Atmospheric Moisture and Stability

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Atmospheric Moisture and Stability

• Lecture 7
• October 21, 2009

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Homework 2 Comments

• Winds are nearly geostrophic in the upper levels
  of the atmosphere because there is less friction.
  

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Homework 2 Comments
Fastest wind at point b because PGF is largest
(isobars closely spaced together)
Fr CF PGF Wind

• Friction in the opposite direction of wind
• CF always at 90 perpendicular to wind
• Wind flows across isobars in lower levels (not in
  geostrophic balance)

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Review from last week

• Energy is the ability or capacity to do work on
  some form of matter
• Kinetic energy the energy an object possesses
  as a result of its motion
• KE ½ mv2
• 1st Law of Thermodynamics Energy cannot be
  created or destroyed.
• Energy lost during one process must equal the
  energy gained during another

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Review

• Therefore the 1st law states that heat is really
  energy in the process of being transferred from a
  high temperature object to a lower temperature
  object.
• Heat can be transferred by
• Conduction transfer of heat from molecule to
  molecule within a substance by direct contact
• Convection the transfer of heat by the mass
  movement of a fluid in the vertical direction (up
  and down)
• Advection the transfer of heat in the horizontal
  direction
• Radiation the transfer of heat through
  electromagnetic wave energy

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• Moving on to moisture and stability

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Water is responsible for many of Earths natural
processes
http//www.srh.weather.gov/jetstream/atmos/hydro.h
tm
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Water can exist in all three phases in our
atmosphere

• What atmospheric variable do we use to quantify
  the amount of water in any given volume of air at
  one time?
• Answer Moisture

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Ways to measure the moisture content of the
atmosphere

• Absolute Humidity- The ratio of the mass of water
  vapor to the volume occupied by a mixture of
  water vapor and dry air.
• Specific Humidity- The mass of water vapor / unit
  mass of air, including the water vapor.
• Mixing Ratio- ratio of the mass of water vapor
  per kg of dry air
• Saturation Mixing Ratio- mass of water vapor when
  a parcel is saturated / mass of dry air in the
  parcel.
• Vapor Pressure- Pressure of water vapor
  constituent of the atmosphere.
• Saturation Vapor Pressure- The pressure of water
  vapor constituent when the atmosphere is
  saturated.

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The variables we will refer to most

• Mixing Ratio-
• ratio of the mass of water vapor per kg of dry
  air
• (does not change with temperature)
• Relative Humidity-
• Vapor Pressure/ Saturation vapor pressure
• Dew Point Temperature-
• The temperature to which a given air parcel must
  be cooled at constant pressure and constant water
  vapor content in order for saturation to occur.

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There are only TWO ways to saturate the air (or
increase the relative humidity)

• 1. Add more water vapor to the air
• 2. Cool the air until its temperature is closer
  to the dew point temperature
• Remember the water vapor molecules are moving
  faster in warm air and less likely to stick
  together and condense. If air cools to the dew
  point temperature, there is saturation.

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Moisture

• An air parcel with a large moisture content has
  the potential for that parcel to produce a great
  amount of precipitation.
• - Air with a mixing ratio of 13 g/kg will likely
  rain a greater amount of water than air with a
  mixing ratio of 6 g/kg.

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Two parcels of air PARCEL 1 Temperature 31
oF, Dew point 28 oF PARCEL 2 Temperature
89 oF, Dew point 43 oF
Parcel 2 contains more water vapor than Parcel 1,
because its dew point is higher. Parcel 1 has a
higher relative humidity, because it wouldnt
take much cooling for the temperature to equal
the dew point! Thus, Parcel 1 is more likely to
become saturated. But if it happened that both
parcels became saturated then Parcel 2 would have
the potential for more precipitation. RH is not
simply equal to the dew point divided by the
temperature but is a good representation.
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Moisture and the Diurnal Temperature Cycle

• Review Water has a high heat capacity (it
  takes lots of energy to change its temperature)
• As a result, a city with a dry climate (like
  Sacramento, CA) will have a very large diurnal
  (daily) temperature cycle
• A city with high water vapor concentration (like
  Key West, FL) will have a small diurnal cycle

Late July averages Sacramento 94/60 Key
West 90/79
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The other key component to the hydrologic cycle-
Stability

• What is stability?
• Stability refers to a condition of equilibrium
• If we apply some perturbation to a system, how
  will that system be affected?
• Stable System returns to original state
• Unstable System continues to move away from
  original state
• Neutral System remains steady after perturbed

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Stability Example
Stable Marble returns to its original position
Unstable Marble rapidly moves away from initial
position
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Stability
How does a bowl and marble relate to the
atmosphere??

• When the atmosphere is stable, a parcel of air
  that is lifted will want to return back to its
  original position

http//www.chitambo.com/clouds/cloudshtml/humilis.
html
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Stability Cont.

• When the atmosphere is unstable (with respect to
  a lifted parcel of air), a parcel will want to
  continue to rise if lifted

http//www.physicalgeography.net/fundamentals/imag
es/cumulonimbus.jpg
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What do we mean by an air parcel?

• Imaginary small body of air a few meters wide
• Can expand and contract freely
• Does not break apart
• Only considered with adiabatic processes -
  External air and heat cannot mix with the air
  inside the parcel
• Space occupied by air molecules inside parcel
  defines the air density
• Average speed of molecules directly related to
  air temperature
• Molecules colliding against parcel walls define
  the air pressure inside

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Buoyancy and Stability

• Imagine a parcel at some pressure level that is
  held constant, density remains the same so the
  only other variable that is changing is
  temperature. (REMEMBER the Ideal Gas Law)
• So if ?parcel lt ?env. Parcel is positively
  buoyant
• In terms of temperature that would mean
• T of parcel gt T of environment buoyant!
  (unstable)
• T of parcel lt T of environment sink! (stable)
• T of parcel T of environment stays put
  (neutral)

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Atmospheric Stability
This is all well and good but what about day to
day applications?
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Review Atmospheric Soundings

• Vertical profiles of the atmosphere are taken
  at 0000 UTC (7 AM CDT) and 1200 UTC (7 PM CDT) at
  80 stations across the country, and many more
  around the world. Sometimes also launched at
  other times when there is weather of interest in
  the area.
• Weather balloons rise to over 50,000 feet and
  take measurements of several meteorological
  variables using a radiosonde.

