Notes on Cyclones and Atmospheric Dynamics: Lecture 4/23

Warm and Cold Columns

• Intensifying Cyclones: The intensity of cyclones increases with altitude; stronger winds are present above the surface level.
• Characteristics of Tropical Cyclones:
  • Warm Core: Hurricanes have a warm core at their center, with cooler air surrounding it.
  • Low Central Pressure: Central pressure is significantly lower than surrounding areas, contributing to strong winds.

Vertical Cross Section Through Tropical Cyclones

• Pressure Profile: A vertical cross-section reveals that the warmest air is at the center, with cooler air on the sides.
• Example: A pressure of 950 millibars represents the lowest pressure at the hurricane's center. The surrounding area has higher pressure, showcasing the pressure gradient necessary for high wind speeds.
• Gas Law Relation: The thickness of air columns varies with temperature, leading to greater distances between pressure levels in warm air than in cold air.

Illustration of Pressure Changes

• Isobars and Pressure Levels:
  • Higher isobar values indicate higher pressure above warm cores, influencing airflow and storm behavior.
  • Upper divergence occurs above lower-level convergence, facilitating upward motion and intensifying storms.

Comparisons Between Tropical and Extratropical Cyclones

• Structural Differences: Tropical cyclones are vertically aligned with no tilt, contrasting with the horizontal tilt seen in extratropical cyclones.
• Wind Rotation: A counterclockwise wind pattern persists at the surface, transitioning to a clockwise rotation aloft, but the intensity diminishes with height.

Observational Techniques

• Hurricane Reconnaissance: Airborne measurements using radar help assess temperatures and pressures within hurricanes, especially when investigating the so-called "eye".
• Challenges: The turbulent nature of high-altitude hurricane observation complicates data collection.

Geostrophic Winds and Temperature Gradients

• Geostrophic Wind Dynamics: In regions with horizontal temperature contrasts, wind speeds increase with altitude, a phenomenon identified as thermal shear.
• Historical Context: The concept of thermal shear originated from meteorological studies in the early 20th century to understand wind behavior associated with temperature changes.

Jet Streams

• Definition: The jet stream is a high-altitude wind band where the maximum horizontal wind speeds occur, typically in the upper troposphere.
• Temperature Gradient Relationship: Close spacing of isobars signifies a strong temperature gradient, correlating directly to powerful jet stream winds.

Seasonal Variation in Jet Streams

• Winds Across Seasons: Observations indicate seasonal variations in wind speeds, influenced by temperature contrasts as the earth orbits the sun.
• Geographical Impact: The jet stream flows from regions of warm air to colder polar air, which can vary considerably across different seasons.