ATOC 181 Final Exam Concepts

ATOC 181 Final Exam Concepts 1: THE EARTH AND ITS ATMOSPHERE Depth and Structure of Atmosphere 

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atmosphere: thin film of gases & tiny particles (aerosols) surrounding the earth o 99% of mass confined to layer of 0.25% earth’s radius o shields humans from ultraviolet radiation o essential for life (H2O, CO2, O2) o depth = 30 km; weather contained in lower 10-15 km spheres: layers pauses: boundaries between layers

Layers of the Atmosphere Layer troposphere

stratosphere mesosphere thermosphere

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Characteristics - surface up to 12 km - temperature decreases with height (~6.5°C/km) - where weather occurs - thunderstorm clouds often reach tropopause - temperature initially isothermal (constant) then increased with height up to ~50 km - site of ozone layer - may impact weather over season time-scales - temperature decreases with height up to ~80 km - temperature increases with height

homosphere: well mixed “lower” atmosphere o 8% N2, 21% O2 heterosphere: poorly mixed “upper” atmosphere ionsphere: electrically charged region with ions & free electrons o plays major role in radiocommunications since it reflects AM signal, allowing transmission over large distances o D region absorbs radio waves, weakening surface signal; much stronger during daytime due to photo ionization

Atmospheric Composition     

78% N2, 21% O2 H2O ranges from -4% (0 in poles, 4 in tropics) trace amounts of CO2, O3 & other gases aerosol particles: suspended tiny soil, salt, ash particles pollutants: sulphur, nitrogen oxides & hydrocarbons

Importance of Ozone, Carbon Dioxide & Water Vapour 

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ozone (O3) o 97% in upper atmosphere where it forms naturally o shields earth from ultraviolet radiation o layer being depleted near poles by chlorofluorocarbons (CFCs) from spray cans/refrigerants o at surface, primary ingredient in photochemical smog o irritates eyes/throat, damages vegetation carbon dioxide (CO2) o used by plants for photosynthesis to produce oxygen o absorbs portion of earth’s outgoing radiation & radiates it back to earth (greenhouse gas) o concentration increasing rapidly water vapour (H2O) o produced by evaporation/sublimation; lost by condensation/deposition o condenses to form clouds o stores latent heat (released in thunderstorms/hurricanes) o critical global circulation factor o highly effective greenhouse gas o initially water vapour produced by volcanoes, then evaporation of liquid & sublimation of ice contributes to formation of water vapour

Greenhouse Effect & Greenhouse Gases    

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gases & clouds absorb radiation emitted by earth & reradiate it towards earth some IR is absorbed by atmosphere & is emitted back to surface since doesn’t escape back to space greenhouses heat up due to lack of vertical mixing selective absorbers o greenhouse gases generally poor shortwave absorbers but good long wave absorbers o CO2, H2O, CH4, O3 o O3 absorbs best in UV o H2O most important/effective greenhouse gas role of clouds o selective absorbers with competing effects  absorbs IR even in atmospheric window  reflects solar radiation  warms @ night, cools during daytime o large droplets scatter all light wavelengths so clouds appear white o reflection/transmission/absorption depends on cloud type  cirrus: transmission of shortwave (enhances greenhouse effect)

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low clouds: reflection of shortwave (opposes greenhouse effect)

Mass, Density, Weight & Pressure    

mass: the amount of matter/material in the sample volume (kg) density: the concentration of mass; mass ÷ volume (kg/m3); how much matter there is in a given space o important for buoyancy weight: the force exerted by the mass due to gravity; mass x gravitational acceleration (kg – m/s2) pressure: the weight of the overlying air column; force per unit area o not felt because it acts in all directions o body’s internal pressure adjusts to atmosphere, so there is no pressure difference

Pressure, Density & Temperature Profiles    

pressure & density decrease rapidly with height density may be uniform in well-mixed layers density decreases with height because heavier particles of air descend temperature generally decreases with height but is reliant on the layer of the atmosphere o decreases, increases, decreases, increases

2: ENERGY: WARMING THE EARTH AND ITS ATMOSPHERE Energy  

energy: the capacity to do mechanical work on some object or fluid; constantly being transformed work: the motion of an object or fluid resulting from an applied force Forms of Energy Characteristics - energy that results from objects/fluids in motion kinetic energy - ½ mv2 - energy a body possesses in virtue of its position with respect to other gravitational potential energy bodies in the field of gravity - PE = mgh - the collective microscopic kinetic & potential energy of molecules in a substance internal energy - action controls temperature/air - not that different from kinetic, just microscopic - energy propagated in form of electromagnetic waves - all bodies emit electromagnetic waves (ie/ sun) radiant energy - vibration of charged particles within atoms generative electromagnetic fields - every object with a temperature is emitting electromagnetic radiation

Conservation of Energy & Adiabatic Processes    

1st law of thermodynamics: energy can’t be created nor destroyed, it merely changes form heating to system must be balanced by work and/or increase in internal energy without external heating/cooling, work is balanced by changes in internal energy (adiabatic processes) adiabatic = no heat exchange with surroundings

Temperature & Heat 

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temperature: measure of the kinetic energy of the atoms/molecules within a substance o closely related to internal energy (kinetic part) o slower atomic motions = cooler temperature o molecules move faster as temperatures increase o units: °C, K, °F o at 0 K = -273°C = -495°F, molecular motion ceases (no movement, coldest temperature possible) o thermometer uses fluids/metals that expand as temperature increases & contract as temperature decreases heat: energy in transit from a hot body to a cold body o after, transfer energy is stored as internal energy o units: calorie (energy needed to heat 1 g of H2O by 1°C)

Specific Heat   

heat capacity: ratio of heat added to a substance to its corresponding temperature rise specific heat capacity: amount of heat required to raise the temperature of 1g by 1°C takes 4x more energy to heat water than air o explains why oceans take longer to heat up than the air in the summer o UK has much milder winters than Canada since it is surrounded by water to temperature doesn’t change as fast; Canada gets cold air from Arctic & since there isn’t a lot of water, the process isn’t slowed down

