Extratropical Cyclones: Life Cycle and Dynamics

Extratropical Cyclones: Life Cycle and Dynamics

Introduction to Extratropical Cyclones

  • Definition: Extratropical cyclones are storm systems that occur outside of the tropics, distinguishing them from tropical cyclones (hurricanes, typhoons). These are the storms commonly experienced in regions like North Georgia, where there is a lot of heat and moisture.

  • Focus: This discussion will cover the life cycle of an extratropical cyclone, exploring air, moisture, and energy flows, and how to interpret associated weather phenomena seen on weather maps.

Motivating Question: Formation of Extratropical Cyclones

  • Question: How do latitudinal or extratropical cyclones form?

  • Prerequisite Knowledge: Understanding air masses, discussed in previous lectures.

Air Masses and Their Properties

  • Tropical Air Masses (denoted by 'T'):

    • Origin: Tropics.

    • Characteristics: Warmer and more moist. Hotter air holds more water vapor than colder air.

  • Polar Air Masses (denoted by 'P'):

    • Origin: High latitudes.

    • Characteristics: Colder and therefore hold less moisture.

  • Further Classification:

    • Marine Air Masses: Come from over oceans; hold more water vapor.

    • Continental Air Masses: Come from continental areas; hold less water vapor.

  • Significance: These differences in air mass properties are the fundamental drivers of extratropical cyclone formation.

Energy Imbalance and Transport

  • Fundamental Role: Extratropical cyclones play a crucial role in balancing Earth's energy budget.

  • Mechanism: They transport energy from the tropics (which receive more solar energy) northward or southward to higher latitudes, thus helping to mitigate the global energy imbalance.

  • Key Driver: The mixture of air masses with different properties prevents immediate mixing, creating unstable conditions that lead to storm formation.

Basic Facts about Mid-latitude Cyclones (Extratropical Cyclones)

  • Scale: Small-scale atmospheric science uses the term "synoptic" to describe these systems.

  • Control: They are controlled by prevailing winds.

    • Prevailing Winds: Also known as average winds or daily winds. In the mid-latitudes, these are the Westerlies.

    • Movement: Extratropical cyclones typically move from west to east, driven by the Westerlies.

    • Observation: This explains why cold fronts (and other features) consistently arrive from the west.

  • Common Life Cycle: All extratropical cyclones share a common life cycle, allowing for a general understanding of their formation mechanisms.

  • Prerequisite: Begin with air masses of different characteristics:

    • Temperature.

    • Water vapor amount.

    • Density.

The Common Life Cycle of an Extratropical Cyclone

1. Cyclogenesis (Beginning of the Life Cycle)

  • Description: This is the initial stage where warm and cold air masses meet.

  • Characteristics:

    • Warm air mass: Warm and moist.

    • Cold air mass: Cold and dry.

    • Non-mixing: Due to significant differences in density (like oil and water), they do not mix immediately.

  • Initiation: The meeting of these air masses creates an unstable condition due to temperature and density differences.

  • Spinning: When these differences combine with average wind conditions and the Coriolis force, the air masses begin to spin, leading to cyclone formation.

2. Open Stage

  • Development: The cold front, typically originating from polar latitudes, advances and often comes up to the west of the warm front. The warm air, originating from tropical sources, gets