synoptic meteorology: a review

Mesoscale M. D. Eastin

Synoptic Meteorology:A Review


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!



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



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


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



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


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


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


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”)



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?)


PGF PGFCo Co

Cf Cf

V

V


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


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


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


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


FrontsPlacing Fronts on Weather Maps: An Example

Fronts andPressure Temperature

Dewpoint Streamlines


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


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


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


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


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


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


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


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



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




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)


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




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




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



Skew-T Log-P Diagrams

A convenient way to: Examine the vertical structure of the atmosphereDetermine unreported meteorological quantitiesAssess air parcel stability



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




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 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


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



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


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

synoptic meteorology: a review

Documents

density air pressure

pressure gradientm

unit area air pressure

given location air pressure

regions of high pressure

air parcels

regions of lower pressure

heat balance