Latent Heat  

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latent heat: the energy required to change a substance from one phase to another at a constant temperature different phases are associated with different molecular configurations o solid: most orderly, least energetic o gas: least orderly, most energetic to make shift, heat must be absorbed or released condensation of liquid (from vapour) and ice (from liquid) releases heat o drives cumulus convection o intensifies mid-latitude cyclones o global circulation control evaporation from surface & clouds/precipitation provides vapour & colds the air tropics have highest air & sea surface temperature o air can hold more water vapour which it receives through evaporation from warm ocean

Heat Transfer Mechanisms  

convection + conduction = sensible heat transfer (direct) conduction: molecular transfer of heat from warm to cold regions o warmer substances = faster molecular motion o collide with nearby molecules, imparting momentum & energy o heat spreads towards colder regions o conductivity: to ability for a material to conduct electricity/heat; speed depends on the material

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convection: transfer of heat by mass movement of a fluid; separated into convection & advection; matters more than conduction o relies on instability of atmosphere o occurs naturally in daytime due to solar heating  air near surface warms & becomes lighter  lighter air is buoyant, causing it to rise  cooler, heavier air sinks to take it places o convection: vertical circulations driven by thermal heterogeneities o advection: transfer of properties from one region to another due to bulk motion of air; for warm & cold advection, wind direction must cross isotherms (lines of constant temperatures)  warm advection: wind blows from warm to cold places, warming it up  cold advection: wind blows from cold to warm places, cooling down o convection & adiabatic heating  adiabatic: no heat exchange between parcel and surroundings  rising branches  moves into lower pressure  expands to equilibrate pressure with surroundings  expansion takes work which cools the air  sinking branches  moves into higher pressure  atmosphere does work on parcel, so temperature increases radiation: all bodies warmer than absolute zero emit electromagnetic radiation o associated with random vibrations of electronics o propagates waves or photons (discrete packets of energy) o properties of waves  wavelength (λ): distance between wave crests  frequency (ƒ): rate of oscillation; rate at which wave crests pass a fixed point  phase speed (v): phase at which a wave travels o warmer = fast electron vibration, shorter wavelengths, higher frequencies, faster speeds, more energetic o Wien’s Displacement Law: relates wavelength of maximum radiation emission to temperature  λ = C/T  sun emits higher energy radiation at a much shorter wavelength  higher temperature = smaller wavelength  lower temperature = larger wavelength o Stefan-Boltzmann Law: radiant energy is proportional to T4  small increase in temperature leads to a large increase in electromagnetic radiation

Solar Radiation in the Atmosphere   

transmission: passes straight through scattering: deflection of light in all directions by small particles; produces ‘diffuse’ radiation; more effective at short (blue) wavelengths reflection: light sent mainly backwards instead of all directions

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absorption: light absorbed by molecules in the air; ozone is a molecule that absorbs ultraviolet light

Absorption, Emission, Transmission, Reflection 

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absorption & emission o if body emits more energy than it absorbs, it cools; if body absorbs more energy than it emits, it warms o at any time, only ½ the earth is under sunlight  light half: shortwave absorption > infrared emission  dark half: only infrared emission o shortwave: visible light o longwave: infrared light; emitted by the earth o blackbody: an object that absorbs all incoming radiation & emits the maximum radiation possible; ie/ sun & earth’s surface reflection: waves bounce against objects & return to space depending on surface cover & angle of sun o snow reflects 95% of radiation o water reflects 98% of radiation o albedo: fraction of incident radiation that is reflected  earth primarily ocean & forest so albedo low  4% earth, 20% clouds, 6% scattered back to space  higher/larger albedo = higher reflection

Earth’s Energy Balance   

equator-to-pole heat transfer needed for steady climate if not, poles would cool forever & equator would heat forever point: understand there is stuff coming down & stuff coming up

3: SEASONAL & DAILY TEMPERATURES The Seasons   

annual variability in earth-sun distance earth has less elliptical (non-circular) orbit o 6.5% less radiation in Northern Hemisphere summer controlled by 2 things: o intensity of radiation: radiant energy received per unit area

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amount of daylight: length of time between sunrise & sunset both larger in the summer

Differences in Day Lengths 

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solstices: longest/shortest days of the year; either 24 hours of light or 24 hours of darkness at poles; lasts for 1 day at arctic circle, 6 months at poles o Arctic circle = 1st latitude getting full day of sunlight (June 21)& full day of darkness (December 21) equinoxes: 12 hours light, 12 hours darkness everywhere equator: 12 hours light, 12 hours darkness year round; gets same amount of sunlight all year round in Canada we get more sunlight in summer & less in the winter days longer at poles

Diurnal Cycle of Temperature 

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landscape variations o south facing landscapes receive more insulation/sensible warming/evaporation  differences in North & South lead to difference in snow depth, vegetation type, etc. o vegetation cover moderates temperatures  dry soil: nothing for sunlight to evaporate so temperature warms up  moist soil: sun will evaporate the water & create humidity daytime heating o ground absorbs shortwave radiation causing it to warm  conduction: heat transferred to lower atmosphere  convection: thermals transfer heat vertically  te1mperature rises until long wave emission exceeds shortwave insulation  heat that is lost is mainly due to long wave radiation; amount depends on temperature  temperature heats up as you receive more radiation keeps warming throughout day o in weak winds, strong temperature gradients may exist near surface  convective eddies transport some heat  in strong winds, wind shear & surface drives eddies that effectively mix heat vertically  decreasing temperature with height because with height, temperature from earth mixes with atmosphere nighttime cooling o earth & atmosphere are always emitting heat o efficient radiator = surface; cools down faster o convection suppressed by strong stability  cold/heavy air beneath warm/light air o radiation inversion forms on calm/clear nights  no thermals that help to mix air vertically  thermals rely on having light air trying to rise & heavy air sinking, so they mix  at night, heavy air below & light air above

Controls of Diurnal Cycle  

higher solar angle = more heating; lower solar angle = less heating vegetation type

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o controls ratio of sensible heat to latent heat o controls moisture content near surface o forests don’t heat up as much as pavement water vapour & clouds o reduce heating rate (day), cooling rate (night) o moisture in air means radiation that leaves earth comes back because of water o cloudy night is warmer because warm air trapped winds o stronger winds diminish extremes due to mixing of winds

