A Dynamic Planet: Weather and Weather Forecasting (Lecture 4)

Describe the structure and evolution of extratropical cyclones

Structure:

• Giant spinning storms.

• Centre is a low-pressure area where air rises. Around it, there are swirling winds.

• Warm air from the south and cold air from the north meet in these storms, creating fronts.

Evolution:

• Start as small disturbances in the atmosphere.

• As warm and cold air collide, they grow larger and stronger. Over time, the warm front catches up with the cold front, and they merge forming an occluded front.

• As the storm matures, it weakens and eventually breaks apart

• Anticlockwise circulation in Northern Hemisphere

• Clockwise circulation in Southern Hemisphere

Why does jet stream exist?

• Temperature Contrast - sharp temperature contrasts between cold polar air and warmer air from the mid-latitudes or tropics. These temperature differences create a pressure gradient, which is a force that drives air from high pressure to low pressure areas.

• Coriolis Effect: As air moves from high to low pressure, the Coriolis effect—caused by the Earth's rotation—causes the air to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection leads to the creation of strong, fast-moving winds in a relatively narrow band (jet stream)

How does jet stream control weather in the midlatitudes?

• Steers the movement of weather systems - extratropical cyclones tend to follow the path of the jet stream

• Defines frontal boundaries - separates warm and cold fronts

  • Cold front interacts with warm, moist air ahead of the jet stream, it can lead to the development of thunderstorms and heavy rain.

  • Warm front advances over cooler air behind the jet stream, it can bring prolonged periods of light to moderate precipitation.

• Maintains temperature contrasts - transport warm air poleward and cold air equatorward creates areas of high and low pressures

• Transports moisture - carry moisture-laden air masses from the tropics toward higher latitudes or steer them away influences precipitation patterns

What’s the difference between satellite and radar imagery?

• Satellites: detect visible light reflected (left image) or infrared emitted by the Earth (right image)

  • Very good at seeing clouds, space-based

• Radar: emits and detect scattered microwaves; beam of precipitation-sized particles emitted then reflected

  • Time lag tells how far away clouds/precipitation particles are and their intensity

Explain the relationship between the general circulation of the atmosphere and weather phenomena.

• General circulation establishes the broad-scale patterns of air movement that influence the distribution of temperature, precipitation, and wind patterns around the globe.

• Weather phenomena are shaped by the interactions between these circulation patterns and local factors such as topography, land-sea distribution, and surface heating.

Identify different cloud types and the environments in which they form

• Stratus – low clouds, layered

  • Stratocumulus – low laying clouds, clumped together

• Cumulus – clouds that clump together

• Alto – middle height clouds

  • Altocumulus lenticularis – often stacked in layers at mid-high altitudes

• Cirrus – high clouds, wispy, composed of ice crystals

Explain the scientific basis for weather forecasting and how weather forecasts are made

• To obtain a sufficiently accurate knowledge of the state of the atmosphere at the initial time:

  • Surface observations from airports and ships (193 282) and buoys (1900)

  • Upper air observations from radiosondes (1062) and aircrafts (504 265)

  • Satellite observations (>9 600 000)

• To obtain a sufficiently accurate knowledge of the laws according to which one state of the atmosphere develops from another:

  • Ideal Gas Law

  • Conservation of mass (water vapour in atmosphere)

  • Conservation of mass (air)

  • How temperature changes

  • How wind changes

Why do good forecasts go bad?

• Imperfect knowledge of current atmospheric state

  • Data void

  • Instrument sensitivity and error

  • No observations of important quantities

  • Interesting weather in between observing locations

• Imperfect computer model

  • Need faster, more powerful computers for more grid points

  • Need better understanding of physical processes

• Chaos

  • “Butterfly effect”

  • Sensitive dependence to initial conditions

  • Small differences in in initial conditions lead to large differences later

EVEN WITH PERFECT MODEL WITH PERFECT INITIAL CONDITIONS…WEATHER FORECASTING LIMITED TO 2 WEEKS