water (ocean) “c” indicates continental origin, “m” (for „maritime) indicates the air mass formed over water o Second letter (upper case) =>whether the air mass formed in polar latitudes, “P”, or tropical latitudes, “T” o Combining these two letters allows for four possibilities  mT air, referred to as maritime tropical air, forms in the tropics over the ocean  For North America this can occur over the subtropical Atlantic, the Gulf of Mexico and Caribbean, and the subtropical Pacific  Air mass is warm & moist  mP air, maritime polar air, is formed over the ocean in polar latitudes o For North America, this occurs in the North Atlantic and North Pacific  Air mass is cool & moist  cP air, continental polar air, is formed over continents in the polar regions o The North American source region is the vast tundra of Canada’s Northwest Territories and Nunavut  Air mass is dry & cold  cT air, continental tropical air, forms over a continent in tropical latitudes o Observed in summer forming over Mexico =>not present at other times of the year o Some weather analysis => “A” for arctic  Colder air mass forming over the Canadian Arctic archipelago  cA air, continental Arctic, is similar to cP air but colder  mT air from the Gulf of Mexico and the subtropical Atlantic dominates the weather in the southeastern United States producing warm and very humid weather in the summer o Winter => mT air interacts in the midlatitudes with cP from northern Canada o Major winter storms forming at the boundary of the two air masses =>along the boundary =>snowfall in central & eastern United States and Canada  West coast of North America => heavily influenced by mT and mP air forming over the Pacific Ocean o Air masses do not penetrate in to the North American continent due to the presence of the Rocky Mountains and other west coast mountain ranges  Mountains=> barrier o As air moves up a mountain slope => pressure drops =>some of the moisture in air mass =>converted from vapour to liquid and the mountains experience rain and snow  The air on shielded side of the mountain is considerably dryer than that formed over the Pacific  Warmer =>release of latent heat during the condensation process Stratospheric Winds Dobson-Brewer Circulation  Stratospheric winds => one-cell circulation: Dobson-Brewer circulation  Air rises at the equator & sinks at the poles  The Coriolis force does act upon these winds and this produces easterly (flowing from the east) in the midlatitudes  At equator =>longitudinal (east/west) flow can be in either direction & swings in a 2-3 year cycle: quasi-biennial oscillation (QBO) Quasi-biennial oscillation  The quasi-biennial oscillation is a variation of the lower stratospheric winds in the equatorial region  Winds switch from being easterly (from the east) to westerly (from the west) every two years or so  The QBO winds also exhibit an asymmetry; the easterly phase tends to have stronger winds than the westerly phase  The QBO was discovered in the late 1950.s using rawinsonde data  Rawinsonde: package of weather instruments carried into the atmosphere by a weather balloon o Transmits pressure, temperature, humidity, wind speed, and wind direction data to the surface o The observation of the oscillation helped to explain earlier observations, such as volcanic ash dispersion  The QBO =>linked to several tropospheric weather phenomena, including tropical cyclones (hurricanes) in the Atlantic basin, the northwest Pacific, and Indian Oceans => affects stratospheric winds Research – New air mass classification system  Lawrence Kalkstein & Scott Sheridan o Introduced seven categories of air masses for North America  The first letter is either a D(dry) or M (moist) and the second letter is P (polar), M (moderate), or T (tropical) Combining these: 1) DP – cool, dry, little cloud – northern Canada/Alaska – similar to cP 2) DM – mild, dry – no traditional source, Rocky Mts. Modified air or mixtures of other air masses 3) DT – hot, dry – similar to cT – source SW USA, Mexico (deserts) 4) MP – cool, cloudy, humid – source: North Atlantic, North Pacific, Great Lakes 5) MM – cloudy, warmer and more humid than MP – modified mP air 6) MT – similar to mT – warm, humid – Gulf of Mexico, tropical Atlantic MT+ – subcategory of MT – hot and exceptionally humid 7) TR – transitional air mass Lecture 3 What is a tropical storm?  Tropical storm is defined as a storm occurring in the tropical region (20oN to 20oS) in which the sustained winds range from 18 to 33 m/s What is a hurricane?  Tropical storm in which the sustained winds range from 33 m/s to 50 m/s What is a major hurricane?  