Notes on Cyclones and Weather Systems: 4/14 Discussion

Cyclones Overview

• Definition: Low pressure systems where winds rotate in the same direction as the Earth.
  • In the Northern Hemisphere (NH): Spins counter-clockwise.
  • In the Southern Hemisphere (SH): Spins clockwise.
  • Typical Lifetime: 5-7 days.
  • Average Horizontal Size: ~1,000 km.

Anticyclones Overview

• Definition: High pressure systems where winds flow in the opposite direction of the Earth.
  • In the Northern Hemisphere: Spins clockwise.
  • In the Southern Hemisphere: Spins counter-clockwise.
  • Generally longer-lived and similar in size to cyclones.

Midlatitude Cyclones

• Occur in regions with large temperature gradients.
• Characterized by frontal circulations.

Weather Discussions and Presentations

• Quality of Presentation (75%)
  • Completeness, time adherence, understanding, relevance to class.
• Group Evaluation (25%)
• Tips:
  • Presentation time: 9-10 mins.
  • Ensure completeness in figures and descriptions.
  • Avoid reading verbatim from slides or notes.
  • Expected to engage in Q&A about the event.

Cyclone Lifecycle according to the Norwegian Cyclone Model

1. Initial Stage: Boundary typically starts as a stationary front.
2. First Wave: Development initiated by upper-level lows or disturbances, creating cold and warm fronts.
3. Intensification: Waves sharpen and become more distinct.
4. Maturity: Cold front overtakes the warm front creating an occluded front.
5. Decay: Increased occlusion leads to the dissipation of the cyclone.

Front Types

• Cold Front:
  • Cold air advances and forces warm air to rise, resulting in vigorous rising air, thunderstorms, and precipitation.
• Warm Front:
  • Warm air advances to cooler air, resulting in gentle rising, stratus clouds, and light precipitation.
• Occluded Front:
  • A cold front overtakes a warm front, pushing warm air aloft, signaling peak intensity before dissipation.

Mass Continuity

• Core Principle: Mass must be conserved. If air is forced into a region, it must rise to maintain balance.
• Convergence: Associated with low pressure; air from different regions moves into the same region.

Vorticity

• Definition: Measure of fluid rotation at a point.
  • Cyclones exhibit positive vorticity (counter-clockwise flow), while anticyclones show negative vorticity (clockwise flow).

Vorticity Advection

• Refers to the horizontal transport of vorticity by wind.
  • Positive Vorticity Advection (PVA): Moves positive vorticity horizontally.
  • Negative Vorticity Advection (NVA): Moves negative vorticity horizontally.

Cyclogenesis

• The process through which cyclones form and intensify.
• Favorable conditions for cyclogenesis involve strong divergence occurring over the surface cyclone, thereby reducing surface pressure and intensifying the cyclone.
  • Key Factors:
  • Upper-level divergence must be stronger than lower-level convergence.
  • Eastward tilt aloft leads to weakening the cyclone, while westward tilt promotes intensification.

Cyclone Life Cycle - Pressure Perspective

• Initial Storm Orientation: Westward tilting storm leading to pressure systems.
• Cyclone Stacking: When an upper-level trough catches up with a surface cyclone, growth is halted.
• Post-Stacking Behavior: The potential transition to eastward tilt leads to cyclone weakening.

Responsibilities and Reminders

• Prepare for presentations next week.
• Group submission for the case study and practical required in upcoming weeks.
• Open invitation for office hour discussions about course materials.