Synoptic Meteorology:
A Review
Mesoscale
M. D. Eastin
The Role of Mid-Latitude Synoptic Systems
The Earth must maintain an Energy (or Heat) Balance!
Earth gains energy from the
Sun (or warms) via Incoming
Shortwave Radiation
Earth
Mesoscale
Sun
Earth emits energy to
space (or cools) via
Outgoing Longwave
Radiation
M. D. Eastin
The Role of Mid-Latitude Synoptic Systems
Maintaining the Global Heat Balance
• The Earth cool everywhere via radiation
(the tropics, midlatitudes, and poles)
• Due to the Earth’s small tilt with respect
to its axis of rotation, the tropics
received much more energy (heat)
from the Sun than the polar regions
• Thus, over time, a strong north-south
gradient in temperature develops
Nature hates strong gradients!!!
The role of mid-latitude synoptic weather
systems is to remove this gradient by
transporting (or advecting) warm air
poleward and cold air equatorward
Mesoscale
M. D. Eastin
The Role of Mid-Latitude Synoptic Systems
Maintaining the Global Heat Balance
• The gradients are continuously removed through the interaction of upper-level waves
in the jet stream with large-scale high and low pressure systems near the surface
Mesoscale
M. D. Eastin
Forces Driving Synoptic-Scale Air Motions
There are five primary forces that govern large-scale atmospheric motions:
Pressure Gradient Force
Gravity
Coriolis Force
Friction
Centrifugal Force
Pressure Gradient Force (PGF)
• Pressure is a force per unit area
• Air pressure is the weight (mass) of the atmosphere
above a given location
• Air pressure is a function of temperature and density
• Air pressure decreases with altitude at a decreasing rate
• Air always tries to flow down the pressure gradient
from regions of high pressure toward regions of
lower pressure → the pressure gradient force
• The pressure gradient force is directly proportional
to the magnitude of the gradient
Mesoscale
M. D. Eastin
Forces Driving Synoptic-Scale Air Motions
Vertical PGF – Gravity – Hydrostatic Balance
• The vertical PGF acts to accelerate air upward
• Gravity acts to accelerate air parcels downward
(or toward the Earth’s center of mass)
• These two forces largely balance each other
• This balance is called Hydrostatic Balance
• For large-scale (or synoptic-scale) atmospheric
motions vertical accelerations are negligible,
observed vertical motions are weak (1-5 cm/s),
and thus, hydrostatic balance is a valid assumption
Mesoscale
M. D. Eastin
Forces Driving Synoptic-Scale Air Motions
Horizontal PGF – Coriolis Force – Geostrophic Balance
Horizontal PGF:
• Acts to accelerate air from regions of high
pressure toward regions of low pressure
Flow down the
pressure gradient
Coriolis Force:
• Apparent force (Earth is rotating reference frame)
• Always acts to accelerate air 90° to the right
of the wind vector in the northern hemisphere
• Magnitude is proportional to the wind speed
Geostrophic Balance
• When the Coriolis and horizontal PGF are equal
and opposite → Geostrophic Wind
• Results in air flow parallel to isobars on height
surfaces and height contours on pressure surfaces
• Geostrophic balance is valid assumption for
large-scale atmospheric motions above the
surface (where friction plays a role…)
Mesoscale
M. D. Eastin
Forces Driving Synoptic-Scale Air Motions
Horizontal PGF – Coriolis Force – Friction
Horizontal PGF:
• Acts to accelerate air from regions of high to low pressure
Coriolis Force:
• Always acts to accelerate air 90° to the right (left)
of the wind vector in the northern (southern) hemisphere
Friction
• Always acts to slow air parcels down
as they move over rough terrain
(land, trees, buildings, hills, etc.)
• Only affects air in the lowest 1-2 km
near the surface
• Results in large-scale convergence
(divergence) in association with low (high)
pressure systems near the surface
Mesoscale
M. D. Eastin
Forces Driving Synoptic-Scale Air Motions
Cyclonic Flow
• Counter-clockwise flow (N Hemisphere)
• Occurs in association with low pressure
systems (called “cyclones”)
Anticyclonic Flow
• Clockwise flow (N Hemisphere)
• Occurs in association with high pressure
systems (called “anticyclones”)
Mesoscale
M. D. Eastin
Forces Driving Synoptic-Scale Air Motions
Horizontal PGF – Coriolis Force – Centrifugal – Gradient Balance
Horizontal PGF:
Accelerates air from regions of high to low pressure
Coriolis Force (Co):
Accelerates air 90° to the right of the wind vector
Centrifugal Force (Cf): Results from and applies to curvature in the flow
Accelerates air outward away from the center of rotation
Magnitude is proportional to wind speed
When (Cf + Co) are equal and opposite PGF for flow
around a cyclone → Gradient Wind
The physics involved in gradient balance allow cyclones
to become smaller and more intense than anticyclones (Why?)
