Climatology Lecture 4

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ClimatologyLecture 4

• Michael Palmer
• Vertical Motion in the Atmosphere
• Continued...

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Turning the atmosphere upside down
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Rules about rising and sinking air

• Pressure decreases vertically
• As air rises, it expands because it moves into
  regions of lower pressure
• As air expands, it cools..At a rate set by the
  dry adiabatic lapse rate (for as long as no water
  vapour condenses)

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Parcel temperature decreases upwards
Pressure decreases upwards
SURFACE
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Adiabatic Lapse Rates

• Rate of change of temperature with
  heightassuming no external heating of
  parcel no mixing of parcel with
  surrounding air dry air
• Dry Adiabatic Lapse Rate 1oC/100m

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Environmental Lapse Rates

• Actual measured rate of change of temperature
  with height
• Measured by balloons

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Instability and Stability

• Determined by comparing the temperature of a
  parcel of air with its surrounding air
  (environmental) temperature at a common height
• if the parcel is cooler than its surrounding air,
  the parcel will sink - the air is stable(parcel
  is more dense and therefore not buoyant))
• if parcel is warmer than its surrounding air, the
  parcel will rise - the air is unstable(parcel is
  less dense and therefore buoyant)

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Instability and Stability

• parcel temperature determined by adiabatic lapse
  rates
• environmental temperature must be measured as it
  changes all the time...

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Dry Air
Environmental Lapse Rate
Dry Adiabatic Lapse Rate
Temperature
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Dry Example Surface Temp 34 oC
Environ Temp
Parcel Temp
Temperature
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Dry Example Surface Temp 34 oC
Stable Air No convection No Rain
Environ Temp
Parcel Temp
Temperature
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Water in the Atmosphere

• Can exist as gas (water vapour), liquid, or solid
  (ice).
• Important in a number of ways
• Formation of precipitation (rain/snow..)
• Has significant radiative effects (lecture 2)
• Polar stratospheric clouds (high altitude ice
  clouds in the stratosphere Important chemical
  reactions on the surface of ice particles
• Ozone destrcution (ozone hole)

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Water in the Atmosphere

• Clouds consist of liquid water droplets and
  ice.. But,
• Much more water is held in the atmosphere as
  vapour than as liquid or ice
• the presence of water vapour in the atmosphere is
  measured in several ways such as vapour pressure,
  mixing ratio, specific humidity, relative
  humidity, precipitable water and dew point
  temperature

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Dew Point Temperature

• Dew Point (Td) is the temperature to which air
  at constant pressure and water vapour content
  must be cooled in order to become saturated and
  for dew to precipitate
• dry air example T20 Td5
• moist air example T20 Td19
• If air cools to dew point, condensation
  begins

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Heat

• Latent Heat - Invisible Heat Heat released or
  absorbed during a phase change evaporation -
  cooling condensation - heating
• Sensible Heat Heat that can be felt and
  measured directly

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...
...
freezing
condensation
Liquid
Gas
Solid
Ice
Water
Water Vapour
melting
evaporation
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deposition
...
...
freezing
condensation
Liquid
Gas
Solid
Ice
Water
Water Vapour
melting
evaporation
Sublimation
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Release of latent heat from environment
deposition
...
...
freezing
condensation
Liquid
Gas
Solid
Ice
Water
Water Vapour
melting
evaporation
Sublimation
Release of latent heat to environment
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Latent heat

• If water changes phase, then the role of latent
  heat has to be considered
• in cloud formation, the release of latent heat
  has to be considered
• the release of latent heat can influence
  stability/instability
• after condensation is reached during ascent,
  latent heat release slows down the rate of
  cooling with ascent to theSaturated Adiabatic
  Lapse Rate (SALR)

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Saturated Adiabatic Lapse Rate (SALR)

• Not a constant rate of cooling since rate of
  change of temperature with height depends on
  latent heat release following condensation, which
  depends on the moisture content of the air
• On average the SALR 6oC/km
• Now we can go on to look at
• Stability vs. Instability The role of water

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Dry Air
Environmental Lapse Rate
Dry Adiabatic Lapse Rate
Temperature
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Dry Example Surface Temp 34 oC Surface Dew
Point 0 oC
Temperature
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Dry Example Surface Temp 34 oC Surface Dew
Point 0 oC
Temperature
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Dry Example Surface Temp 34 oC Surface Dew
Point 0 oC
Environ Temp
Parcel Temp
Temperature
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Dry Example Surface Temp 34 oC Surface Dew
Point 0 oC
Stable Air No convection No Rain
Environ Temp
Parcel Temp
Temperature
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Wet Example Surface Temp 34 oC Surface Dew
Point 29 oC
Dew Point
Temperature
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Wet Example Surface Temp 34 oC Surface Dew
Point 29 oC
Saturated Adiabatic Lapse Rate
Temperature
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Wet Example Surface Temp 34 oC Surface Dew
Point 29 oC
Parcel Temp
Temperature
Environ Temp
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Wet Example Surface Temp 34 oC Surface Dew
Point 29 oC
Unstable Air Convection Rain
Parcel Temp
Temperature
Environ Temp
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Wet Example
Unstable Air Convection Rain
Level of free convection
Condensation Level
Temperature
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Features of Conditionally Unstable Air

• Lifting Condensation Level (LCL)close to the
  surface in moist airfar above the surface in dry
  air
• Level of Free Convectioninstability is unlikely
  to be released if this is a long way above the
  surface
• Degree of instabilitydetermined by the
  difference in temperature between the parcel and
  the environment in the region of maximum
  instability

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Wet Example Surface Temp 34 oC Surface Dew
Point 29 oC
Temperature
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Space Shuttle Pics
Great intro to atmospheric stability http//apoll
o.lsc.vsc.edu/classes/met130/notes/chapter7/index.
html

• http//cass.jsc.nasa.gov/publications/slidesets/cl
  ouds.html
• http//www.ncdc.noaa.gov/ol/satellite/olimages.htm
  l
• http//earth.jsc.nasa.gov/categories.html

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Readings for todays lecture

• Barry and Chorley 1997 p76-86
• Briggs et al. 1997 Fundamentals of the Physical
  Environment p78-88
• Henderson-Sellers and Robinson 1999 p56-74
• Linacre and Geerts 1997 Climates and Weather
  Explained p127-145
• McIlveen 1992 p109-139