Controls on Temperature 

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latitude o controls sun & amount of daily sunlight o temperatures decrease towards poles from tropics in subtropics o greater variation in solar radiation between low & high attitudes in winter than summer (isotherms closer together (tighter gradient) in January than in July land & water distribution (continentality) o temperatures are lower in middle of continents that near the ocean in January; reverse for July o attributed to unequal heating & cooling properties of land & water o solar energy reaching land is absorbed, reaching water is penetrated o water has a high specific heat capacity o hurricane season is September-October because that is when the water is warmest ocean currents o eastward: warm ocean currents transport warm water polewards o westward: cold water transport equatorward elevation vegetation cover

4: ATMOSPHERIC HUMIDITY The Hydrologic Cycle

Phases of Water    

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gas: molecules far apart, moving about freely (highest energy) liquid: joined together but still constantly jostle & bump into each other solid: rigidly locked into place with some vibrational activity (lowest energy) phase changes: liquid to vapour o sealed beaker with dry air above water surface o initially, fast moving liquid molecules escape (evaporation) o as vapour builds up in air, slower moving vapour molecules begin to enter liquid (condensation) o vapour increases until these two processes come into equilibrium (saturation) controls on evaporation o solar radiation: excites liquid water molecules allowing them to escape (increases evaporation) o warmer temperatures: faster molecular motion (increases evaporation) o stronger winds: blow/mix near-surface moistened layers away (increases evaporation) o humidity moister air closer to saturation, less capacity for vapour (decreases evaporation)

*Quantifying Atmospheric Moisture 

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absolute humidity: total water vapour mass in a volume of air o depends on density of water vapour & volume o parcel size = parcel volume o doesn’t give a real intuition of how much humidity in air specific humidity: water vapour mass divided by total air mass o highest in tropics since warm air can absorb more water vapour before reaching saturation mixing ratio: the amount of water vapour in the air divided by the dry air mass o very similar to specific humidity since total air mass dominated by dry contribution relative humidity: ratio of the amount of water vapour in the air to the maximum amount of water vapour required for the saturation at that temperature & pressure o RH = vapour pressure ÷ saturation vapour pressure = (mixing ratio ÷ saturation mixing ratio) x 100% o if RH = 100% air is saturated o if RH > 100% air is supersaturated o if RH < 100% air is subsaturated relative vs. specific humidity o specific humidity follows temperature o relative humidity trends more reflective of climate

Vapour Pressure 

vapour pressure: total air pressure comprised of many individual components o if water vapour high enough, air reaches saturation (reaches capacity to hold water vapour)  strongly depends on temperature  depends on phase (lower for ice)  boiling occurs when saturation vapour pressure matches environments air pressure

Dew Point vs. Wet Bulb 

dew point: temperature to which air must be cooled at constant pressure to reach saturation

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wet bulb: coolest temperature that can be reached through evaporation of water vapour

*Moisture Source Regions  

south-central & eastern Canada affected by hot, humid air originating in Gulf of Mexico other parts of Canada affted by cooler, less humid air from Pacific or cold air from Arctic oceans

5: CONDENSATION, FEW, FOG & CLOUDS Dew & Frost    

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surface cools as it emits long wave radiation through conduction, air lays in contact with surface also cools liquid/ice may condense/deposit onto surfaces that transpires moisture/poor at storing heat at above/below 0°C controls o clouds: trap heat in atmosphere preventing surface from effectively cooling o water vapour: traps heat limited dew/frost potential, but increases dew point o winds: turbulently mix near surface air with warmer air above limiting dew/frost potential frozen dew: liquid dew formation followed by freezing; ie/ black ice dry freeze/black frost: plants freeze due to subfreezing air hoarfrost/white frost: deposition forms delicate tree-like crystals

Condensation Nuclei         

provides surfaces for water vapour to condense on water molecules don’t like to stay together when they collide, so attracted to impurities in the air have difficulty forming in clean are due to surface tension largest concentrations of nuclei in lower atmosphere near earth’s surface light so remain suspended in air for many days hygroscopic (water-seeking) vs. hydrophobic (water-repelling) heterogeneous nucleation at RH = 100% homogeneous nucleation of droplets in clean air require RH > 100% hygroscopic nuclei attract water vapour, allowing condensation at RH < 100%

Sizes & Concentration of CCN & Cloud Droplets Type of Particle small (Aitken) condensation nuclei large condensation nuclei giant condensation nuclei fog & cloud droplets

Approx. Radius < 0.1 0.1 – 1.0 > 1.0 > 10

# of Particles 1000 – 10 000 1 – 1000 < 1 – 10 10 – 1000

Per Cubic 1000 100 1 300

Haze     

haze: a layer of dust or salt particles suspended above a region dry: particles scatter light reducing visibility wet: condensation on hygroscopic nuclei @ RH < 100% caused by pollution & other aerosols particles make the air not clear since they deflect light

Fog  

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cloud: region of condensed water droplets suspended in the air; scatter full spectrum of light so they look white clouds based at ground are fog or mist o fog: lower visibility (< 0.8 km) o mist: higher visibility (> 0.8 km and < 10 km) radiation fog: forms due to radiation cooling at surface; aided by weak winds that mix up low levels of spread cooling over deeper level; forms best on clear nights when shallow layer (doesn’t absorb outgoing infrared radiation) of moist air near ground is overlain by drier air advection fog: warm air passes over cold; surface must be sufficiently cooler than air above so transfer of heat from air to surface will cool air to dew point & produce fog upslope fog: air cools as it ascends up a slope; forms as moist air flows up along elevated plain, hill or mountain; evaporation fog: water content increased by evaporation from water surface or from raindrops o steam fog: above warm body of water; forms over lakes on autumn mornings as cold air settles over water still warm from long summer o frontal fog: rain falls through a cold lower layer; develops in shallow layer of cold air just ahead of approaching warm front of behind cold front mixing fog: mixing of 2 unsaturated parcels of different temperature can produce a cloud

Basic Cloud Types 

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cumulus: white to light gray, puffy; vary in shape due to vertical development; often have rounded domes/towers at top; on humid days composed of water; when they reach the tropopause they can be composed of water & ice; appear harmless but turn into most severe thunderstorms stratus: stable layer cloud; sheets of clouds; most general type; generally don’t produce precipitation, but sometimes light drizzle over coastal waters; low, uniform bases; composed of water cirrus: most common type of high clouds; thin & wispy; made of ice; sun shines through them; if thicker would be stratus; moves west to east; movement indicates prevailing winds at their elevation & point to good weather; formed in lower temperatures