V
PGF
Co
Co
PGF
Cf
Cf
V
Mesoscale
M. D. Eastin
Pressure and Temperature
How does Pressure respond to Temperature Changes?
• Increase (decreases) in temperature cause a column of air to expand (contract)
• The pressure surfaces move according to this expansion or contraction
Warming
Cooling
Altitude
Upper-level pressure
surfaces rise
Upper-level pressure
surfaces sink
200mb
Warming results from:
Advection
Latent heat release
Incoming solar radiation
Cooling results from:
Warming
Cooling
Leads to increases in the
1000-500 mb Thickness
Pressures near the
surface decrease
Mesoscale
Leads to decreases in the
1000-500mb Thickness
850mb
L
Advection
Outgoing radiation
H
Pressures near the
surface increase
M. D. Eastin
Jet Streams and Jet Streaks
The Midlatitude Jet Stream:
• 100-400 miles wide
• 500-3000 miles long (non-continuous around the globe)
• Winds speeds range 50-200 knots
• Located between 200-400 mb (altitude of largest N-S pressure gradient)
• Westerly flow in northern hemisphere (Why?)
 Moves/Bends/Buckles from day to day in response to large-scale heating / cooling
 Play important roles in steering and intensifying/weakening surface highs and lows
Jet
Stream
250mb
Warm
Cold
Equator
Mesoscale
North Pole
M. D. Eastin
Jet Streams and Jet Streaks
The Jet Streaks:
• Faster moving pockets of air embedded within the Jet Stream
 Produce regions of upper-level divergence and convergence
 Have a strong influence on the evolution surface highs and lows
 Upper-level divergence will enhance / promote convection and severe weather
Divergence and Convergence
• Speed divergence (convergence) results
in a mass decrease (increase), and hence
a surface pressure decrease (increase)
due to air parcel accelerations
Entrance Region
Speed Divergence
Confluence
Exit Region
Speed Convergence
Diffluence
• Diffluence (Confluence) results in a local
mass and surface pressure decrease
(increase) due to flow in opposite
directions
 A net divergence (convergence) often
Jet Streak
occurs in the exit (entrance) region,
but this varies from jet streak to jet streak
Mesoscale
M. D. Eastin
Fronts
• Fronts separate different large-scale
air masses (Recall: cP, mT, etc.)
• Characterized by large density gradients
whereby the more dense air undercuts
the less dense air
• “Local regions” of considerable convective
overturning (i.e. storms)
• Fronts are always defined by which way
the more dense (cold and/or dry) air
is moving
Placing Fronts on Weather Maps
• Most often defined by strong horizontal
gradients in temperature and/or moisture
• Prominent shifts in the wind direction
and elongated “notches” in the pressure
field can help pinpoint a front’s location
Mesoscale
Different types of fronts on weather maps
•
•
•
•
•
•
•
•
Cold front
Warm front
Stationary front
Occluded front
Surface trough (little or no vertical structure)
Squall line
Dryline
Tropical Wave
M. D. Eastin
Fronts
Placing Fronts on Weather Maps: An Example
Mesoscale
Fronts and
Pressure
Temperature
Dewpoint
Streamlines
M. D. Eastin
Fronts
Warm Fronts
• Slope is gentle and shallow
• Move much slower than cold fronts
• Warm air overruns the more dense
cold air, rising “slowly” and forming
shallow convection:
Low stratiform clouds
Light precipitation
• Precipitation is often well in advance
of the surface frontal boundary
Mesoscale
M. D. Eastin
Fronts
Cold Fronts
• Slope is steeper than warm fronts
• Move much faster than warm fronts
• Warm air is “rapidly” forced over the
cold air, forming deep convection:
Cumulonimbus Clouds
Heavy precipitation
Severe weather
• Precipitation is often just ahead of,
along, or just behind the surface
frontal boundary
Mesoscale
M. D. Eastin
Fronts
Occluded Fronts
• Occurs when the cold front “catches”
the warm front
• The relatively “warm” air (cool air) is
gradually lifted over the cold air
forming shallow convection
Low stratiform clouds
Generally light precipitation
• Precipitation is often along the
surface frontal boundary
Mesoscale
M. D. Eastin
The Norwegian Cyclone Model
 Provides the basic lifecycle of low pressure
systems in the mid-latitudes
Cold Air
Stage 1: The Stationary Front
• Boundary between a cold and war air mass
• Air moving parallel to the front
Warm Air
Stage 2: Upper Level Support
• A region of upper level divergence moves
over the front and forms an area of surface
low pressure
Cold Air
• The circulation around the low starts to bring
cold air south and warm air north
• Movement of the air masses forms the cold
and warm fronts
Mesoscale
Warm Air
M. D. Eastin
The Norwegian Cyclone Model
Stage 2: Upper-Level Support
• Upper-level support is important if the surface
low is to intensify (decrease in pressure)
since friction produces convergence that will
“fill-in” or weaken any low
• Support is provided by an eastward moving
synoptic-scale trough (or Rossby wave) in
the jet stream
• Divergence, warm air advection, and moist
convection often dominate the region east of
the trough axis (all promote lower surface
