Advanced SynopticM. D. Eastin Surface Pressure Systems Will these Surface Lows Intensify or Weaken?...

Advanced Synoptic M. D. Eastin

Surface Pressure Systems

Will these Surface Lows Intensify or Weaken?

Where will they Move?



Formation

• Climatology• Use of QG Theory• Effects of Vorticity Advection• Effects of Temperature Advection• Effects of Diabatic Heating• Effects of Topography• Common Favorable and Unfavorable Combinations of Effects• An Example Case

Movement

• Climatology• QG Forcing• Aspects of Topography• An Example Case


Where do Surface Cyclones Occur?

Genesis

More frequent in the winter than the summer Occurs further south during the winter

In the lee of the northern Rockies from Alberta to Montana. In the lee of the southern Rockies from Colorado to New Mexico. These two are related to flow over

topography

Off the east coast from the mid-Atlantic states to New England. Off the Texas coast in the Gulf of Mexico These two are related to cold air flowing over relatively warm waters (diabatic)

Formation Climatology

From Zishka and Smith (1980)


Where do Surface Cyclones Occur?

Decay (Lysis)

Along the Pacific northwest coast Related to flow over topography

Over New England and eastern Canada Related to warm air flowing over a relatively cold land surface (diabatic) • Many do not decay until well off the east coast over the north-central Atlantic when they become occluded and are cut-off their source of warm moist (tropical) air




Where do Surface Anticyclones Occur?

Genesis

Again, more frequent and further south in the winter than in the summer

In the Southern plains from Texas to Kansas and North Dakota In the Northern Rockies Related to strong cold air advection to the west of developing Colorado lows and Alberta Clippers (that are moving east)




Where do Surface Anticyclones Occur?

Decay (Lysis)

Over the central Rockies Over the central and southern Appalachians Related to flow over topography




Hoskins and Hodges (2002)

Attributes for tracking of positive vorticity of 850 hPa level

Upper panel: Probability Density Function (PDE) of cyclogeneis

Lower panel: PDE of cyclolysis


Goal:

We want to use QG theory to diagnose, understand, and predict the formation and evolution (genesis and lysis) of surface cyclones and surface anticyclones

What aspects of QG Theory can be Applied?

• We can not apply the QG height-tendency equation

• Lower boundary condition assumes no height tendency at the surface• Contrary to what we are trying to infer…

We can use the QG omega equation evaluated above the surface Then we can use the vorticity equation locally evaluated at the surface

Use of QG Theory


Use of QG Theory

Tp

Rf

p

f

p

fppgpp

gg vv 202

2202

Use of the QG Omega Equation above the Surface:

Term A Term B Term C

• Evaluate both Terms B and C using the three-dimensional fields of height, vorticity, and temperature on upper-air pressure surfaces

• Recall:

• Rising Motion → An increase in PVA with height → WAA (Warm Air Advection)

• Sinking Motion → An increase in NVA with height → CAA (Cold Air Advection)

Look for regions: Rising Motion → Surface Cyclone formation / intensifySinking Motion → Surface Anticyclone formation / intensify How?


How?... Local application of the QG Vorticity Equation at the Surface:

• If rising motion (ω < 0) is present above the surface (where ω = 0), then we know:

• We can then infer from the QG vorticity equation that:

Recall (at the surface):

• Using the relationship between vorticity tendency and height tendency we thus know:

Recall: and

• Finally, using the relation between the height tendency and the pressure tendency:

Since: (see your text)

Therefore: Rising motions aloft → Surface pressure decreasesSinking Motions aloft → Surface pressure increases

Use of QG Theory

pf

tg

0

0p

Note: From the continuity equation, this relationship is equivalent to convergence at the surface

0

tg

0t

2

0

1p

g

ft t

0t

pzp t

p

t

1


For a Typical Synoptic Wave:

• Areas of PVA are often located east of the trough axis• PVA often increases with height in this region (especially below 500mb)

• An increase in PVA with height results in rising motion aloft, which produces a surface pressure decrease, and the formation of a surface cyclone (or low)

• Areas of NVA are often located east of a ridge axis• NVA often increases with height in this region (especially below 500mb)

• An increase in NVA with height results in sinking motion aloft, which produces a surface pressure increase, and the formation of a surface anticyclone (or high)

Effects of Vorticity Advection

From Bluestein (1993)