Satellite Images         

geosynchronous: moves through space at same rate as earth rotates; remains above fixed spot on equator & monitors one area constantly polar orbiting: see whole earth; goes around & around the earth; scans from north to south; on each successive orbit, satellite scans an area farther to the west visible IR IR enhanced infrared water vapour images: detects radiation at wavelengths of water vapour emission water vapour images: shows amount of moisture in mid-to-upper troposphere TRMM: polar orbiting; visible, infrared scanners, microwave images & radar; 3D CloudSat: mm wavelength radar on satellite; provides detailed cloud structure

6: STABILITY AND CLOUD DEVELOPMENT Adiabatic Processes     

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parcel: a small, coherent mass (bubble) or air o when forced to rise/sink, it expands/compresses & changes temperature if no heat exchanged with environment, process is adiabatic work done to equilibrate pressure between air parcel & surroundings changes parcel temperature adiabatic process: parcel that expands & cools, or compresses & warms with no interchange of heat with its surroundings the dry adiabatic lapse rate o approximately 10°C/km o rising air cools, sinking air warms  specific humidity doesn’t change, but relative humidity does  rising parcel brought closer to saturation o compression will warm up parcel by 10°C, it is a constant o further lifting results in condensation (cloud forms) & latent heat released o since heat added during condensation offsets cooling due to expansion, air now cools at moist adiabatic rate

saturated adiabatic lapse rate o lapse rate decreases for saturated parcels  rising: latent heat release offsets cooling

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 sinking: evaporative cooling offsets warming depends on temperature: warm air carries more water so it experiences more latent heat release/evaporation this is why thunderstorms & hurricanes happen released latent heat makes them rise through atmosphere if saturated parcel with water droplets were to sink, it would compress & warm at moist adiabatic rate because evaporation of liquid droplets would offset rate of compressional warming not constant, varies with temperature & moisture content

Density, Buoyancy and Stability  

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at fixed pressure, warm air is less dense than cold air buoyancy force related to difference between parcel density & that of surrounding environment o warmer, lighter: positive buoyancy, accelerates up o colder, heavier: negative buoyancy, accelerates down if parcel displaced upward is colder than environment, it will accelerate back down (stable)

Stability   

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lapse rate: rate at which air temperature changes with elevation environmental lapse rate: rate at which air temperature surrounding us will be changing if we were to climb upward in atmosphere an absolutely stable atmosphere o occurs when environmental lapse rate < saturated adiabatic lapse rate o comparing parcel against temperature profile of theoretical parcel o will start with same temperature as surroundings o rising parcel is colder & denser than air surrounding it o if given the chance, will return to its original position

an absolutely unstable environment o occurs when environment lapse rate > dry adiabatic lapse rate o rising air will continue to rise because it is warmer & less dense than surrounding air o layers will immediately overturn (warm air rises, cold air sinks) restoring stability

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a conditionally unstable atmosphere o occurs when environmental lapse rate is between saturated adiabatic lapse rate & dry adiabatic lapse rate o atmosphere is stable if rising parcel is unsaturated o atmosphere is unstable if rising parcel is saturated o real atmosphere contains layer of differing stability

*Processes Influencing Stability 

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general considerations o warmer air below & colder air aloft = less stable (larger environment lapse rate) o colder air below & warmer air aloft = more stable (smaller environment lapse rate) o increased moisture: parcel saturates more easily, then follows reduced adiabatic lapse rate (less stable) advection o cold advection aloft and/or warm advection @ surface = less stable o warm advection aloft and/or cold advection @ surface = more stable  opposite process (stabilizing process) happens after cold front comes through environmental subsidence o in large scale subsidence, troposphere often extremely stable  warms air aloft, leads to strong inversions atop boundary layer  parcels lifted through this environmental o aversion issue  when weather disturbed, atmosphere is in a rising motion (destabilizing effect)  dry air for awhile, air will drop trapping air at low levels  sinking air = fast rate of warming layer stretching & compressing

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descent & compressing = stabilization  top layer descends & warms more than bottom o ascent & stretching = destabilization  top layer ascends & cools more than bottom o get aversion when take layer down & compress is convective (potential) instability o when lifted, parts of layer may saturate before others depending on moisture profile o if bottom saturates 1st, will cool less rapidly than upper part  continued lifting destabilizes layer mixing brings layer closer to dry adiabatic o destabilized with respect to saturated parcel motions o stable inversion forms a top of mixed layer, capping the parcel ascent deep convection o convective storms often result from upper level lifting o leads to layer stretching: cooling aloft, weakening of stable layers o also moistens mid-level flow by lifting moist air upwards from boundary level

Cloud Development 

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mechanisms for cloud development: o surface heating & free convection o uplift along topography o widespread ascent due to convergence of surface air o uplift along weather fronts earth doesn’t warm universally thermals created (pockets of air that are warm & rise) & cool as they rise as temperature changes, air holds less & less water so eventually a cloud forms cloud-layer stability o fate of cloud depends on saturated lapse rate of stability of cloud-bearing layer o thunderstorms only develop within deep conditionally unstable layers moist convection o cumulus congestus, cumulonimbus o when we get warm arm, it blows over shore, so have warm air over cold air (unstable) orographic lifting o air forced upwards as it impinges over a mountain o precipitation happens so air loses moisture o when air goes over side of mountain it is unsaturated mountain winds/lenticular clouds generated by stable flow over mountains

7: PRECIPITATION Cloud Precipitation  

clouds required for precipitation, but not all make it depends on many factors like temperature, cloud width/thickness, internal cloud motions, etc.

