Download The Atmosphere 2

EG4508: Issues in environmental science
Meteorology and Climate
Dr Mark Cresswell
The Atmosphere 2
Phases of water
• Water (H2O) is the most important
material on the planet
• Water can exist in solid, liquid and gas
phases
• Water molecules are free to move in a
gas, are closer together in a liquid and
are locked in an orderly pattern as a
solid
Phases of water
• As a solid, water forms hexagonal (6sided) crystals we call ice
• In freezing air, if enough energy is
available, ice can change directly into
gas (water vapour). This is called
sublimation
• If a water vapour molecule combined
with ice crystals it is deposition
Phases of water
• Applying warmth (energy) to an ice
crystal means that the molecules
vibrate faster - so much so that they
can vibrate out of their hexagonal
crystal structure - the ice melts
• At the surface of water, some molecules
have just enough energy to break free
from the rest - called evaporation
Phases of water
• Some water vapour molecules with very
little energy can combine with other
water molecules on the surface of water
- called condensing
• If a cover is placed over a beaker of
water, eventually an equilibrium
between escaping and returning water
molecules is reached.
Phases of water
• When this state of equilibrium is
reached the air in the beaker is said to
be saturated with water vapour
• Removing the cover from the beaker
would allow some molecules to be
blown away - so the air would no longer
be saturated and more would have to
evaporate to take their place
Phases of water
• This explains why evaporation occurs
more readily when there is wind than
on a still day
• Temperature also affects evaporation
• Warm water means that molecules have
more energy and speed up. These
molecules are more likely to escape
from the liquid surface
Linacre et al, 1997
Vapour pressure
• The air’s moisture content may also be
expressed in terms of the pressure exerted by
the water molecules within it
• Air pressure at sea level is the result of
pressure exerted by all gas molecules
(nitrogen and oxygen included). The total
pressure is equal to the sum of all pressures
from all gases - known as Dalton’s law of
partial pressure
Vapour pressure
• An increase in the number of water vapour
molecules will tend to increase the total
vapour pressure
• Actual vapour pressure indicates the air’s
total water vapour content. Saturation vapour
pressure describes how much water vapour is
necessary in order to make the air saturated
at any given temperature (remember the
beaker hypothesis).
Vapour pressure
• Saturation vapour pressure is the pressure
that the water vapour molecules would exert
if the air were saturated with vapour at a
given temperature
Dew point
• The dew point is the temperature to
which air must be cooled (with no
change in air pressure or moisture
content) for saturation to occur
• When the dew point temperature is
reached on a surface, dew, frost or fog
forms
• Lifting condensation level for air aloft
Measuring humidity
• Humidity is measured using a
psychrometer/hygrometer
• Wet and dry bulb thermometers based
in a Stevenson screen use the same
principle
• Difference between wet and dry bulb
temperatures indicates water vapour
content of the air
Formation of FOG #1
• The process of condensation that forms
fog and clouds is not so simple. It is not
simply the case that saturation (dew
point) must be reached
• There must be airborne particles on
which water vapour can condense
Formation of FOG #2
• Although air looks clean - it never really is. Air
contains many tiny particles (impurities)
• many of these particles serve as a surface on
which condensation can occur
• These particles are called condensation
nuclei
• Some condensation nuclei are very small with
a radius of < 0.2µm (Aitken nuclei)
• Particles 0.2 - 1µm are called large nuclei
• Particles > 1µm giant nuclei
Formation of FOG #3
• As the relative humidity reaches 75 100% (saturation) water condenses
onto condensation nuclei
• As the air cools and becomes more
saturated the droplets of suspended
condensed water get larger until visible
to the naked eye
• We can see these clouds of droplets as
fog
Radiation fog
Formation of FOG #4
• City air (with its extra impurities)
produces a thicker fog as there are
more condensation nuclei
• London often suffered from very thick
