Transcript Chapter 7 & 8 PowerPoint - University of South Australia

Synoptic Meteorology and
Climatology
ATC Chapter 7 & 8
Aim
To learn how to decipher a meteorological chart
including all weather systems and to learn
about climatology in Australia
Objectives
1. Explain what an air mass is
2. State and explain the different types of fronts
3. State the different pressure systems
4. Decipher a synoptic chart
5.Explain the seasonal weather of Australia
6. Describe the life cycle of a Tropical Cyclone
1. Air masses
Air masses
If air remains over a surface for long enough, it takes on the
characteristics of that surface
Once it has taken on the characteristics, it is called an air
mass
Ie if a cold body of air moves over a warm surface, it will
gradually become warmer
1. Air masses
Air masses
Air masses can be classified as either polar or tropical
The air within the polar cell is called polar air, while all other
air is tropical
Air that remains over land for a period, gradually loses its
moisture and is classified as continental
Air that remains over sea gradually increases its moisture
and is classified as maritime
Therefore, air masses can be
classified as:
• Tropical maritime
• Tropical continental
• Polar maritime
• Polar continental
2. Fronts
Fronts
When an air mass moves over a different region, it often
encounters air of different temperatures
Because the air has different densities, the warmer air tends
to rise and the colder air tends to sink
The boundary of these air masses is called a front
The different types of fronts are:
• Cold fronts
• Warm fronts
• Occluded fronts
• Quasi stationary fronts
2. Fronts
Cold Fronts
A cold front occurs when a cold air mass moves into a region
previously occupied by warm air
The boundary of these air masses is called a cold front
The cold front on a synoptic chart is shown as a line with solid
barbs pointing in the direction of travel
2. Fronts
Cold Fronts
As the cold air advances, it lifts the warm air above
The exact cloud formed depends on the stability and humidity
of the warm air and the speed at which the cold air is
advancing
Generally as the warm air is forced to rise, it promotes
instability and cumuliform clouds are likely
The passage of a cold front can be characterised by:
• The temperature falling
• The pressure rising
• The wind backing
2. Fronts
Warm Fronts
A warm front is characterised by cold air retreating from
advancing warm air
The position of a warm front is shown on a synoptic chart as a
line with solid semicircles pointing in the direction of travel, ie
into the cold air
2. Fronts
Warm Fronts
As the warm front approaches, it is characterised by:
• The temperature rises
• Wind backs
• Pressure falls
Cloud associated with a warm front are usually cirrus first,
followed by a gradual lowering of the base to form middle
level altostratus. Nimbostratus can then form to produce rain
until the front passes
2. Fronts
Occluded Fronts
Because cold air is heavier, it tends to surge forward much
more rapidly than a warm air
As days go by, the cold front moves faster over the ground
than the warm front
Eventually the cold front overtakes the warm front producing
an occluded front
An occluded front is shown on a synoptic chart as a solid line
with a mixture of barbs and semi circles
2. Fronts
Occluded Fronts
The cloud associated with an occluded front depends on what
clouds were associated with the cold and warm fronts
It is not unusual for stratiform clouds from the warm front to
conceal cumuliform clouds such as cumulonimbus from the
cold front making flight very dangerous
2. Fronts
Quasi stationary Fronts
A quasi stationary front is a front that tends not to move
These types of fronts tend to align themselves with the
isobars and have generally benign weather
The quasi stationary front is shown on a synoptic chart as a
solid line with barbs and semicircles on either side of the line
3. Pressure systems
Pressure systems
To understand how to read synoptic charts, we must first be
able to understand the pressure systems, how they are
indicated, and what weather is associated with them
On a synoptic chart, the pressure systems which we see are:
• High pressures or anticyclones
• Low pressures or depressions
3. Pressure systems
Anticyclones
When isobars close around a region of high pressure, the
feature is called a high pressure system or anticyclone
In the southern hemisphere, the rotation of winds around a
high pressure are anticlockwise
In the surface layer, the result of friction causes the wind to
blow slightly outwards and across the isobars
The flow associated with a high pressure system is:
• An outflow of air in the lower layers
• A subsidence of air over the wide area from above
• Convergence of air
in the upper layers
3. Pressure systems
Weather associated with anticyclones
As air is subsiding, the air in the top layers will be warming
quicker than the air in the lower layers due to compression
This can cause a subsidence inversion and trap pollutants such
as smoke or haze beneath it
This can affect flight visibility
3. Pressure systems
Ridges of high pressure
A ridge is an elongated area of high pressure
A ridge is generally associated with fine weather conditions
3. Pressure systems
Depressions
When isobars form around a region of low pressure it is called
a low pressure system or depression
In the southern hemisphere, the winds about a low tend to
blow clockwise
Due to surface friction, the winds tend to blow inwards and
across the isobars
3. Pressure systems
Depressions
Compared with a high pressure, a low pressure has a stronger
pressure gradient, that is a greater change of pressure with
distance
Because of the inflow of air towards the centre of a low
pressure system, the wind tends to get stronger at the centre
• This is called convergence and causes the air to also rise
at the centre due to the lower pressure
• This in turn causes a divergence in the upper layers
3. Pressure systems
Weather associated with a Depression
In a depression, the rising air in towards the centre will also be
cooling
If the air reaches dew point temperature, cloud will form
Instability in the rising air can cause a large vertical development
of cumuliform clouds accompanied by showers
Because of the vertical motion of air, good visibility can result due
to the vertical motion taking away all the particles suspended in
the air
This can also cause turbulence
3. Pressure systems
Troughs of low pressure
An elongated area of low pressure is known as a trough
Convergence will occur and the air will rise
If the air is unstable, large cumuliform clouds can occur
3. Pressure systems
Col
A col is an area between 2 areas of low pressure and 2 areas of
high pressure, or 2 anticyclones and 2 depressions.
