ATS/ESS 452: Synoptic Meteorology

- Cyclone Structure

Fig. 6.1 (Meridional Cross Sections of Temperature and Zonal Winds)

• Meridional temperature gradient characteristics

- Compare winter vs. summer in NH

- Temperature gradient

• Larger in NH winter, less in NH summer

• Not as much seasonal change in SH• SH has more water,

heats up faster/cools off slower… moderates temperature

• Can see migration of ITCZ (Thermal Equator)

• Sfc temps are warmer in NH summer than SH summer

Observed Structure of Extratropical Systems

DJF

JJA

Fig. 6.1 (Meridional Cross Sections of Temperature and Zonal Winds)

• Zonal wind characteristics

- Jet stream much stronger in NH winter than in NH summer

- Smaller seasonal difference in SH winter vs. summer

- Jet core is located just below the tropopause

- Jet core is found at the latitude where the thermal gradient (as averaged through the troposphere) is greatest:

• 30-35 degrees N during winter

• 40-45 degrees N during summer

• Deduce that the jet stream has a lot to do with frontal boundaries!

Observed Structure of Extratropical Systems

DJF

JJA

Fig. 6.2 (Longitudinal Distribution of Average Zonal 200 mb Winds for NH Winter)• Characteristics

• Large differences in zonal 200-mb wind speed with longtiude; strongest winds found near 30° N latitude

• Two NH jet maxima in mid latitudes

East Asian Jet and Eastern North America Jet Synoptic disturbances frequently develop near these maxima Semi-permanent low surface pressure is observed near the left exit region of each jet maximum (Aleutian

and Icelandic Lows) Semi-permanent high pressure is observed near the left entrance region of each jet maximum (Siberian

and NW Canada Highs) Develop 4-quadrant jet theory for straight flow… correlate with semi-permanent surface pressure features.

• Two NH jet minima in mid-latitudes

L LH HL

Fig. 6.2 (Longitudinal Distribution of Average Zonal 200 mb Winds for NH Winter)• And remember jet streams are exhibitive of strong thermal

gradients… notice where both average jet maxes are located• Along eastern coasts of continents were strong warm ocean

currents are riding along continental boundaries• Strong thermal gradients in those locations!

Fig. 6.3 (Mean 500-mb Height Contours for January in the NH)• Characteristics

• Large asymmetries in this pattern are observed from one longitude to another Mean troughs are located just

east of the North American and Asian Continents. These mean troughs correspond to jet maxima in Fig. 6.2

Mean ridges are found off the west coasts of North America and Asia. These ridges correspond to wind minimums in Fig. 6.2.

• Relating the trough and ridge positions to surface weather patterns, we can see that Low sfc pressure is located east

(downstream) of 500-mb troughs

High sfc pressure is located east (downstream) of 500-mb ridges

How might this 500-mb wave pattern be linked to the distribution and influence of ocean and continent positions in the mid-latitudes?

• During winter, the atmosphere cools as it flows from west to east across cold continents, leading to lower 500-mb heights on the east coasts

• However, the atmosphere warms as it flows west to east across warmer oceans, leading to higher 500-mb heights over eastern ocean basins

• Winds are stronger near troughs because the height gradient between tropics and poles is very strong

Fig. 6.4 (Vertical Cross Sections from Charleston SC to Omaha NE across a Jet Stream Wind Maximum)

a) Wind and Temperatures (top panel)• Characteristics:

Strong jet core around 300-mb (Polar Jet) Tropopause Break Jet core will cause a break or fold in the tropopause Warm at Charleston tropopause is high

o (warm temperature = large thickness) Cool at Omaha tropopuase is low

o (low temperature = low thickness) Jet sits between warm/cold areas; also notice how the frontal surface slopes to the west and is

connected to the jet Jet stream is over maximum thermal gradient

Fig. 6.4 (Vertical Cross Sections from Charleston SC to Omaha NE across a Jet Stream Wind Maximum)

b) Wind and Potential Temperature (bottom panel)• Characteristics:

Potential Temp increases with height This gradient is less in warm air; greater in cool air

is very large in the stratosphere; less in troposphere

Unsaturated parcel will follow the isentropes up/down; so you know vertical motion of parcel if you know what the winds are doing Rising air east of front; Subsidence (sinking air) behind front

Stability and Isentropic Spacing

Static Stability and Potential Temp

Very Stable Conditions

• When there is a tight vertical packing of isentropes (θ quickly increasing with height), then there exists a very stable atmosphere

• This tight vertical gradient of θ indicates an inversion

• In Fig. 6.4b, the greatest stability is indicated at:– The tropopause and upward into the stratosphere– On the cold side of the polar front

Conditional Stability

• When there is a weak vertical gradient of isentropes (θ slowly increases with height), then there exists a conditionally stable atmosphere– A dry parcel in this environment is stable– A moist parcel in this environment is unstable

• Conditional stability is observed:– In the warm sector ahead of the polar front, particularly

• Between the surface and 850-mb• In the upper troposphere between 500-mb and the

tropopause

– In a shallow layer below 850-mb behind the polar front

• On the synoptic scale, the atmosphere is always stably stratified (because convection would immediately stabilize any unstable regions)

• So, (dθ/dz) > 0

Fig. 6.5 (Growth of a Mid-Latitude Baroclinic Cyclone)

• Baroclinic vs. Barotropic Atmosphereso Barotropic Atmosphere

No intersection of isoheights and isotherms are allowed This means no temperature advection (isotherms parallel

isoheights) No thermal wind; no vertical wind shear Tropics are very near barotropic

o Baroclinic Atmosphere Allows vertical wind shear Has thermal wind Allows temperature advection Main region of baroclinic instability is in mid-latitudes

• Large horizontal temperature gradients!

Fig. 6.5 (Growth of a Mid-Latitude Baroclinic Cyclone)

• Frontal zones exhibiting strong thermal gradients and vertical wind shear (i.e. thermal wind) are called “baroclinic zones”

• This thermal wind can reach excessive values, and “baroclinic instability” may result. This is normally associated with a jet streak (i.e. wind maximum)

• In a baroclinically-unstable environment, a small disturbance with a developing baroclinic zone (i.e. thermal gradient) will amplify and develop by drawing energy from the jet stream flow. Thus, you have a down-scale cascade of energy from the mean flow to the eddies

Cyclone = eddy; mean flow = jet stream The jet stream controls surface cyclones/anti-cyclones

• Most mid-latitude synoptic-scale systems are the result of baroclinic instability (see figure of hyper-baroclinic zone).

• Figure 6.5 shows 500-mb heights (heavy solid lines), 1000-mb heights (thin lines) and 1000-500-mb thickness (dashed lines) Frontal locations are also indicated

• What is happening in this figure (6.5)?

Temperature advection is represented by thickness advection

Open 500-mb shortwave trough amplifies, then begins to close off (aloft)

Surface frontal wave cyclone amplifies and becomes more vertically stacked

Surface cyclone becomes occluded

• The fact that the developing surface low and upper-level trough are off-set from one another guarantees strong thermal advection

Heavy solid lines = 500-mb contours

Thin lines = 1000-mb contours

Dashed lines = thickness

More detail…

• Panel a) The developing mid-latitude cyclone is downstream of the upper-level trough Sfc isobars don’t quite form advection boxes with the thickness contours

• Panel b) Advection boxes are now formed (stronger temp advection) Surface low deepens Surface cyclone moves closer to upper-level trough

• Panel c) Occulusion occurs significant temperature advection Upper level low forms and cyclone is becoming vertically stacked (i.e. sfc low directly underneath upper-level

low) Lose westward tilt with height eventually lose strong thermal advection

Heavy solid lines = 500-mb contours

Thin lines = 1000-mb contours

Dashed lines = thickness

• Boundary b/w cold, dry air and warm, moist air

• Thermal gradient is already an area of natural lower pressures, causing air to blow towards the boundary