fog as a result of pollution and industrial
activity until legislation was introduced
early in the 20th century
City fog – exacerbated by pollutant particulates
Formation of FOG #5
• Fog often forms near the ground on a
natural surface (e.g. football pitch)
• This is exacerbated on clear nights
when radiation leaves rapidly and cools
the ground down and the moist air
directly above it
• This is known as radiation fog
Formation of FOG #6
• Fog is usually seen in low lying areas as
it is denser than the surrounding air and
is pulled to the surface by gravity
• When fog is seen to "burn off" by
sunlight it is actually the heating of the
ground which rises the air temperature
above causing the air to become
unsaturated and the fog dissipates
Formation of Clouds #1
• Clouds form in a similar way to fog - except
that the process takes place aloft
• In the case of cloud formation, the cooling
required to cause water to condense on
particulate nuclei is due to adiabatic cooling
• Clouds consist of tiny particles of ice or water
droplets (formed around condensation nuclei)
so small and light in weight that impacts from
the air's randomly moving molecules are
sufficient to keep them aloft
Formation of Clouds #2
• Cloud formation may be convectional,
orographic or frontal
• Convectional clouds form when moist air is
carried upwards by the action of vertical
convection (due to solar heating of the
surface). Moist air cools as it ascends until it
becomes saturated. At the point of saturation,
the moist air condenses to form cloud
(Convectional Condensation Level)
Formation of Clouds #4
• Orographic cloud forms when moist air is forced
upwards - usually when it flows over a plateau or
mountain. The air forced upslope cools until it
becomes saturated forming clouds near to or above
the mountainous structure (Lifting Condensation
Level).
• The air (now free of moisture) flows down the lee side
of the mountain at a higher temperature as energy
lost during condensation is carried away by the wind.
Rainfall usually occurs on the lee side, forming a rain
shadow on the upslope side.
Formation of Clouds #5
• With frontal cloud formation, moist warm air is
forced above cooler air (the cooler air acting
like a wedge). As this moist air is forced
upwards it cools until the air becomes
saturated and condenses into clouds. The
clouds are usually easily seen as a visible
ridge along the line of an active front.
Precipitation processes
• Small cloud droplets have a greater curvature
which causes a more rapid rate of
evaporation. As a result of this process
(curvature effect) smaller droplets require
an even greater vapour pressure to keep
them from evaporating away. This requires
the air to be supersaturated - with a relative
humidity greater than 100%. The smaller the
droplet, the greater the supersaturation
needed to keep it in equilibrium
Precipitation processes
• How do droplets with a diameter of <1µm grow to
the size of a cloud droplet?
• The answer lies with the cloud condensation nuclei.
Many of these nuclei are hygroscopic (have an
affinity for water vapour)
• Condensation may begin when the vapour pressure is
much lower than the saturated vapour pressure
• This reduces the equilibrium vapour pressure
required and is known as the solute effect
Precipitation processes
• In warm clouds (tops warmer than -15ºC) the action
of collisions between droplets is important
• Random collisions with already large droplets
mediated by salt particles (hygroscopic condensation
nuclei) produce larger droplets when they collide
• Large droplets begin to reach terminal velocity and
collide with smaller droplets in their wake - merging
together in a process called coalescence
• Falling droplets may evaporate on their way down, or
reach the ground as drizzle if the air below is moist
Precipitation processes
• In very deep convective clouds the ice-crystal process is an
important factor in precipitation
• Ice crystals may form nuclei upon which other ice crystals may
form
• These are deposition nuclei as water vapour changes directly
into ice without passing through the liquid phase
• The constant supply of moisture to an ice crystal allows it to
enlarge rapidly, it becomes heavy enough to overcome updrafts
and begins to fall
• If these crystals stick together (accretion) the icy matter
(rime) that forms is called graupel (or snow pellets).