Tropical storm, which has sustained winds which exceed 50 m/s.  Another way to classify and quantify hurricanes is the Saffir-Simpson Scale o Rating scale spanning from 1 to 5 and is based on wind speed and the central pressure of the hurricane Saffir-Simpson Scale Dynamics How do hurricanes form?  Tropical storms are fueled by sea surface temperatures & latent heat release Category One Category Two Category Three Category Four Category Five Hurricane Hurricane Hurricane Hurricane Hurricane 119-153 km/h 154-177 km/h 178-209 km/h 210-249 km/h Greater than 249 (33 – (42.5 -49 m/s), (49 – (58 – km/h (> 69 m/s), 42.5 m/s), > 980 965 – 58 m/s), 945 – 69 m/s), 920 – < 920 mb mb 979 mb 964 mb 944 mb o 26.5.C =>threshold for tropical storm formation o Necessary but not sufficient condition for tropical storms to form  Also needed is a relatively quiescent atmosphere (low winds) initially What is latent heat?  Energy released due to a change of phase of matter  Heat released due to change of phase: Heat that is absorbed when water evaporates, and released when water condenses  Major fuel source for storms such as Hurricanes, Midlatitudes cyclones Tropical Storm Development  Tropical storms begin as a tropical wave in the Intertropical convergence zone (ITCZ) near the equator  The storms spawn between latitudes 5– 20 in either of the northern and southern hemispheres  Groupings of thunderstorms => organized and self-sustaining => form tropical cyclone that can persist up to two weeks Requirements for Tropical Cyclones  For a tropical storm to occur, convergence must occur at the surface  This is a common feature of the ITCZ o Convergence =>focusing of surface winds o Above the surface divergence must occur in order to balance the surface convergence (otherwise the storm will not be self-sustaining) o Rising air releases latent heat, which causes the upper atmosphere to warm and expand resulting in divergence o The eye of the storm occurs at the point of upper level divergence and is characterized by subsidence (sinking air) What affects the length and strength of a tropical cyclone?  The sea surface temperatures both affect genesis, strength and length of tropical storms  Warmer surfaces result in the more frequent and stronger and longer storms  Upper wind structure plays an important role  Factors =>affect their length and strength: o SST: warmer temperatures mean stronger hurricanes o Upper level winds: strong winds can disrupt the hurricane’s organization o QBO: in the Atlantic basin, hurricane frequency and intensity are stronger when the QBO is in its westerly phase and weaker when in its easterly phase o El Nino: affects winds in Eastern Pacific (EP) and Atlantic (Atl) hurricanes enhances EP, suppresses Atl o Landfall: cuts off the hurricane from the warm SSTs and water vapour that fuel it Distribution Why do the storms occur where they do?  Tropical cyclones occur in the North Atlantic, the northeastern Pacific, the Indian Ocean and both sides of Australia  Why do they not occur in the South Atlantic and the southern Eastern Pacific? o Tropical cyclones form in tropical oceans (5 – 20 latitude) with a sea surface temperature of at o least 26.5 C Canadian Hurricanes  Hurricanes are felt in the Great Lakes region once in about every five years, once in every two years in the Eastern provinces  Emergency Preparedness Canada has analyzed natural disasters  Tropical storms accounts for the 8th costliest natural disaster in Canada Forecasting  Atlantic basin hurricanes have been successfully forecasted for the last twenty years  Dr. William Gray of the University of Colorado => Gray Index o Index is based on ENSO (El Niño-Southern Oscillation), QBO (Quasi-biennial Oscillation), the SSTs (sea surface temperatures) and Sahel rainfall (related to larger scale air flow which affects hurricane development)  Under prediction in 2005 and an over prediction in 2006  Possible=>effect of changing sea surface temperatures is not being incorporated properly – or perhaps it cannot be incorporated properly in a statistical method with no historical precedence for the current SSTs and hurricane activity Katrina and recent hurricane activity  2005 was the most active hurricane year in the North Atlantic in recorded history  Hurricanes frequency has been above average since 1995  Hurricane Katrina occurred from August 23 to 31 of 2005 o It hit the city of New Orleans as a category 3 storm Hurricanes and Climatic Change A scientific controversy  The scientific community =>badly divided over the influence of climate change/ global warming on hurricane activity  Dr. William Gray of Gray Index fame => feels that global warming