It is an area of almost constant pressure
A col usually has light and variable winds
4. Synoptic Charts
Synoptic Charts
When presented with a synoptic chart you must be able to
identify:
• Pressure systems
• Fronts
• Troughs and ridges
• Surface wind and pressure at a particular location
4. Synoptic Charts
Synoptic Charts
When analysing a chart we need to remember that:
• A front ALWAYS originates from a low
• The circulation around a low is clockwise
• Pressure gradient runs down into a low (up into a high)
• The gradient wind cannot blow in a reverse direction
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, assuming an isobar spacing of
4hPa determine:
1. The pressure systems marked: A, B, C
2. The name of the features: BD, BE, CF
3. The surface wind and pressure in Hobart
A
C
D
B
E
F
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, determine:
1. The pressure systems marked: A, B, C
- Fronts ALWAYS start from a low, therefore B is a Low
- Once we have identified one pressure system we can
draw arrows to indicate wind direction – clockwise
around a low
A
C
D
B
E
F
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, determine:
1. The pressure systems marked: A, B, C
- The wind is blowing in an anti-clockwise direction
around A and C therefore they are Highs
A
C
D
B
E
F
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, determine:
2. The name of the features: BD, BE, CF
- Looking at the symbols on the chart, BD is a cold front,
BE is a warm front
A
C
D
B
E
F
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, determine:
2. The name of the features: BD, BE, CF
- CF is an elongated section originating from a high,
therefore it is a Ridge
A
C
D
B
E
F
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, determine:
3. The surface wind and pressure in Hobart
- Wind: Gradient wind is a South Westerly, the surface
wind will be Westerly due to veering from surface friction
A
C
D
B
E
F
4. Synoptic Charts
Synoptic Charts - Example
Example, given the chart below, determine:
3. The surface wind and pressure in Hobart
- Pressure: Start from the known pressure then write what the
pressure is at each isobar, remember pressure decreases as
we move further from the high. Hobart = 1018
A
C
D
1022
1018
B
E
F
5. Climatology
Weather in Australia
Australia is a vast country, and with it carries a vast range of
weather
This weather is created on a large scale and depends on the
seasons and airstreams
Generally speaking we can divide the weather in Australia into
two categories:
• Tropical – Northern Australia
• Sub-Tropical – Southern Australia
5. Climatology
Sub-Tropical Meteorology
The weather in Southern Australia is classified as Sub-Tropical
The weather in this region is mainly influenced by the subtropical ridge
This causes us to have the four seasons:
• Summer
• Autumn
• Winter
• Spring
5. Climatology
Sub-Tropical Meteorology - Summer
Summer comprises the months December to February
In summer the highs which form the subtropical ridge move to
the south
Winds are generally south easterly
In Western Australia the wind is generally very hot and dry
5. Climatology
Sub-Tropical Meteorology - Autumn
Autumn comprises the months March to May
Autumn generally brings long periods of fine weather and is
often considered the best time to go flying
Although passage of a cold front can bring cold conditions and
the possibility of frontal thunderstorms
Fog can be expected
during the mornings as
overnight temperatures
can get low
5. Climatology
Sub-Tropical Meteorology - Winter
Winter consists of the months June to August
During the winter months the sub-tropical ridge moves to the
north causing the mid-latitude depressions and the associated
fronts to lie just to the south of the continent
Winds are generally westerly
Cloud is often widespread
stratocumulus
Passing fronts bring
periods of rain
5. Climatology
Sub-Tropical Meteorology - Spring
Spring consists of the months September to November
Spring weather is often quite variable
Conditions can be generally fine one minute then can be unflyable the next
This is due to fast moving cold fronts and troughs
5. Climatology
Tropical Meteorology
The weather in the North of Australia is classified as Tropical
There are 2 major wind streams in the tropics and they are
largely responsible for the weather in these regions
The wind streams are the trade winds and the monsoons
5. Climatology
Tropical Meteorology - Trade Winds
The regions between the subtropical high pressure belts and
the equatorial trough are occupied by the trade winds
In the southern hemisphere, Coriolis deflection produces the