L H

PVA NVA

Rising Sinking

Convergence Divergence

Trough

Ridge

500 mb


Effects of Vorticity Advection: Mechanism

PVA

STEP ONE1- Cyclonic vorticity advection2- Divergence = 0

From QG vorticity equation:

Cyclonic vertical vorticity advection Decrease in THICKNESS



STEP TWOIn the absence of temperature advection in the thermodynamic energy equation:

Ascending air is the only way to decrease the temperature of the layer:



The QG vorticity equation results: Secondary Circulation

In the level of a: - +In the level of b: + + Secondary circulation returns the

atmosphere to QG equilibrium


For a Typical Synoptic Wave:

• Areas of strong WAA aloft are often below 500mb east of trough axes and north of closed lows

• WAA results in rising motion aloft, which produces a surface pressure decrease, and the formation of a surface cyclone (or low)

• Areas of strong CAA aloft are often below 500mb west of trough axes and south of closed lows

• CAA results in to sinking motion aloft, which produces a surface pressure increase, and the formation of a surface anticyclone (or high)

Effects of Temperature Advection

From Bluestein (1993)

L

H

RisingWAA

CAASinking


What effect does diabatic heating or cooling have?

Diabatic Heating: Latent heat release due to condensation (Ex: Cumulus convection)

Strong surfaces fluxes (e.g. Cold air advection over the Gulf Stream) (e.g. Intense solar heating in the desert)

• Diabatic heating always leads to temperature increases → thickness increases• Consider the three-layer model with a deep cumulus cloud

• The maintenance of geostrophic balance requires rising motion through the layer• Identical to the physical processes induced by WAA

• Therefore, diabatic heating aloft induces rising motion → surface cyclone formation

Effects of Diabatic Heating

ΔZ increasesΔZ

SurfaceRose

SurfaceFell

Z-400mb

Z-700mb

Z-bottom

Z-top


What effect does diabatic heating or cooling have?

Diabatic Cooling: Evaporation (e.g. Precipitation falling through sub-saturated air)

Radiation (e.g. Large temperature decreases on clear nights)

Strong surface fluxes (e.g. Warm air flowing over a cold surface)

• Diabatic cooling always leads to temperature decreases → thickness decreases• Consider the three-layer model with evaporational cooling aloft

• The maintenance of geostrophic balance requires sinking motion through the layer• Identical to the physical processes induced by CAA

• Therefore, diabatic cooling aloft induces sinking motion → surface anticyclone formation

Effects of Diabatic Heating

ΔZ decreasesΔZSurfaceRose

SurfaceFell Z-400mb

Z-700mb

Z-bottom

Z-top


What effect does flow over topography have?

Downslope Motions: Flow away from the Rockies Mountains

Flow away from the Appalachian Mountains

• Subsiding air always adiabatically warms• Subsidence leads to temperature increases → thickness increases• Consider the three-layer model with downslope motion at mid-levels

• The maintenance of geostrophic balance requires rising motion through the layer• Identical to the physical processes induced by WAA and diabatic heating

• Therefore, downslope flow induces rising motion → surface cyclone formation

Recall: Lee-side cyclogenesis locations that were often observed

Effects of Topography

ΔZ increasesΔZ

SurfaceRose

SurfaceFell

Z-400mb

Z-700mb

Z-bottom

Z-top


What effect does flow over topography have?

Upslope Motions: Flow toward the Rockies Mountains

Flow toward the Appalachian Mountains

• Rising air always adiabatically cools• Ascent leads to temperature decreases → thickness decreases• Consider the three-layer model with upslope motion at mid-levels

• The maintenance of geostrophic balance requires sinking motion through the layer• Identical to the physical processes induced by CAA and diabatic cooling

• Therefore, upslope flow induces sinking motion → surface anticyclone formation

Effects of Topography

ΔZ decreasesΔZSurfaceRose

SurfaceFell Z-400mb

Z-700mb

Z-bottom

Z-top


Use of QG Theory

Tp

Rf

p

f

p

fppgpp

gg vv 202

2202

The Modified QG Omega Equation for Surface System Evolution:

+ Diabatic + Topographic Forcing Forcing

VerticalMotion

TemperatureAdvection

Change in VorticityAdvection with Height


Combined Effects of Forcing You must consider the combined effects from each forcing type in order to infer the expected total surface pressure change

• Sometimes one forcing will “precondition” or “ripen” the atmosphere for another so as to enhance surface cyclogenesis or anticyclogenesis

• Other times forcing types will oppose each other, thereby inhibiting surface cyclogenesis or anticyclogenesis