pressures)
• Surface lows tend to move toward locations
of maximum pressure decrease (which is
often in the area of maximum warm air
advection east of the low)
• Similar support is provided for surface highs
in the region east of an upper-level ridge
Mesoscale
M. D. Eastin
The Norwegian Cyclone Model
Stage 3: Organization
• Enhanced upper-level divergence continues to
strengthen the surface low
• As the low strengthen (pressure drop), the fronts
become better organized
• The stronger winds around a stronger low allows
more cold and warm air advection to occur
Cold Air
Cool Air
Warm Air
Stage 4: Maturity
• The upper-level low begins to catch the surface
low and the divergence wanes
• The surface low reaches its lowest pressure
Cool Air
• The cold front (moving faster) “catches” the warm
front, forming an occluded front
• In the occluded front, the cold air undercuts the
“cooler air” ahead of the warm front, so lifting still
occurs but it is less vigorous (stratiform clouds
and precipitation)
Cold Air
Warm Air
Mesoscale
M. D. Eastin
The Norwegian Cyclone Model
Stage 4: Maturity
• Once the occlusion forms, the low pressure is
separated from the warm air mass and thus the
warm air advection
• The motion of the surface system slows
• The upper-level low “catches” the surface low
and is now located directly above (the system
is now “stacked”)
Cold Air
• What happens next?
Warm Air
Stage 5: Dissipation
• With the upper-level low and its associated
convergence directly above the surface low,
the surface low begins to “fill-in” and
eventually dissipates
• The remains of the warm and cold front will form
a stationary boundary and await the next
upper-level trough….
Mesoscale
Cold Air
Warm Air
M. D. Eastin
Thermodynamic Concepts and Stability
Basic Definitions of Meteorological Quantities:
Vapor Pressure (e):
Partial pressure of water vapor in the air
Mixing Ratio (w):
Mass of vapor to the mass of dry air
Saturation Mixing Ratio (ws): Maximum vapor mass for a given temperature and pressure
Relative Humidity:
100 x (mixing ratio / saturation mixing ratio)
Specific Humidity (q):
Mass of vapor to the mass of moist air
Dewpoint Temperature (Td):
Temperature of a air parcel if cooled to saturation
at constant pressure
Virtual Temperature (Tv):
Temperature a dry air parcel would have if its density
equaled that of a moist air parcel at the same temperature
and pressure
Mesoscale
M. D. Eastin
Thermodynamic Concepts and Stability
Basic Definitions of Meteorological Quantities:
Potential Temperature (θ):
Temperature of air if brought dry-adiabatically to 1000 mb
Wet-bulb Temperature (Tw):
Equilibrium temperature whereby the latent heat loss
(or cooling) induced by evaporation is balanced by the
flow of heat (warming) from the surrounding warmer air
Equivalent Temperature (Te): Temperature an air parcel would have if all of its
latent heat was released
Equivalent Potential Temperature (θe): Temperature of a parcel if all moisture is
condensed out (latent heat release) and the
parcel is brought dry-adiabatically to 1000 mb
Mesoscale
M. D. Eastin
Thermodynamic Concepts and Stability
Adiabatic Processes:
Parcels of air expand and cool, or compress and warm, with
no exchange of heat with the environment
A reversible process
Dry Adiabatic Process:
If the parcel does not saturate, then the warming or cooling
occurs at the dry adiabatic lapse rate, which is constant
in our atmosphere (~0.1ºC/mb or ~10ºC/km in the vertical)
Moist Adiabatic Process:
If the parcel saturates, condensation and latent heat release
occurs, and the parcel cools at the slower moist adiabatic
lapse rate, which varies with temperature and moisture
(but averages ~0.06ºC/mb or ~6.5ºC/km in the vertical)
Pseudo-Adiabatic Process: If an air parcel saturates, condenses water, and the water is
removed from the parcel (i.e., rainfall), the parcel will still
cool at the moist adiabatic lapse rate, but the process is
no longer reversible (loss of heat energy in the liquid water)
Mesoscale
M. D. Eastin
Thermodynamic Concepts and Stability
Five Types of Thermal Stability in the Atmosphere:
1. Absolute Instability:
z
Moist
• Environmental (or observed) lapse rate is greater
than the dry adiabatic lapse rate
• Air parcels will spontaneously accelerate (via dry
convection) to remove the “super-adiabatic” layer
Dry
Env
T
2. Dry Neutral Stability:
• Environmental and dry adiabatic lapse rates are equal
• Often observed in the boundary layer (“mixed” layer)
z
Env
Dry
Moist
T
Mesoscale
M. D. Eastin
Thermodynamic Concepts and Stability
Five Types of Thermal Stability in the Atmosphere:
3. Conditional Instability:
• Environmental (or observed) lapse rate is less than
the dry adiabatic lapse rate but greater than the
moist-adiabatic lapse rate
• Air parcels will spontaneously accelerate (via moist
convection) once the parcel becomes saturated
• Any vertical displacement of an unsaturated parcel
will result in an acceleration back toward its original
location.