south east tradewinds, and in the northern hemisphere, the
northeast trade winds
The winds do not always blow
exactly in these directions, changes
due to pressure gradients and the
Coriolis force cause the wind
direction and speed to change from
place to place
5. Climatology
Tropical Meteorology - Trade Winds
The Trade Winds are responsible for the “Dry Season” in Northern
Australia
The characteristics of the trade winds are:
• Winds are South-Easterly (in the southern hemisphere)
• It blows at a consistent velocity for very long periods of time
(months)
• Becomes weaker at lower latitudes – due to less Coriolis force
• Generally stable
• Relative humidity is high at low levels
• Often produces showers near the coast but is dry inland
5. Climatology
Tropical Meteorology - The monsoons
Due to specific heats, continents tend to warm up quickly in
summer and cool rapidly in winter, whereas oceans tend to
maintain a more even contrast throughout the year
When large continental and ocean areas are involved,
atmospheric circulations may develop on a large scale
A monsoon is the persistence of maritime airflow over a hot
continent
Moist maritime air crosses the
coast to the hot land and
becomes more and more
unstable as it is heated from below
5. Climatology
Tropical Meteorology - The Monsoons
The Monsoons are responsible for the “Wet Season” in Northern
Australia
The characteristics of the Monsoons are:
• Winds are North-Westerly (in the southern hemisphere)
• Generally unstable
• It is moist near the coast but dry inland
• Produces areas of widespread rainfall up to 300nm inland
5. Climatology
Tropical Meteorology - The equatorial trough
The equatorial trough is the name given to the area between
the trade winds of each hemisphere
Generally, the weather consists of light winds and fine
weather, if there is a large level of convergence, instability can
exist with large cloud formations
The equatorial trough follows the passage of the sun
throughout summer to winter
In summer, it is lower down and covers parts of northern
Australia
In winter, it is further north
5. Climatology
The Intertropical Convergence Zone (ITCZ)
Convergence occurs on a large scale when the tradewinds of
each hemisphere meet in a narrow zone
This zone is called the Intertropical Convergence Zone (ITCZ)
It can occur when the north east trade winds and the
southeast tradewinds approach each other between two nearequatorial troughs
The ITCZ can produce extreme weather conditions over a wide
area
Weather can include heavy rain,
frequent thunderstorms and
violent wind squalls
6. Tropical Cyclones
Tropical Cyclones
Tropical Cyclones are strong, deep depressions that form over
the ocean
Tropical cyclones derive their energy from warm waters,
usually above 28 degrees Celsius
Tropical cyclones form usually between 5-15 degrees latitude
This is due to the coriolis force required to develop the
balanced circulation flow
Tropical cyclones can decay if they move into colder waters
(further away from equator), or if
they move inland
The stages of a tropical cyclone are:
• Formative stage
• Immature stage
• Mature stage
• Decaying stage
6. Tropical Cyclones
Tropical Cyclones - lifecycle
Formative stage – A tropical cyclone usually develops in a pre
existing low pressure tough
Atmospheric wind shear is low and there is an abundance of
moisture for vigorous convective cloud growth
The system becomes more circular, the pressure in the centre
decreases, still above 1000hPa, and wind increases
6. Tropical Cyclones
Tropical Cyclones - lifecycle
Immature stage – The pressure falls below 1000hPa and winds
increase to 64kts
The convective cloud forms into spirals and the eye develops
Pressure continues to fall, and winds continue to increase in
strength
6. Tropical Cyclones
Tropical Cyclones - lifecycle
Mature Stage – At this stage the cyclone is at its maximum size
and strength and the centre pressure is steady, at around
950hPa
The hurricane force winds and bad weather extend to around
150nm from the centre with the strongest winds being in the
left forward quadrant
6. Tropical Cyclones
Tropical Cyclones - lifecycle
Decaying Stage – When the cyclone has its energy supply
disrupted, by either reaching colder water or land, the
decaying stage begins
The system may continue over land as a rain bearing
depression or move into higher latitudes and interact with
frontal systems
6. Tropical Cyclones
Tropical Cyclones – The eye
The eye of the tropical cyclone is an indication of a well
developed storm
The conditions are benign with a constant pressure and light
winds
There is generally clear skies with no precipitation
Questions?