Chapter 1
Anatomy of a
Cyclone
“The Beast in the East” – 15-17 February 2003
Extra-tropical cyclone – an area of low pressure outside of the tropics
Other names for extra-tropical cyclones:
Cyclone
Mid-latitude cyclone
Frontal cyclone
Storm system
Severe weather associated with this storm system:
Rain
Floods
Mudslides
Tornadoes
Heavy snow
What were the atmospheric processes responsible for the formation and
evolution of this storm as it passed over the United States?
Why were such diverse severe weather events as heavy snow and
tornadoes reported with this one system?
See weather maps of this storm on the Applied Atmospheric Dynamics
CD-ROM
Describing the Atmosphere
Troposphere – lowest layer of the atmosphere, where temperature normally
decreases with increasing altitude
SI (Système Internationale) Units
Length: meter (m)
Mass: kilogram (kg)
Time: second (s)
Temperature: Kelvin (K)
(see Appendix 2 for additional derived SI units)
Atmospheric Temperature
Atmospheric Pressure
Horizontal and vertical variations in pressure give rise to atmospheric
motions (which are the focus of ATOC 4720).
What is atmospheric pressure?
A molecular perspective…
SI Units for pressure
Pressure is a force per unit area:
From Newton’s second law: F=ma
Force has units of kg m s-2 (or Netwons – N)
Force per unit area has units of: kg m-1 s-2 = N m-2 (or Pascals – Pa)
Other commonly used units of pressure:
1 hPa = 100 Pa
1 millibar (mb) = 1 hPa
In the atmosphere the observed pressure is also equal to the weight of air
above the observation point.
Therefore, pressure decreases with increasing altitude in the atmosphere.
Surface pressure – the actual air pressure measured at the surface of the
earth
Mean sea-level pressure (MSLP) – surface pressure interpolated to zero
elevation (sea level)
Why do meteorologists use MSLP instead of surface pressure?
Isobar – contour line of
constant pressure
Station Reports
What are the typical weather observations made at surface weather
stations around the world?
Dew point temperature – the temperature to which a small volume of air
must be cooled at constant pressure in order for that air parcel to become
saturated
Dew point temperature indicates the amount of moisture contained in the
air.
What units are used for temperature and dew point temperature reported
on station models?
Significant weather
Cloud cover
Wind direction – the direction the wind is coming from
Wind speed
Units for wind speed
SI: m s-1
Meteorological convention: Knots (kts)
1 kt = 0.51 m s-1
Station model wind speed plotting convention:
- Wind speed symbols shown in Figure 1.4 are additive
- Actual wind speed is within 2 kts of plotted wind speed
Sea level pressure
Decoding sea level pressure reported on station models
If coded SLP is greater than 500:
Put a 9 in front of the 3 digit coded SLP
Insert a decimal point between the last two digits
Add units of mb
Example: coded SLP = 956
Decoded SLP = 995.6 mb
If coded SLP is less than 500:
Put a 10 in front of the 3 digit coded SLP
Insert a decimal point between the last two digits
Add units of mb
Example: coded SLP = 052
Decoded SLP = 1005.2 mb
The normal range of sea level pressure is 950 to 1050 mb.
Record high sea level pressure: 1086 mb (Tosontsengel, Mongolia)
Record low sea level pressure: 870 mb (Typhoon Tip, western Pacific)
Time and weather observations
UTC – Universal time coordinate
GMT – Greenwich Mean Time
Z – Zulu time
Quick facts about Universal Time Coordinate (UTC)
UTC is based on a 24 hour clock (so add 12 to any times after 12:59PM)
6AM UTC would be written as 06 UTC
12 noon UTC would be written as 12UTC
6PM UTC would be written as 18UTC
If UTC time is given as both hours and minutes it looks like this:
2:15AM UTC would be written as 0215 UTC
12:00 noon UTC would be written as 1200 UTC
10:20PM UTC would be written as 2220 UTC
UTC never switches from standard time to daylight savings time
This means we need to change how we convert between UTC and
Mountain time depending on whether we are on standard time or
daylight savings time
How do I convert from UTC to MST or MDT?
MST = UTC - 7 hours
MDT = UTC - 6 hours
How do I convert from MST or MDT to UTC?