z
Moist
Dry
Env
T
• Most common state of the atmosphere
Mesoscale
M. D. Eastin
Thermodynamic Concepts and Stability
Five Types of Thermal Stability in the Atmosphere:
4. Moist Neutral Stability:
z
• Environmental lapse rate is equal to the moist
adiabatic lapse rate
• Often observed in the tropical cyclones or the
center of large thunderstorms
Env
Moist
Dry
T
5. Absolute Stability:
• Environmental lapse rate is less than both the dry and
moist adiabatic lapse rates
• Temperature inversions are an example
• Any vertical displacement of an air parcel will result in
an acceleration back toward its original location
Mesoscale
z
Moist
Dry
Env
T
M. D. Eastin
Skew-T Log-P Diagrams
A convenient way to:
Mesoscale
Examine the vertical structure of the atmosphere
Determine unreported meteorological quantities
Assess air parcel stability
M. D. Eastin
Skew-T Log-P Diagrams
Basic Definitions of Meteorological Quantities:
Lifting Condensation Level (LCL):
Altitude whereby a parcel becomes saturated
by lifting dry-adiabatically
Mixing Condensation level (MCL):
Altitude whereby a parcel with the mean
properties of a layer (a mixed layer) becomes
saturated by lifting dry-adiabatically
Convective Condensation Level (CCL): Altitude whereby a parcel, if heated sufficiently,
will rise dry-adiabatically until it becomes saturated
Convective Temperature (Tc):
Temperature that must be reached for a parcel
to rise to its CCL
Level of Free Convection (LFC):
Altitude whereby a parcel lifted dry-adiabatically to
saturation, and then moist adiabatically, first
becomes warmer than the surrounding air
Equilibrium Level (EL):
Altitude where the temperature of a buoyant air
parcel (above an LFC) becomes equal to the
surrounding air temperature
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Basic Definitions of Meteorological Quantities:
Positive Area (or CAPE):
Area between the environmental sounding and the moist
adiabat of the parcel at altitudes between the LFC and the EL
Proportional to the amount of energy the parcel gains from the
environment.
Large values often lead to deep convection with strong updrafts
Negative Area (or CIN):
Area between the environmental sounding and the moist
adiabat of the parcel at altitudes between the condensation
level (either LCL, MCL, or CCL) and the LFC
Proportional to the amount of energy need to move to parcel
Large values can inhibit deep convection
Wet-Bulb Zero:
Altitude where the wet-bulb temperature first reaches 0ºC
This is the altitude where ice (snow and hail) begins to melt
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
200
Pressure (mb)
Equilibrium
Level
300
400
Positive
Area
500
Level of Free
Convection
600
700
800
900
1000
LCL
Td
T
Temperature (oC)
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
200
Pressure (mb)
Equilibrium
Level
300
400
500
Level of Free
Convection
600
700
800
900
1000
LCL
Negative
Area
Td
T
Temperature (oC)
Mesoscale
M. D. Eastin
Skew-T Log-P Diagrams
Stability Indices:
• Help the forecaster evaluate the likelihood and type of thunderstorms and severe weather
from an individual sounding
• Each index has advantages and disadvantages, but all are relatively easy to compute
Showalter Index (SI)
Lifted Index (LI)
K-Index (KI)
Total Totals (TT)
SWEAT Index
Convective Available Potential Energy (CAPE)
Convective Inhibition (CIN)
Bulk Richardson Number (BRN)
Environmental Helicity
Storm-relative Helicity
Note: Detailed descriptions of how to calculate the first five indices and their resulting
index values associated with thunderstorm potential are in the Skew-T manuals
available on the course website
Mesoscale
M. D. Eastin
Synoptic Meteorology:
The “Stage” for Mesoscale Events
Summary of Concepts and Tools:
•
•
•
•
•
•
•
•
•
Mesoscale
Role of Mid-Latitude Weather Systems
Five Forces Driving Synoptic Scale Air Motions
Relationship between Pressure and Temperature
Jet Streams and Jet Streaks
Fronts (Cold, Warm, Occluded)
Norwegian Cyclone Model
Thermodynamic Concepts
Five Types of Stability
Skew-T Log-P Diagrams
M. D. Eastin