Mesoscale M. D. Eastin
Synoptic Meteorology:A Review
Mesoscale M. D. Eastin
The Role of Mid-Latitude Synoptic Systems
Earth emits energy to space (or cools) via Outgoing Longwave Radiation
Earth gains energy from the
Sun (or warms) via Incoming
Shortwave RadiationSun
Earth
The Earth must maintain an Energy (or Heat) Balance!
Mesoscale 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 weathersystems 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 ForceGravityCoriolis ForceFrictionCentrifugal 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 MotionsHorizontal PGF – Coriolis Force – Geostrophic Balance
Horizontal PGF:
• Acts to accelerate air from regions of high pressure toward regions of low pressure
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…)
Flow down the pressure gradient
Mesoscale M. D. Eastin
Forces Driving Synoptic-Scale Air MotionsHorizontal 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
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”)
Forces Driving Synoptic-Scale Air Motions
Mesoscale M. D. Eastin
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?)
Forces Driving Synoptic-Scale Air Motions
PGF PGFCo Co
Cf Cf
V
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
200mb
850mb
Altitude
L H
WarmingCooling
Warming
Upper-level pressure surfaces rise
Warming results from:
Advection Latent heat release Incoming solar radiation
Leads to increases in the 1000-500 mb Thickness
Pressures near the surface decrease
Cooling
Upper-level pressure surfaces sink
Cooling results from:
Advection Outgoing radiation Leads to decreases in the 1000-500mb Thickness
Pressures near the surface increase
Mesoscale M. D. Eastin
Jet Streams and Jet StreaksThe 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
Equator North Pole
250mb
Warm
Cold
JetStream
Mesoscale M. D. Eastin
Jet Streams and Jet StreaksThe 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
• Diffluence (Confluence) results in a local mass and surface pressure decrease (increase) due to flow in opposite directions
A net divergence (convergence) often occurs in the exit (entrance) region, but this varies from jet streak to jet streak
Jet Streak
Exit RegionSpeed Convergence
DiffluenceEntrance RegionSpeed Divergence
Confluence
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
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
Mesoscale M. D. Eastin
FrontsPlacing Fronts on Weather Maps: An Example
Fronts andPressure Temperature
Dewpoint Streamlines
Mesoscale 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
Stage 1: The Stationary Front
• Boundary between a cold and war air mass
• Air moving parallel to the front
Stage 2: Upper Level Support
• A region of upper level divergence moves over the front and forms an area of surface low pressure
• 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
Cold Air
Cold Air
Warm Air
Warm Air
Mesoscale 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 ModelStage 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
Stage 4: Maturity
• The upper-level low begins to catch the surface low and the divergence wanes
• The surface low reaches its lowest pressure
• 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
Cold Air
Warm Air
Warm Air
Cool Air
Cool Air
Mesoscale M. D. Eastin
The Norwegian Cyclone ModelStage 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”)
• What happens next?
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….
Cold Air
Warm Air
Cold Air
Warm Air
Mesoscale M. D. Eastin
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
Thermodynamic Concepts and Stability
Mesoscale M. D. Eastin
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
Thermodynamic Concepts and Stability
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 coolingoccurs at the dry adiabatic lapse rate, which is constantin 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 adiabaticlapse 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 isremoved from the parcel (i.e., rainfall), the parcel will stillcool at the moist adiabatic lapse rate, but the process isno longer reversible (loss of heat energy in the liquid water)
Mesoscale M. D. Eastin
Five Types of Thermal Stability in the Atmosphere:
1. Absolute Instability:
• 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
2. Dry Neutral Stability:
• Environmental and dry adiabatic lapse rates are equal• Often observed in the boundary layer (“mixed” layer)
DryMoist
Env
T
z
DryMoistEnv
T
z
Thermodynamic Concepts and Stability
Mesoscale M. D. Eastin
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.
• Most common state of the atmosphere
DryMoist
Env
T
z
Thermodynamic Concepts and Stability
Mesoscale M. D. Eastin
Five Types of Thermal Stability in the Atmosphere:
4. Moist Neutral Stability:
• Environmental lapse rate is equal to the moist adiabatic lapse rate • Often observed in the tropical cyclones or the center of large thunderstorms
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
DryMoist
Env
T
z
DryMoist
Env
T
z
Thermodynamic Concepts and Stability
Mesoscale M. D. Eastin
Skew-T Log-P Diagrams
A convenient way to: Examine the vertical structure of the atmosphereDetermine unreported meteorological quantitiesAssess air parcel stability
Mesoscale M. D. Eastin
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
Skew-T Log-P Diagrams
Mesoscale M. D. Eastin
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 LFCProportional to the amount of energy need to move to parcelLarge values can inhibit deep convection
Wet-Bulb Zero: Altitude where the wet-bulb temperature first reaches 0ºCThis is the altitude where ice (snow and hail) begins to melt
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
Mesoscale M. D. Eastin
Skew-T Log-P DiagramsP
ress
ure
(m
b)
Pre
ssu
re (
mb
)
10001000900900800800700700
600600
500500
300300
200200
400400
Temperature (Temperature (ooC)C)
30304040
2020
101000
-10-10
-20-20-30-30
-40-40
-50-50
-60-60
LCLLCL
Level of FreeLevel of FreeConvectionConvection
EquilibriumEquilibriumLevelLevel
PositivePositiveAreaArea
TTdd TT
Mesoscale M. D. Eastin
Skew-T Log-P DiagramsP
ress
ure
(m
b)
Pre
ssu
re (
mb
)
10001000900900800800700700
600600
500500
300300
200200
400400
Temperature (Temperature (ooC)C)
30304040
2020
101000
-10-10
-20-20-30-30
-40-40
-50-50
-60-60
LCLLCL
Level of FreeLevel of FreeConvectionConvection
EquilibriumEquilibriumLevelLevel
Negative Negative AreaArea
TTdd TT
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 HelicityStorm-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
Summary of Concepts and Tools:
• 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
Synoptic Meteorology:The “Stage” for Mesoscale Events
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