Ice crystals
No
precipitation
Altocumulus Cloud
Water and Ice clouds – usually bring precipitation
After 15 – 20 hours
Stratus Cloud
Typically overcast or drizzle conditions
Cumulus Cloud
Associated with gusty winds and heavy precipitation
ITCZ (Inter-Tropical Convergence Zone)
The global winds
• The driving force of ANY wind is the
local pressure gradient expressed as:
• ∆p/ ∆n
where:
• ∆p is the difference between the
pressures at points separated
horizontally by a distance ∆n
Hadley Cells
The global winds
• Winds within 30º of the equator in the
Atlantic and Indian oceans were first
mapped by Edmund Halley in 1686
• The maps which developed from this
time were invaluable for shipping and
hence commercial and military
strategies
The global winds
• Winds between the tropics converge on
a line called the Inter-Tropical
Convergence Zone (ITCZ)
• This line of convergence can be
discerned on a map of streamlines and
visualised on a satellite image from
space
ITCZ (Inter-Tropical Convergence Zone)
The global winds
• Winds are mainly easterly at latitudes
between 10-30º - these are known as
the trade winds or trades
• Westerly winds prevail at about 35-60º
and are known as the midlatitude
winds
• There are polar easterlies at latitudes
above 60º
Ancient climate prediction
• The earliest attempts to predict the weather
were by farmers and the military
• The Greeks successfully used predictions
about the wind to defeat the Turkish during
sea battles
• Predicting weather could make the difference
between life and death for farmers
Weather vs Climate
• Weather forecasting is concerned with
accurate descriptions of weather type for a
short period of time
• Climate forecasting deals with how different
future conditions may be from those
expected in an average year
• Weather describes specific conditions
(raining, wind speed and direction, dew-point
etc). Climate discusses anomalies
Short, medium and long-range
•
•
•
•
Short-range is between 3 and 72 hours
Medium-range is between 3 days an a week
Long-range is a month or more ahead
Experimental-range (X-range) includes new
seasonal forecasts up to 6 months ahead
• Global climate prediction looks at climate out
to 50 to 100 years
Short, medium and long-range
forecast accuracy
good
poor
short
Range
long
Statistical Forecasting
• The oldest form of formal weather forecasting
• A statistical model is constructed from
regression and correlation analyses
• Model is trained on past (historical) weather
observations
• Model is given data relating to patterns of
SST or other large-scale conditions prior to
the period the weather changed
Statistical Forecasting
• The model thus learns what sets of conditions
(certain SST pattern, persistence of pressure,
timing of snowmelt etc) are associated with a
particular weather regime
• To use a statistical model you enter details
about large-scale conditions and it matches
those with its historical database to give a
prediction
• Drawback - can only “see” extremes
encountered in training data
Chaos theory
• One of the most fundamental advances
in the prediction of any natural process
(climate and weather included)
occurred after the discovery of chaos
• Chaos theory is an amalgamation of
game theory, probability theory and
fluid dynamics
Chaos theory
• Edward Lorenz realised that although
the atmosphere behaved as a chaotic
and random system, there were aspects
of it which could be solved within a
phase-space
• The strange attractor (Lorenz attractor)
was his visualisation of this hyperspace
and initialised fractal theory
Dynamical forecasting
• Dynamical forecasting is the most advanced
and current method of weather/climate
prediction
• Unlike a statistical forecast, it is based on the
calculation of weather/climate conditions
from first principles (Physics)
• Calculation is undertaken for each time-step
for regularly spaced grid-points across the
earth and up through the atmosphere
Dynamical forecasting
• A modern Atmospheric Global Circulation
Model (AGCM) solves many equations for
each grid-point for the earth surface,
atmosphere and oceans
• This type of model requires extremely
powerful computers (supercomputers) and
the science of GCMs only developed after
such computers became available
Dynamical forecasting
• A single model integration provides a
deterministic solution
• A better approach (originally proposed
by Lorenz) was to use a probabilistic
ensemble approach
• Ensemble forecasting strategy allows
greater uncertainty to be sampled
rainfall quantity
Dynamical forecasting
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Dynamical forecasting
rainfall quantity
Probability = f/n
3
where f is number of members in a category
where n is total number of integrations
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Dynamical forecasting
rainfall quantity
Probability = f/n
3
where f is number of members in a category
where n is total number of integrations
2
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Dynamical forecasting
rainfall quantity
Probability = f/n
3
Prob of Rain = 0.3 (30%)
Prob of NO rain = 0.7 (70%)
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Dynamical forecasting
• The UKMO and ECMWF utilise the
ensemble forecasting approach to
determine rainfall, temperature, El Niño
and general weather patterns many
weeks or even months in advance
• El Niño forecasting is considered VERY
important.......
WWW.IPCC.CH
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