Precipitation in Warm Clouds  

clouds droplet a lot smaller than raindrops the curvature effect: in cloud physics, as cloud droplets decrease in size, they exhibit greater surface curvature that causes a more rapid rate of evaporation o for droplets the size of CCN (0.2 um), curvature effect is important o large supersaturation needed to keep droplet alive o for larger droplet (20 um), curvature effect is minimal o curved line represents the relative humidity needed to keep droplet in equilibrium with its environment o for given droplet size, droplet will evaporate & shrink when relative humidity is less than that given by a curve. droplet will grow by condensation when relative humidity is greater than the value on the curve

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the solute effect: the dissolving of hygroscopic particles thus reducing relative humidity required for the onset of condensation o remember hygroscopic nuclei allows condensation for RH > 80% o also reduce equilibrium vapour pressure by binding tightly with liquid-water molecules o red line shows equilibrium relative humidity for a droplet that forms on a tiny, hygroscopic, salt, cloud condensation nucleus relative to the droplet’s diameter. line represents combined impact on droplet of the curvature & solute effects o when humidity is higher than red line, droplet will grow o when humidity is lower than red line, droplet will diminish o peak in the curve represents minimum environmental humidity needed for droplet to grow beyong 0.39 um in diameter o if we separate impacts of combined effects, blue line illustrates how solute effect allows droplets to grown even when relative humidity is < 100% o purple line illustrates impact of curvature effect (only effect when droplet has no condensation nucleus ie/ in pure water) o in this case, high supersaturation levels (humidity higher than green line) are needed for droplets to grow

Influences of Continentality and Droplet Size 

number of CCN strongly dependent on geography o over land, many CCN due to dust & pollution

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o over water, fewer CCN in “clean” air leads to differences in droplet size (and precipitation formation) o over land, liquid distributed over many droplets; droplets smaller & easier to suspend o over water, liquid distributed over fewer droplets; droplets larger & harder to suspend

Collision and Coalescence       

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significant role in producing precipitation different terminal velocities lead to collisions between different-sized droplets terminal velocities: droplet fall speed; dependent on diameter larger drops overtake & collide with smaller drops coalescence: merging of drops by collision not 100% effective (droplets don’t always stick together) in warm cloud composed only of small cloud droplets of uniform size, droplets less likely to collide as they all fall very slowly at about same speed o droplets that don’t collide frequently don’t coalesce because of strong surface tension that holds each tiny droplet together in cloud composed of different-sized droplets, larger droplets fall faster than smaller droplets o some tiny droplets swept aside, some collect on larger droplet’s forward edges o others coalesce on droplet’s backside

Cold (Ice) Clouds    

clouds don’t freeze immediately when subzero supercooled: subfreezing liquid homogeneous nucleation: freezing of pure water requiring very low temperatures heterogeneous nucleation: freezing around ice nucleus (IN); ice nuclei nowhere near as abundant as CCN o deposition nuclei: deposition onto particle o freezing nuclei: embedded IN causes ice activation within supercooled drops o contact nuclei: supercooled drop freezes on contact

The Ice Crystal (Bergeron) Process     

Bergeron process: explains how supercooled clouds turn into ice through addition of small ice crystals which grow at expense of droplets saturation vapour pressure over ice is less that that over liquid surface o less energetic molecules, less tendency to escape when droplets & crystals present, ice grows at expense of liquid causes ice crystals to grow rapidly, until they become heavy enough to fall through cloud need many, many cloud droplets to grow a single ice crystal to precipitation size

Methods of Ice Growth   

precipitation forms in cold clouds, not warm ones forms by accretion convective clouds: liquid water content causes collision-coalescence process

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stratus clouds: thick enough to extend levels where air temperatures quite low; last long enough for the Bergeron process to initiate precipitation graupel: falling ice crystals may freeze supercooled droplets on contact (accretion) producing larger ice particles; happens in high liquid water (cumulonimbus) clouds secondary initiation: falling ice particles may collide and fracture into many tiny (secondary) ice particles snow growth: falling ice crystals may collide & stick to other ice crystals (aggregation) producing snowflakes; happens in low liquid water (nimbostratus) clouds

Cloud Seeding    

exploits the Bergeron process inject solid particles (dry ice or silver iodide) into cloud heterogeneous nucleation occurs & crystals grow via Bergeron process this occurs naturally when upper layer “seeds” water-heavy lower layer

Types of Precipitation 

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rain o falling drop with diameter > 0.5 mm o smaller diameters = drizzle o rain full evaporated in sub-cloud air = virga o can accompany convective (unstable, heavier & sporadic) or stratiform (stable, continuous) clouds snow o most precipitation begins as snow in subfreezing air o usually reaches ~300 m below freezing level before melting o can extend much lower in very dry air snowflakes o when snow falls through above-freezing air, edges melt  helps cling flakes together upon contact  leads to giant snowflakes  differs from colder, drier air where flakes are smaller & more powdery  explains why skiing conditions are better as terrain gets higher (when air is drier) ice crystals habits o Bergeron process maximized at -14°C snowfall intensity o flurries: lightest (convective or stratiform)

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snow showers: heavier but usually short-lived (convective); pockets of heavy snow & periods of no snow snow squalls: intense but very brief (convective) blizzard: combination of prolonged (4h) heavy snow (vis < 1km) & strong winds (> 40 km/h) blowing snow: wind lifts flakes off ground & blows them around

Riming, Snow Pellets (Graupel) & Hail 

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riming o rime forms due to supercooled water (SCW) freezing on contact with subfreezing objects o rimed snowfall, formed due to accretion of SCW, becomes graupel o also forms on other objects exposed to SCW snow grains o snow grains: small, light grains o thin stratus clouds o snow equivalent of drizzle snow pellets (graupel) o snow pellets: larger, denser o riming in cold cumulonimbi: accumulation of supercooled droplets by snow hail o hail: pieces of ice > 5 mm in diameter o may be round, irregular or slushy o caused by heavy riming in cumulonimbus clouds o may involve multiple cycles of uplift & sedimentation o may involve horizontal motions through regions of varying humidity o layered structure due to alternating wet & dry growth regimes  wet: high supercooled liquid water content; rapid growth, strong latent-heat release from freezing, outer shell melts  dry: low supercooled liquid water content; outer shell stays frozen as it grows due to limited latent heat release

Radar 

 

powerful microwave pulses transmitted in all directions o targets (including raindrops) scatter energy o precipitation rate related to intensity of return signal o target distance related to elapsed time between transmission & reception Doppler radar o sense target motion by detecting change in frequency (Doppler shift) Polarimetric o send both horizontal & vertical polarized signals o determines aspect ratio o spheres have equal energy in both directions