is not taking place and that changes in the sea surface temperatures are a result of a multi-decadal oscillation in the North Atlantic Ocean  Prof. Kerry Emanuel of MIT => global warming is causing the warming North Atlantic SSTs and increased hurricane activity Dr. Gray in part states: The Atlantic has seen a very large increase in major hurricanes during the last 11-year period of 1995-2005 (average 4.0 per year) in comparison to the prior 25-year period of 1970-1994 (average 1.5 per year). This large increase in Atlantic major hurricanes is primarily a result of the multi-decadal increase in strength of the Atlantic Ocean thermohaline circulation (THC), which is not directly related to global temperature increase. Changes in ocean salinity are believed to be the driving mechanism. These multi- decadal changes have also been termed the Atlantic Multi-Decadal Oscillation (AMO). In contrast, Prof, Emanuel states: There has been a large upswing in the frequency of Atlantic hurricanes, beginning in 1995. This corresponds to an upswing in tropical North Atlantic sea surface temperature, which is very likely a response to increasing anthropogenic greenhouse gases. It is important to note that the late summer and early fall tropical Atlantic sea surface temperature closely follows the Northern Hemisphere mean surface temperature (including land), which makes it unlikely that regional Atlantic climate phenomena are affecting tropical sea surface temperatures (and thereby affecting hurricanes) on time scales of more than a few years. In particular, there is no evidence for "natural cycles" of either Atlantic hurricane activity or tropical Atlantic sea surface temperature.  The Atlantic Multidecadal Oscillation (AMO) is a proposed variation in the density driven circulation of the North Atlantic, which has lead to variations of sea surface temperatures with periods of twenty to thirty years  Since 1995 => warm phase  Gray’s position has been criticized because his reasoning suggestions a stronger thermohaline circulation in recent years  However observation evidence suggests the opposite Human folly? 1) Increasing population of the US and its coastalization – larger numbers and a greater percentage of the population now live in vulnerable sea coast areas 2) Greater insurable wealth of Americans a. At present these factors are more important than changes in the nature of hurricanes Lecture 4 What are midlatitude cyclones?  Midlatitude cyclones occur within the moving boundary of the Ferrel and Polar cells referred to as the Polar front o A series of low and high pressures circulate along the polar front o Lows => midlatitude cyclones & largely characterize the weather conditions of the midlatitudes in the fall, winter & spring o During the summer the polar front often lies to the north of the Great Lakes region o Storms during the summer are either mid-latitude cyclones or convective storms arising from surface heating  Midlatitude cyclones: a.k.a: low pressures =>major source of weather variation in the midlatitudes (30. to 60.) o Occur approximately every four to seven days  In North America, they occur at the boundary of two major air masses, a cold, polar air mass (cP) and a moist, tropical air mass (mT) o Warmth and latent heat (in water vapour) of the mT air provide the energy for the storms  In North America, there are four major regions of cyclone development: o Two of the regions are on the lee-side (eastern side) of the Rocky Mountains producing Alberta Clippers and Colorado Lows o The other two regions have cyclones developing over ocean bodies  Gulf Lows begin over the Gulf of Mexico and Hatteras Lows (Nor’easters) off the eastern coast of the United States  Midlatitude cyclones are typically 100s to 1000s km in extent (larger than hurricanes)  Less intense winds than hurricanes o May have thunderstorms and tornadoes associated with them (along the cold front)  Atlantic Tropical Cyclone => Steenhof and Gough How are they formed?  