• Temperature
• Dew point temperature
• Wind
• - Direction and Speed
• Pressure

http//www2.ljworld.com/photos/2006/may/24/98598/
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Vertical Profile of Atmospheric Temperature
allows us to assess stability of the atmosphere
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Lapse Rates

• Lapse Rate The rate at which temperature
  decreases with height (Remember the inherent
  negative wording to it)
• Environmental Lapse Rate Lapse rates associated
  with an observed atmospheric sounding (negative
  for an inversion layer)
• Parcel Lapse Rate Lapse rate of a parcel of air
  as it rises or falls (either saturated or not)
• Moist Adiabatic Lapse Rate (MALR) Saturated air
  parcel
• Dry Adiabatic Lapse Rate (DALR) Dry air parcel

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DALR

• Air in parcel must be unsaturated (Relative
  Humidity lt 100)
• Rate of adiabatic heating or cooling 10C for
  every 1000 meter (1 kilometer) change in
  elevation
• Parcel temperature decreases by about 10 if
  parcel is raised by 1km, and increases about 10
  if it is lowered by 1km

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MALR

• As rising air cools, its RH increases because the
  temperature approaches the dew point temperature,
  Td
• If T Td at some elevation, the air in the
  parcel will be saturated (RH 100)
• If parcel is raised further, condensation will
  occur and the temperature of the parcel will cool
  at the rate of about 6.5C per 1km in the
  mid-latitudes

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DALR vs. MALR

• The MALR is less than the DALR because of latent
  heating
• As water vapor condenses into liquid water for a
  saturated parcel, LH is released, lessening the
  adiabatic cooling

Remember no heat exchanged with environment
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DALR vs. MALR
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Absolute Stability

• The atmosphere is absolutely stable when the
  environmental lapse rate (ELR) is less than the
  MALR
• ELR lt MALR ltDALR
• A saturated OR unsaturated parcel will be cooler
  than the surrounding environment and will sink,
  if raised

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Absolute Stability

• Inversion layers are always absolutely stable
• Temperature increases with height
• Warm air above cold air very stable

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Absolute Instability

• The atmosphere is absolutely unstable when the
  ELR is greater than the DALR
• ELR gt DALR gt MALR
• An unsaturated OR saturated parcel will always be
  warmer than the surrounding environment and will
  continue to ascend, if raised

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Conditional Instability

• The atmosphere is conditionally unstable when the
  ELR is greater than the MALR but less than the
  DALR
• MALR lt ELR lt DALR
• An unsaturated parcel will be cooler and will
  sink, if raised
• A saturated parcel will be warmer and will
  continue to ascend, if raised

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Conditional Instability

• Example parcel at surface
• T(p) 30C, Td(p) 14C (unsaturated)
• ELR 8C/km for first 8km
• Parcel is forced upward, following DALR
• Parcel saturated at 2km, begins to rise at MALR
• At 4km, T(p) T(e)this is the level of free
  convection (LFC)

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Conditional Instability

• Example continued
• Now, parcel will rise on its own because T(p) gt
  T(e) after 4km
• The parcel will freely rise until T(p) T(e),
  again
• This is the equilibrium level (EL)
• In this case, this point is reached at 9km
• Thus, parcel is stable from 0 4km and unstable
  from 4 9km

EL
LCL
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Rising Air

• Consider an air parcel rising through the
  atmosphere
• The parcel expands as it rises
• The expansion, or work done on the parcel causes
  the temperature to decrease
• As the parcel rises, humidity increases and
  reaches 100, leading to the formation of cloud
  droplets by condensation

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Rising Air

• If the cloud is sufficiently deep or long lived,
  precipitation develops.
• The upward motions generating clouds and
  precipitation can be produced by
• Convection in unstable air
• Convergence of air near a cloud base
• Lifting of air by fronts
• Lifting over elevated topography

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Lifting by Convection

• As the earth is heated by the sun, thermals
  (bubbles of hot air) rise upward from the surface
• The thermal cools as it rises, losing some of its
  buoyancy (its ability to rise)
• The vertical extent of the cloud is largely
  determined by the stability of the environment

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Lifting by Convection

• A deep stable layer restricts continued vertical
  growth
• A deep unstable layer will likely lead to
  development of rain-producing clouds
• These clouds are more vertically developed than
  clouds developed by convergence lifting

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Lifting by Convergence

• Convergence exists when there is a horizontal net
  inflow into a region
• When air converges along the surface, it is
  forced to rise

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Lifting by Convergence

• Large scale convergence can lift air hundreds of
  kilometers across
• Vertical motions associated with convergence are
  generally much weaker than ones due to convection
• Generally, clouds developed by convergence are
  less vertically developed

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Lifting due to Topography

• This type of lifting occurs when air is
  confronted by a sudden increase in the vertical
  topography of the Earth
• When air comes across a mountain, it is lifted up
  and over, cooling as it is rising
• The type of cloud formed is dependent upon the
  moisture content and stability of the air

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Lifting due to Topography
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Lifting Along Frontal Boundaries

• Will discuss origin more in detail next week as
  we begin to discuss cyclones and fronts