8: AIR PRESSURE AND WINDS Forces 



 

pressure: the force per unit area o moves from high to love o pressure gradient: change in pressure divided by change in distance Coriolis: acts on wind (caused by earth’s rotation) o apparent force, only seen by objects rotating o strength increases with latitude & wind speed o pulls objects to the right (N.H.) & the left (S.H.) centrifugal: pushes objects outward from axis of rotation friction: slows air motion (strongest influence as surface)

Geostrophic Wind       

at upper levels, without friction, PGF & Coriolis largely balance PGF pushes air from H to L as winds strengthen, so does Coriolis, which turns flow to right as winds become parallel to isobars, forces come into balance no net acceleration: air flows along isobars constricting isobars much like water flowing through a river changing width only strictly valid for straight east-west flow o with N-S component, latitude (Coriolis) changes o for curved flow, centrifugal forces becomes important

The Gradient Wind   

winds on curved motion paths still follow isobars 3-way balance between PGF, Coriolis & centrifugal force winds blow to right of PGF

Friction and Surface Winds 



surface friction slows down near-surface winds o reduction of COR destroys geostrophic balance o wind becomes a balance of PGF, COR & friction with cross-isobaric component cross-isobaric component important for cyclones o blows into low centres & out of high centres

Frictional Convergence/Divergence    

horizontal convergence & divergence lead to vertical air motions ascent above low important for cyclonic development descent above high leads to clear air also causes pressure to evolve

9: WINDS AT DIFFERENT SCALES: SMALL & LOCAL Scales of Atmospheric Processes  

different processes occur on different scales relevant forces may depend on these scales

The Boundary Layer            

boundary layer: atmospheric layer near surface that is influenced by friction (turbulence) depth ranges from a few meters at night to several km in the afternoon in desert regions deeper for stronger winds & more strongly heated surfaces (larger instability) air generally well mixed by turbulent eddies slows down winds through viscous dissipation, but also mixes stronger winds from aloft down towards the surface we live in the boundary layer (only outside it if in a plane) more turbulence means deeper boundary layers turbulent eddies bring air up & downs causing unorganized mixing making it uniform throughout layers thin at night due to radiative cooling; as sun comes up, it grows mechanical turbulence develops due to flow separation past bluff body o enhanced by stronger surface winds induced by thermal turbulence in the afternoon winds strengthen so there is more turbulence winds increase where airflow isn’t slowed by friction

Mountain Phenomena (not a deep understanding, but know what they are) 





lee vortices o precise mechanisms complex o flow separation + lee dissipation downwind + wave instability? lee waves o form in statically stable flow past terrain obstacles o oscillate downwind, forming wave clouds to parallel to ridge o may also form turbulent rotors beneath wave crest mountain waves



o form in strong, statically stable flow over large obstacles o wave breaking creates strong surface winds & transports warm, dry air to the surface Chinook winds o warm, dry wind that descends eastern slope of Rocky Mountains o similar winds occur in leeward slopes of mountains o occur when strong westerly winds aloft flow over north-south-trending mountain range producing a trough of low pressure on mountain’s east side that forces air downslope

Thermal Circulations   

caused by horizontal changes in air temperature warm air rises & cold air sinks need to understand & explain how horizontal temperature gradients give rise to pressure gradients o cold, dry air is less dense, so pressure is not as great o warm, moist air is dense, so pressure is higher o could have warm air in the north and cold air in the south creating horizontal gradient

Sea Breezes, Land Breezes, Monsoons & the Initiation of Clouds/Convection 



sea & land breezes o caused by temperature gradient between land and sea o in summer, land warmer in daytime & colder at night monsoon o large-scale (synoptic-scale) wind system that varies direction seasonally o affected by Coriolis force o occurs over all major continents o North American monsoon combines with mountain lifting effects to produce heavy mountain precipitation

Wind Measurements   

anemometer: measure wind speed & direction lidar: uses laser to generate intense pulses that are reflected from atmospheric particles of dust & smoke rawindsondes (balloons) & Doppler radar/sodar measure variations for horizontal winds in height & time

10: GLOBAL WIND SYSTEMS General Circulation      

circulation that depends on latitude of earth represents a time-averaged (smoothed) picture of global wind systems (not day-to-day) local weather varies considerably from this average driven by unequal heating of earth’s surface although earth is in quasi-radiative balance, some regions experience net gain/loss in radiation (net gain in tropics, loss in polar regions) circulation acts to maintain climate in steady state (moves tropical air poleward, polar air equatorward)

Single Cell Model & Three Cell Model 

single cell model o if earth wasn’t rotating, would have simple thermally direct circulation & no Coriolis force & constant heating over equator o tropics go from 10°N to 10°S o air circulation in Hadley Cell



three cell model o with rotation, breaks down in 3 cells (per. hemisphere) o trade wins come from NE o since it is large scale, Coriolis force is important o air doesn’t go directly from high to low pressure, tries to go but is deflected (this is why winds are westerly; there is a semi-permanent high pressure system over tropics so winds are deflected to right) o still not a satisfactory model, particularly in midlatitudes o midlatitude westerly winds increase with height (implied they decrease in model)

Semi-Permanent Highs & Lows  



semipermanent highs and lows: areas of high pressure and low pressure that tend to persist at a particular latitude belt throughout the year highs: subtropical anticyclones develop in response to the convergence of air aloft near an upper level jet stream o Bermuda high & Pacific high lows: cyclonic activity o Icelandic low & Aleutian low



in summer subtropical high-pressure belt girdles world aloft near 30° latitude; air sinks produces clear skies; air near ground warms rapidly

The Jet Stream    





jet stream: a swiftly flowing current of air that moves in a wavy, west to east direction westerly winds in N & S hemispheres caused by S-N temperature/pressure gradients & Coriolis strongest winds in mid-latitudes during winter tropopause jets o polar & subtropical jet streams encircle the globe (generally W-E but with embedded wave activity) o jet dynamics strongly impact global heat transfer o mid-latitude cyclones develop on jet, which cause meridional (N-S) heat transfer via advection o needed to compensate for radiative imbalance between equator & poles polar jet o concentrated S-N temperature gradient along polar front o warmer air light & “thicker” colder air denser & “thinner” o creates strong N-S PGF (steep slope of pressure surfaces) o geostrophic balance: strong westerly flow subtropical jet o conservation of angular momentum = mVr ( m = conserved mass, V = wind speed, r = radius to axis of rotation) o upper-level outflow from ITCZ travels north/south o r decreases, so V must increase o like spinning an ice skater