• A counter-clockwise circulation may develop, which will act to take warm air up from the south and cold air down from the north– This is called cyclogenesis

• Mass convergence in the center of developing circulation, causing the air to want to rise

• IF the upper-levels are favorable, then we should have a region of divergence aloft, above the developing low pressure center.– This will help pull the air that is converging

at the surface upward and continue to develop the low pressure area

– If the upper-levels are unfavorable, then the cyclone will not grow and the mass convergence at the surface will just pile up and fill in the low, causing it to decay

• If the upper levels are favorable, the mid-latitude cyclone will continue to develop, via increasing temperature advection– This will act to strengthen the temperature

gradient, and thus strengthening the upper-levels (i.e., divergence), and deepening the low

• As the cyclone reaches maturity, the central pressure will be at its lowest and occlusion will begin– Cold front catches up to

warm front• Once the system is fully

occluded (warm air is above cold air), temperature advection weakens, and mass convergence acts to fill in the low and the system decays

• The west-ward tilt with height observed in developing baroclinic systems means that:

The surface trough is overlain by an upper-level ridge (between 200 and 300 mb)

The surface ridge is overlain by an upper-level trough (between 200 and 300 mb)

• See Handout

Fig. 6.6 (West-to-East Cross Section through a Developing Baroclinic Wave)

• This figure shows the westward vertical tilt of pressure troughs and ridges with height

This westward tilt of troughs and ridges is necessary in order for the mean jet stream flow to give up potential and kinetic energy to the developing baroclinic system

**Westward tilt allows jet stream to give up energy to eddies at the sfc**

Healthy Westward Tilt of Pressure Systems with Height

Z

W E

H L

H L

• Note that the thermal ridges and troughs depicted in Fig. 6.6 tilt eastward with height and are out of phase with the pressure ridges and troughs. This means that:

Baroclinic ridges (highs) are cold at the surface and warm aloft

Baroclinic troughs (lows) are warm at the surface and cold aloft

Healthy Westward Tilt of Pressure Systems with Height

Vertically Stacked Systems

Z

W E

H L

H L

• As cyclones mature and become occluded, they become vertically stacked with their upper-level system

Vertically Stacked Systems

• As a result, the pressure trough and temperature trough lose their vertical tilt. They also become aligned. (This is common as cyclones move poleward toward 60N and 60S latitudes)

• The same happens with the pressure ridge and temperature ridge as an aging polar high moves equatorward and merges with the subtropical high near 30N and 30S

i.e. systems began to lose their baroclinic nature

• As a consequence, the thermal advection with these systems becomes quite weak (or non-existent). Thus, energy conversion from the jet stream to these surface systems becomes negligible

No thermal contrasts, then the jet stream retreats

Vertically Stacked Systems

• These vertically-stacked systems become more barotropic in nature (equivalent barotropic) with the wind direction being uniform with height (no wind shear)

Westward tilt of pressure systems with height

Vertically stacked

Unhealthy Eastward Tilt

Z

W E

H L

H L

• If the pressure trough displays an eastward tilt with height, there is an up-scale energy cascade whereby the eddies must give their energy back to the mean flow

• Thus, individual cyclones and anticyclones will weaken or dissipate as they lose energy to the larger jet-stream westerlies

• The surface cyclone or anticyclone must give up its kinetic and potential energy to the jet stream

• The surface system rapidly dissipates• This is characterized on weather maps by the

upper trough (ridge) tending to “outrun” the surface low (high)

• This is common with landfalling cyclones on the West Coast

• A simplistic diagram of baroclinic troughs and ridges is displayed in your handout

– The surface trough (i.e. convergence) is overlain by the upper-tropospheric ridge (i.e. divergence)

– Thus, mass continuity requires that upward vertical motion occur in the vicinity of baroclinic surface lows

– The surface ridge (i.e. divergence) is overlain by the upper-tropospheric trough (i.e. convergence)

– Thus, mass continuity requires that downward vertical motion occur in the vicinity of baroclinic surface highs