NATS 101 Lecture 17 Fronts & cyclogenesis

Review

	Air Masses
	Large regions with ÒuniformÓ temperature and moisture distributions and distinctive weather
	Classified by Source Region
	Continental (c) or Maritime (m)
	Polar (P) or Tropical (T)
	Source Regions
	Big in area (>1600 km by 1600 km)
	Dominated by light winds (long resident times)

Air Mass Characteristics

Air Mass Source Regions

Weather Map with Air Masses, Fronts, Extratropical Cyclone

Extratropical Cyclones and Fronts

	In mid-latitudes, significant weather is often associated with a particular type of storm: Extratropical Cyclone
	Cyclone denotes the circulation around a low pressure center
	The energy for extratropical cyclones comes from horizontal temperature contrasts

Extratropical Cyclones and Fronts

	ET cyclones often form on a boundary between a warm and cold air mass, associated with the jet stream
	They tend to focus temperature contrasts along frontal zones, bands of very rapid horizontal temperature changes

Extratropical Cyclones and Fronts

	Strongest temperature gradients occur at warm edge of frontal zone, called a front
	There are four types of fronts Classified by their movement Each has its own symbol, color scheme
	Cold, Warm, Stationary, Occluded

Slide 9

Frontal Types

Frontal Types

Frontal Types

Frontal Types

Slide 14

Cross-Section: Cold Front

Typical Cold Front Weather

Slide 17

Cross-Section: Warm Front

Typical Warm Front Weather

Occluded Fronts Warm portion lifts off the ground

Typical Occluded Front Weather

Summary Fronts

	ET cyclones tend to focus temperature contrasts along frontal zones
	Strongest temperature gradients occur at warm edge of frontal zone, called a front
	Fronts classified by movement, each has own symbol and characteristic weather
	Cold, Warm, Stationary, Occluded

Summary: Frontal Weather

Summary: Frontal Weather

Cyclone Genesis & Evolution

Cyclone Family

Decaying Situation

Where Winds are Divergent

What Increases Divergence?

What Increases Divergence?

What Increases Divergence?

Longwaves and Shortwaves

Longwaves and Shortwaves

Vertical Structure

	Cyclone deepens only if divergence in column exceeds convergence
	System tilts toward the west with height
	Tilt would align upper-level (UL) divergence over the surface low
	Results in deepening of the surface low

Storm of Century

Storm of Century

Storm of Century (not shown)

Storm of Century

Track of Storm of Century

Cyclone-Anticyclone Tracks

Summary: Extratropical Cyclones

	In mid-latitudes, much of our weather is associated with the Extratropical Cyclone
	Cyclone denotes the circulation around a low pressure center
	Their circulation affects an area of 1000 km (or more) across, through entire troposphere

Summary: Extratropical Cyclones

	ET cyclones derive their energy from horizontal temperature contrasts.
	Not surprisingly, they generally form on a boundary between a warm and cold air mass, near the jet stream axis
	They transport warm air poleward and upward, and cold air equatorward and downward.

Summary: Extratropical Cyclones

	They tend to follow a similar lifecycle
	They form underneath areas of UL divergence, downwind of SW troughs
	The low pressure center tends to move with speed and direction of 500 mb flow
	They enhance temperature contrasts into frontal zones, which act as a breeding ground for future ET cyclones

Assignment

	Topic – Forecasting Part I
	Reading - Ahrens pg 231-248
	Problems - 9.3, 9.4, 9.5, 9.6