Ocean Currents        

atmosphere & ocean interacts via exchanges of energy & momentum oceans warmer/colder than land o higher heat capacity, long to heat & slower to cool o high density, restricts motions & heat transfer ocean currents slower than atmospheric winds since atmosphere imparts moment to sea currents spiral in whirls called “gyres” currents move slower than prevailing flow poleward flow along E margins of continents; equatorward flow along W coasts help to explain regional climate variations the Gulf stream o very warm waters off US East Coast o leads to rapid cyclone development in winter o travels across Atlantic to warm N Europe o similar to Kuroshio current in W Pacific

Upwelling       

ocean waters move at angles to prevailing wind frictional drag sets sfc layer into motion, which is deflected by COR drag activates layer below, which turns farther to the right by COR leads to coastal upwelling when wind parallels the coast winds blow parallel to W North America, surface water transported to the right (out to sea) cold water moves up from below (upwells) to replace surface water Ekman spiral: descending from surface, water is slowing & turning to the right until at some depth it moves in the opposite direction; the turning of the water at the depth is the Ekamn spiral

El Nino, La Nina & Southern Oscillation (ENSO)    

      

around Christmas, cold upwelled waters along Peruvian coast replaced by warm current for few weeks (El Nino) sometimes very strong (El Nino) or very weak/non-existent (La Nina) associated with different atmospheric pressure patterns & wind fields the southern oscillation o fluctuating pattern of reversing atmospheric pressure perturbations along equatorial Pacific o La Nina = intensification of “normal” state o during El Nino, pressure decreases over E Pacific, weakening trade winds & ocean current water heat by sun is blown (upwelling up until the surface) adds moisture to air above favoured for conditional instability (which favours condensation) warm water transported E by equatorial Kelvin waves higher waves = warmer water atmosphere & ocean act on different time scales **look at answers to homework question!!**

11: AIR MASSES AND FRONTS 

air masses: a large body of air with relatively uniform thermodynamic (temperature, humidity) properties; doesn’t have to be uniform inside it air  source regions: regions where air masses originate  obtain character by spending long periods over same region  mid-latitudes too variable  mid-latitudes are where tropical, subtropical, polar & arctic air masses meet  pressure gradients stronger  mid-latitudes: where air masses meet Source Region Arctic (A) Polar (P) Tropical (T) Continental Arctic (cA) Continental Polar (cP) Continental Tropical (cT) Continental (c) extremely cold, dry stable; hot, dry; stable air aloft; land cold, dry, stable ice & snow covered surfaces unstable surface air Maritime Arctic (mA) Maritime Polar (mP) Maritime Tropical (mT) Maritime (m) water cold, moist, unstable cool, moist, unstable warm, moist, usually unstable

Air Masses of North America

Lake Effect Snow     

frigid, dry airflow over relatively warm lakes surface heating & moistening creates conditional instability shallow but vigorous cumuli develop (usually banded) need sufficient “lake fetch” for precipitation to form instability released rapidly; snow doesn’t extend far downwind

Air-Mass Characteristics 



winter: the pineapple express o mT air originating in subtropical east Pacific o very warm & humid o generates tremendous precipitation along west coast o warm, humid airflow produces heavy rain & extensive flooding summer: the Gulf of Mexico o brought northward by “Bermuda High” o warm, humid & conditionally unstable air

Fronts      

front: transition zone between two air masses air masses with different densities caused by temperature and/or humidity differences frontal boundary: lies on warm/light edge of density gradient frontal “zone”: a thin region of horizontal density gradient 3d: extend upward along frontal surface ie/ typical winter: o cA air diving down from Canada o mT air coming up from Gulf of Mexico o mP air drawn in from Atlantic

Cold Fronts vs. Warm Fronts 



cold front: cold air advances into relatively warm air o ie/ cold front separates cA/cP from mT (“Arctic front”) o precipitation normally occurs ahead of or immediately behind the front o vertical cross section: sharp lifting along steep frontal surface o clouds are increasing Ci, Cs before, Tcu or Cb during & Cu & Sc when ground is warm o precipitation before is short period of showers; during is heavy showers of rain/snow & after is decreasing intensity of showers, then clear warm front: warm air advances into relatively cold air o ie/ warm front separates mT from mA air o precipitation usually occurs on cold side of warm front o vertical cross section: much gentler slope than cold fronts o clouds before are mixed, during are stratus & after are clear with scattered Sc o precipitation before is rain, snow, ice pellets or drizzle; during is drizzle or none; after is none

Stationary Fronts  

stationary front: stationary boundary between two air masses similar characteristics as cold/warm fronts, but without the motion

Occluded Fronts 

occur in mature stage of cyclone development

  

occluded front: cold front essentially “catches up” to warm front cold occlusion: cold air “catches up” to warm air, lifting warm air above surface warm occlusion: air behind cold front is warmer than air ahead of warm front; air behind cold front lifts over colder air ahead of warm front

Upper Level Front     

upper air front: front that is present aloft form at tropopause, not surface along jet axis where temperature gradients are already large cause stratospheric air to descend to low levels may or may not be connected to surface fronts

14: THUNDERSTORMS AND TORNADOES Thunderstorms  

thunderstorm: a convective cloud (cumulonimbus) containing lightning and thunder; cloud development driven by positive buoyancy force within the cloud most common in afternoon because ground is warm & so good chance there will be instability o sensible & latent heating at low levels increases conditional instability

Favoured Thunderstorm Conditions 





conditional and/or potential instability o steep lapse rate of temperature and/or humidity (warm/moist air at low levels & cold/dry air aloft) o large low-level moisture supply (air needs to saturate to release instability) o some lifting mechanism to bring air to saturation air that is hot & humid o latent heat release stronger at warmer temperatures due to larger water-holding capacity o saturated lapse rate reduced: conditional instability more common o also develop when air aloft is very cold favoured by stronger forced lifting (ie/ fronts) o more cooling aloft and/or latent heat release o more destabilization and/or stronger initiation

3 Storm Types & Vertical Wind Shear   

severe thunderstorms ordinary single-cell thunderstorms o vertical shear: change of horizontal wind with height multi-cell thunderstorms o vertical shear prevents downdraft from cutting off updraft