UTC = MST + 7 hours
UTC = MDT + 6 hours
Air Masses and Fronts
Air mass – a large volume of the atmosphere with relatively uniform
temperature and humidity
What conditions favor the formation of air masses?
What happens to an air mass when it moves away from its source region?
Front – boundary between differing air masses
Fronts are defined based on the
thermodynamic differences
across the front and the
direction of movement of the air
masses on either side of the
front.
Surface Weather Maps
Upper Level Weather Maps
Since pressure always decreases with height, and above any given spot on
the earth each height has a unique pressure, we can use pressure as a
vertical coordinate.
Weather data on upper level weather maps are plotted on constant
pressure surfaces rather than constant height surfaces.
Pressure surface – an imaginary surface above the ground where the
pressure has a constant value
One of the key properties meteorologists are interested in when looking at
a constant pressure map is the height of the constant pressure surface.
The height of a
constant pressure
surface varies with
the temperature of the
column of air below
the pressure surface.
Lower heights
correspond to a
colder column
temperature.
Therefore, we expect
(and do) find lower
constant pressure
heights near the poles
and higher heights in
the tropics.
Commonly Used Constant Pressure Maps
Pressure
Approximate
Level
Altitude (ft)
850 mb
About 5,000 ft
700 mb
About 10,000 ft
500 mb
About 18,000 ft
300 mb
About 30,000 ft
250 mb
About 35,000 ft
200 mb
About 39,000 ft
Approximate
Altitude (km)
About 1.5 km
About 3.0 km
About 5.5 km
About 9.0 km
About 10.5 km
About 12.0 km
Upper Air Station Model
What are the differences between surface and upper air station models?
A sample 500 mb upper level weather map
Trough – region of low heights on a constant pressure map
Ridge – region of high heights on a constant pressure map
Shortwave – a small ripple in the height field
Longwave – a large ripple in the height field
How does the height of the 500 mb surface vary from south to north on this
map?
What is the relationship between winds and height contours on an upper
level constant pressure map?
Does this relationship vary between the Northern and Southern
hemispheres?
What wind direction should we expect to find in the mid-latitudes if lower
constant pressure surface heights are found near the poles and higher
heights are found in the tropics?
Isotherm – contour line of constant temperature
Baroclinic – temperature varies on a constant pressure surface
Barotropic – temperature is constant on a constant pressure surface
The Structure of a Typical Extra-Tropical Cyclone
Extra-tropical cyclone characteristics:
Size
Pressure
Winds
Polar front – boundary between warm tropical and cold polar air masses
Stages in the life cycle of an extra-tropical cyclone
What physical process can cause the surface pressure to decrease?
Convergence – accumulation of atmospheric mass
Divergence – removal of atmospheric mass
Divergence associated with the shortwave shown in Figure 1.9a leads to
the formation of a low pressure center at the surface of the earth.
What is the direction of circulation around this area of low pressure?
How does this circulation alter the distribution of temperature?
What types of fronts form in response to this circulation?
Warm sector – wedge of warm air ahead of an advancing cold front and
behind a warm front
How does the height of the 500 mb constant pressure surface change in
response to the changes in the distribution of temperature near the
surface?
Mature stage of a mid-latitude cyclone
Where are the upper level ridge and trough located relative to the low
pressure center at the surface during the mature stage of the cyclone’s life
cycle?
What determines how much the surface pressure decreases (or
increases)?
See animation of mature mid-latitude cyclone on class web page.
Where are the clouds and precipitation located relative to the fronts in this
cyclone?
How does the speed of movement of the cold and warm fronts differ?
What happens when the cold front catches up to the warm front?
Occluded front – a boundary that separates two cold air masses at the
surface with warm air aloft
How does the position of the upper level trough relative to the surface low
pressure center change once the cyclone has occluded?
What causes the cyclone to weaken and dissipate?
The February 2003 Storm Revisited
Mature stage – 00 UTC 15 February 2003 (see weather maps shown
above)
Occluded stage and the creation of the “Beast in the East” – 00 UTC 17
February 2003
Surface Weather Map
500 mb Weather Map
Which low pressure center at the surface is in the most favorable position
to intensify at this time?