Unlike hurricanes, surface conditions are not the dominant mechanism for midlatitude cyclone formation  Upper level flow triggers storm formation =>jet stream Polar front theory and fronts  Fronts are divisions between masses of air of different characteristics in a midlatitude cyclone o These are storms often referred to as low pressures or extratropical storms that dominate the weather in the midlatitudes  Polar front theory=> a.k.a: Norwegian cyclone model => growth of a kink or disturbance along the Polar Front into a frontal wave characterized by familiar features such as the cold front and warm front  Four types of fronts are: 1) Stationary front a. Exists between air masses but lacks the instability or energy for a storm to develop 2) Cold front a. Division between cP and mT air, although it can occur between mP and mT air b. Result of lifting of mT air =>cumulus & cumulonimbus clouds form resulting: heavy precipitation c. Most violent weather of a midlatitude cyclone occurs along the cold front d. Thunderstorms and tornadoes can spawn from the cold front e. Easy to identify on weather map i. Across the front there is a strong temperature gradient, a natural result of the differences in the two colliding air masses ii. In addition to this, there is a strong gradient in the moisture content, also a result of the meeting of dry and moist air masses iii. The cold front is also denoted by a strong shift in wind direction iv. Winds in the cP are typically heading south or southeast whereas winds in the mT air mass are heading north or northeast => due to air masses rotating around the low pressure in a cyclonic or counterclockwise direction v. Precipitation types include snow, sleet (ice pellets), freezing rain and rain 3) Warm front a. Division between mT and mP or cP air b. Leading edge of a midlatitude cyclone c. mT air overrides the cold air mass to the north and east d. Results in the formation of stratus and nimbostratus clouds in front of the surface warm front => frontal slope shallower than cold front i. Warm clouds that typically produce drizzle (light rain or snow) e. As the air is pushed further aloft in advance (east and north of the front), other higher cloud types form, such as altostratus, altocumulus and cirrus type clouds f. Lying between the cold front and the warm front is a body of mT air called the warm sector g. The warm sector is roughly triangular with the apex at the low center where the warm and cold sectors meet i. Characteristics of mT air, warm and moist ii. After drizzle that precedes the warm front =>surface observers can experience relatively warm and humid conditions 4) Occluded front a. Not directly observable at the surface b. Peak will no longer reside at the center of the low pressure of the midlatitude cyclone=>shifted to the south at the junction of the fronts i. mT air in this area is entirely pushed above the surface ii. Occluded front is the line linking the center of the low pressure to the apex where the cold and warm fronts are now meeting c. Squeezing process of the mT air is not only the most intense phase of the midlatitude cyclone, but also the beginning of the end Polar Front Theory Step 1:  Stationary front with a strong horizontal wind shear Step 2:  Under certain conditions a kink or small disturbance forms along the polar front  A "cold front" of cold air pushes to the south and warm air ("warm front") pushes to the north  The pivot point is the lowest local pressure and is the low pressure centre  Precipitation begins Step 3:  Fully developed wave  The wave moves east or northeast  It takes 12 to 24 hours to reach this stage of development  The centre pressure continues to drop  Large bands of precipitation have formed  A "warm sector" has formed in the region between fronts Step 4:  The faster moving cold front catches up with the warm front, reducing the size of the warm sector Step 5:  Occlusion occurs as cold front catches up with warm front  This is the most intense part of the storm  There is widespread precipitation. Step 6:  Storm dissipates after occlusion  The source of the energy (mT air) has been cutoff (i.e. no more latent heating). Great Lakes Storm Climatology  Midlatitude cyclones are the major precipitation events of the region and largely determine lake levels  Major cause of erosion, sediment transport, and ship and property damage  Angel & Isard (1998) examined cyclone track data since 1900 => seasonal distribution of storms o Storms were found to occur more frequently in the winter and to be more intense (stronger winds, lower pressure) in that season Where do the storms come from?  Most storms that are experienced in the Great Lakes have their origin elsewhere; only 20% are locally generated  The remainder either comes from the Gulf of Mexico (Gulf lows) or from the leeside of the Rockies (Colorado lows and Alberta clippers)  Isard et al. (2000) examined the origin of Great Lakes storms o Found that origin varied with season Famous Midlatitude Cyclones Edmund Fitzgerald  Strong midlatitude cyclone (“the gales of November”) hit Lake Superior on November 10th, 1975 sinking the Edmund Fitzgerald, a Great Lakes ship=> made into song The Perfect Storm => Sebastian Junger  A midlatitude c
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