Ordinary Single-Cell Thunderstorms    

most common may produce heavy rain, frequent lighting & small hail form in conditionally unstable but weakly “sheared” environments sounding & radar o deep layer of conditional instability o weak winds over deep layer





o high humidity at low levels 3 stages o updraft saturates, initiating cumulus o cloud deepens, precipitation forms. cold outflow develops from evaporation within & below cloud o storm decays when cold outflow cuts off warm inflow often only anvil portion remains o cools air near surface, causing relief from hot weather o tends to also make air more humid

Multicell Thunderstorms      

very common but less than ordinary storms contain multiple cells, each in a different stage of development each cell has short lifetime but entire system may last for much longer than ordinary cells new cells initiate at “gust front” (leading edge of cold outflow air) each cell grows, matures & dissipates as it moves from front to rear precipitation, downdraft strongest in rear of storm

Features of a Thunderstorm  



gust front: downdrafts bring cold air to surface propagating outwards away from storm outflow boundaries: individual gust fronts from different cells may merge to form organized “outflow” boundaries o extend outward from storm complex o boundaries often initiate new storms, leading to propagation of entire system downbursts: downdraft beneath thunderstorm that is localized so it hits the ground & spread horizontally in radial burst of wind

Squall Lines   

very common form of multicell storms organization may come from larger-scale ascent (cold fronts, drylines) or from storm outflow boundary structure is a variation on multicell storms o midlevel heating causes “gravity wave” causing alternating pattern of uplift & descent behind the line o descent may transport strong winds down from upper levels

Bow Echoes, Derechoes, Mesoscale Convective Complexes 



bow echoes & derechoes o vertical transport of strong winds may cause segments of line to “bow” forward o straight line winds may cause significant damage o derecho: straight-line wind damage for hundreds of km along squall-line path o happens when winds brought down from aloft mesoscale convective complexes o large-scale region of thunderstorms o consist of clusters of multicell storms & “supercells” o common nocturnal feature over US in summer o convection initiated over mountains, organizes downwind over stationary fronts, travels across US (or Canada) o strongest at night due to supply of moisture from low-level jet o storms often “elevated” – not rooted in boundary layer

Supercells      

  

supercells: most violent thunderstorms produce hail & all violent tornadoes the rotate type depends on rain rate schematic of classic tornado requirements o strong conditional instability o strong vertical wind shear  winds strengthen & turn counter-clockwise with height  shear needs to be stronger than for multicells usually form in warm sector of frontal systems (strong conditional instability and vertical wind shear) mesocyclone: rotating air column on south side of storm rear-flank downdraft

Cloud Electrification    

involves collisions between particles of different size & temperature smaller ice particles lose electrons when they collide with larger graupel/hail lighter (+) particles carried to top; heavier (-) particles suspended in storm centre electric field exists in cloud (can be discharged by lightning)

Tornadoes      

tornadoes: rapidly rotating column of air around a small area of low pressure funnel cloud: tornado whose circulation has not reached the ground most dangerous form in supercells other types o landspouts, waterspouts, gustanodes, etc. o need pre-existing rotation from other features vertical shear is origin of storm rotation o updrafts tilt environmental rotation into vertical axis rear-flank downdraft critically important o descend from aloft; usually precipitation free o acts on contract & stretch mesocyclone near surface, intensifying rotation near ground

Doppler Radar   

extremely useful for storm detection rotation visible in Doppler velocity can measure how fast winds are moving towards/away from radar

15: HURRICANES 

QUESTION 4 ON ASSIGNMENT 4 WILL BE ON FINAL EXAM!!

The Tropics  

tropics: latitude belt between 23.5°S and 23.5°N o warm weather all year round o if cold front brings cold air towards Tropics, by the time it gets there it is warm due to solar radiation much different from latitudes o temperature/pressure gradients weaker

o

Coriolis force weaker

Tropical Cyclone Classifications   

tropical depression: closed circulation with sustained (> 1 minute-averaged) winds at 1m between 0-18 m/s tropical storm: same, but sustained winds at 18-33 m/s; tend to have more intense convection & spiral rainbands hurricane: same, but sustained winds > 33 m/s; intense & organized convection

Anatomy of Hurricane 



  

eye: area of broken clouds in centre o very centre of hurricane o can have absence of clouds o surrounded by EXTREMELY deep convection (18 km) eyewall: ring of intense thunderstorms that whirl around storm’s centre o surrounds eye o strongest wind/rain/waves o where hurricane is most dramatic spiral rainbands o how convection is organized inflow outflow boundary: a surface boundary formed by the horizontal spreading of cool air that originated inside a thunderstorm

Environmental Conditions Favouring Hurricanes 

  

warm ocean temperatures o fluxes of heat & moisture off warm water fuel storm o warmer temperatures = high saturated vapour pressure o air able to receive more moisture from water before saturating o over land, temperatures may be warm but supper of water greatly reduced weak vertical wind shear o vertical wind shear removes heat associated with condensation in hurricane core o tilts hurricane vortex, disrupting organized ventilation pattern high humidity throughout troposphere the Coriolis force o needed for rotation so air doesn’t converge too strongly into centre

Mechanisms for Intensification   

deep convection o can be due to easterly wave, stalled frontal boundary or along inter-tropical convergence zone o cluster of convection serves as seed for tropical cyclone latent heating aloft induces high pressure o evacuates air from central core region o causes low pressure to develop at lower levels low pressure drives cyclonic circulation o stronger airflow picks up more heat & moisture from ocean o convergence into centre intensifies deep convection, latent heating & circulation

Dissipation 

dissipate more easily than they form

   

passage over colder water (< 25°C) strengthening vertical wind shear landfall: lose moisture supply; frictional convergence causes low to “fill” decaying storm may become absorbed into midlatitude cyclones (extratropical transition) o moisture, latent heating may invigorate cyclone

Hurricane Motion   

Genesis regions in subtropical oceans initially move westward under influence of subtropical highs o ultimately swing northward around periphery of high o may get caught in mid-latitude westerlies (where they are often sheared apart) path not always dictated by larger-scale winds o storms may interact with flow to produce erratic paths o flow may be too weak to steer storm

Storm Surge  

low atmospheric pressure at centre causes water to rise Ekman transport o water transported to right of winds o causes high water levels just outside eyewall