IAP - 12.310 Introduction to Weather Forecasting

Lecture 4:The Polar Front and Extra-tropical Cyclones

• Recap on Front experiment• Polar front• Satellite images• Cyclones develop in the Polar Front• Met Ed video of cyclogenesis• Eddies experiment

Global scale: the Polar Front

Transition from dome of 

polar air to tropical air is 

the Polar Front

Real time data, polar view

Fronts experiment

Real data:

Isosurface oftemperature

(note the dome of cold air over the pole)

Loop

Real data:

Isosurfaces of temperatureand wind

(note the dome of cold air over the pole and the accompanying upper level jet, located in the regions of largest frontal slope)

Yesterday: 140121 12z, 500mb temperature in C

Polar front is “wavy” with cold air intrusions over North America and Asia

Yesterday: Water vapor satellite image on 140121 15z

Click on the image to access a loop (PSU)

Note a sequence of High and Low pressure systems developing in the region of the polar front, the so called “weather systems”

Yesterday: The upper level jet on 140121 12z

The weather systems move from west to east with the jet

Cyclone model

Cyclones develop on the Polar Front

Gfks23

Cyclones facts:

1. develop along the Polar Front,

2. stir the north-south temperature gradient, and are responsible for the poleward transfer of heat,

3. move from west to east with the jet,

4. have a lifetime of ~5 days.

Eddies experiment -to understand how the cyclones form

Two main ingredients:

1. Earth rotation

2. North south temperature difference (warming of the equator and cooling of the pole)

For more on the experiment, see http://paoc.mit.edu/labguide/circ_exp_fast.html

Scale of phenomena and precipitation

Precipitation ForecastingFactors to consider:• Availability of moisture

• Magnitude of vertical velocitysynoptic scale: L= 1000Km w = few cm/sfrontal scale: L= 100km w = 10 cm/sconvective scale: L= 100m-1Km w= 1-10 m/s

• Orographic effectsEnhanced precipitation in upslope regionsDeserts on downwind sides of mountains

In general,when forecasting precipitation,

life is complicated