Common Favorable Combinations

• Vorticity advection with temperature advection• Temperature advection with diabatic heating• Vorticity advection with temperature advection and diabatic heating• Downslope motions and vorticity advection

Common Unfavorable Combinations

• Vorticity advection with temperature advection• Temperature advection with diabatic heating

Note: Nature continuously provides us with a wide spectrum of favorable and unfavorable combinations


Favorable Combinations of ForcingVorticity Advection with Temperature Advection:

Scenario: A region of increasing PVA with height (located downstream from a trough) is collocated with a region of strong warm air advection

PVA

Max

Vort

WAA

Upper Levels

Lower Levels


Favorable Combinations of ForcingTemperature Advection with Diabatic Heating:

Scenario: A region of strong warm advection collocated with deep convection Commonly observed near warm fronts and in the warm sector

WAA


Favorable Combinations of ForcingVorticity Advection with Temperature Advection and Diabatic Heating:

Scenario: A region of increasing PVA with height (located downstream from a trough) is collocated with a region of warm air advection and deep convection

Max

Vort

WAA

Upper Levels

Lower Levels

PVA


Favorable Combinations of ForcingVorticity Advection with Downslope Motions:

Scenario: A region of increasing PVA with height (located downstream from a trough) is located over the leeside of a mountain range

PVA

Max

Vort

Downslope Motions

Upper Levels

Lower Levels


Unfavorable Combinations of ForcingVorticity Advection with Temperature Advection:

Scenario: A region of increasing PVA with height (located downstream from a trough) is collocated with a region of strong cold air advection

PVA

Max

Vort

CAA

Upper Levels

Lower Levels


Unfavorable Combinations of ForcingTemperature Advection with Diabatic Heating:

Scenario: Strong cold air advection over a warm surface Commonly observed during the winter months off the east coast as

cold continental air flows over the warm Gulf of Mexico

Warm OceanH

CAA

If the cold air advection effects are greater than the diabatic heating effects, the net result will be a surface pressure increase and anticyclone formation

Surface Fluxes(Diabatic Heating)

Note: The complete opposite scenario could happen, whereby the CAA is weaker than the surface fluxes

This will produce a surface cyclone

Often occurs off the east coast:

“Bomb” Cyclones Noreasters


Example Case: Formation

Will these Surface Lows Intensify or Weaken?


Vorticity Advection:

L

LL




L

L

PVA

Assume NVA below

Expect PressureDecreases

NVA

Assume PVA below

Expect PressureIncreases

L



Temperature Advection:

L

L

L




L

L

L

WAA

Expect PressureDecreases CAA




Diabatic Heating:

L

L

L



Diabatic Heating:

L

L

LDiabatic Cooling


Diabatic Heating


Note the snowand cloud cover

Note: Time is 12Z or 5:00-7:00 am (before sunrise)

Note the clear skies



Flow over Topography:

L

L

L

Note direction of surface winds from the previous slide



Flow over Topography:

L

L

LDownslope Flow


Note direction of surface winds from the previous slide



Moderate NVA RWeak CAA RDiabatic Cooling RDownslope Flow F-----------------------------------------------------------

Net Pressure Decrease R-----------------------------------------------------------

15Z: Pressure rose 2 mb

Moderate NVA RWeak WAA FDiabatic Cooling RDownslope Flow F-----------------------------------------------------------

Net Pressure Increase R-----------------------------------------------------------

15Z: Pressure rose 3 mb

Weak PVA FModerate CAA RDiabatic Heating FDownslope Flow F-----------------------------------------------------------

Net Pressure Decrease F------------------------------------------------------------

15Z: Pressure fell 1 mb



Surface Pressure System Motion

Will this SurfaceLow Move?


Where do Surface Cyclones Move?

• Cyclones move from their genesis regions to their decay (or lysis) regions

Most surface cyclones move to the northeast

Related to motion toward maximum surface pressure decreases

• WAA maximum is often to the northeast• An upper-level PVA maximum is often

to the northeast• The warm front and its associated

convection (or diabatic heating) is often to the northeast

Motion Climatology



Where do Surface Anticyclones Move?

• Anticyclones move from their genesis regions to their decay (or lysis) regions

Most surface anticyclones move to the southeast

Related to motion toward maximum surface pressure increases

• CAA maximum is often to the southeast• An upper-level NVA maximum is often

to the southeast

Motion Climatology



Goal:

• We want to use QG theory to diagnose the motion of surface pressure systems

General Motion Characteristics

• There is no mean flow at the surface (a boundary condition) to advect the systems Surface cyclones (anticyclones) always move away from regions of pressure increases (decreases) toward regions of pressure decreases (increases)

Application of QG Theory and the QG Omega Equation:

• Surface pressure increases (decreases) result from sinking (rising) motion

Cyclone Regions of sinking motion → Regions or rising motion Motion Regions of NVA aloft → Regions of PVA aloft (From → To) Regions of CAA → Regions of WAA

Regions of diabatic cooling → Regions of diabatic heatingRegions of upslope flow → Regions of downslope flow

Anticyclone Regions of rising motion → Regions of sinking motion Motion Regions of PVA aloft → Regions of NVA aloft (From → To) Regions of WAA → Regions of CAA

Regions of diabatic heating → Regions of diabatic coolingRegions of downslope flow → Regions of upslope flow

QG Forcing


Topography can have significant effects on system motion:

• Consider a cyclone (low pressure system) east of a mountain range:

• Motion will be to the south along the range

• Consider an anticyclone east of a mountain range

• Motion will be to the south along the range

Aspects of Topography

L

Upslope Flow → Pressure Increase

Downslope Flow → Pressure Decrease

HUpslope Flow → Pressure Increase



Topography and Temperature Advection:

• Consider a low pressure system initially just east of a mountain range:

• Motion will be to the southeast

• Consider the low at a later time southeast of the mountain range

• Motion will now be to the east-southeast

As the low moves further away from the mountain range, it begins to feel less topographic

effects and more temperature advection effects → acquires a more northeastward motion

Aspects of Topography

L

Upslope Flow → Pressure Increase


WAA → Pressure DecreaseT

T-ΔTT-2ΔT

L

Weaker Upslope Flow → Pressure Increase

Weaker Downslope Flow → Pressure Decrease

WAA → Pressure DecreaseT

T-ΔTT-2ΔT


Example Case: Motion

Where will this Surface

Low Move?



L


Maximum PVA

Assume NVA below

Expect motion toward the south



L

Maximum WAA

Expect motion toward the southeast



Diabatic Heating:

L

Maximum Heating

Expect motion toward the northwest



Flow over Orography:

L

Maximum Downslope Flow

Expect motion toward the southwest



Motion Summary

LL

WAAPVA

Diabatic

Downslope

ExpectedMotion

Initial Location

Later Location




Summary:

Formation (Cyclones and Anticyclones)

• Climatology (primary genesis, occurrence, lysis locations)• Application of QG Omega Equation

• Effects of Temperature and Vorticity Advection• Effects of Diabatic Heating• Effects of Topography

• Common Favorable and Unfavorable Combinations of Effects

Movement (Cyclones and Anticyclones)

• Climatology• Application of QG Omega Equation

• Effects of Temperature and Vorticity Advection• Effects of Topography


ReferencesBluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume I: Principles of Kinematics and Dynamics.

Oxford University Press, New York, 431 pp.

Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of WeatherSystems. Oxford University Press, New York, 594 pp.

Boyle, J. S., and L. F. Bosart: 1983: A cyclone / anticyclone couplet over North America: An example of anticyloneevolution. Mon. Wea. Rev., 111, 1025-1045.

Boyle, J. S., and L. F. Bosart: 1986: Cyclone / anticyclone couplets over North America. Part II: Analysis of majorcyclone event over the eastern United States. Mon. Wea. Rev., 114, 2432-2465.

Charney, J. G., and A. Eliassen, 1964: On the growth of the hurricane depression. J. Atmos. Sci., 21, 68-75.

Pearce, R. P., 1974: The design and interpretation of diagnostic studies of synoptic scale atmospheric systems.Quart. J. Roy. Meteor. Soc., 100, 265--285.

Petterssen, S., and S. J. Smebye, 1971: On the development of extratropical storms. Quart. J. Roy. Meteor. Soc.,97, 457--482.

Tracton, M. S., 1973: The role of cumulus convection in the development of extratropical cyclones. Mon. Wea. Rev.,114, 2432-2465

Zishka, K. M., and P. J. Smith, 1980: the climatology of cyclones and anticyclones over North America and surroundingoceans environs for January and July, 1950-1977. Mon. Wea. Rev., 108, 387-401.

Advanced SynopticM. D. Eastin Surface Pressure Systems Will these Surface Lows Intensify or Weaken?...

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