Notes on Cyclones and Anticyclones: Lecture 4/14

Understanding Fluid Dynamics in Cyclones

• You cannot pile up fluids like air or water due to their properties.
• The concept of surface cyclones relates to how these fluids interact with low-pressure areas:
• Friction from the ground causes convergence of surface air.
• Air must rise towards the center of the cyclone as it cannot accumulate vertically.

Boundary Layer Dynamics

• The boundary layer is about 1 km deep where friction is minimal among air particles.
• Surface cyclones and anticyclones display a sequential pattern in atmospheric observations, noted as occurring on a day in April 2015.
• Each event interconnected on surface maps aids in comprehending cyclonic structures.

Structure of Cyclones

• Cyclones exhibit a three-dimensional structure that is essential for analyzing weather patterns.
• Observation levels extend up to approximately 500 millibars (around 3 miles above ground), where the structure of the atmosphere is analyzed above and below.
• The relationship between the surface conditions and the atmospheric layers is critical for understanding cyclonic behavior.

Air Flow and Pressure Dynamics

• Surface cyclones are associated with positive precipitation due to rising air.
• The interplay of mass transfer in the column of air is defined by the divergence and convergence at upper and lower levels:
• Upper divergence indicates mass being evacuated from the top of a column while surface convergence refers to air being pushed into the column.
• If upper-level divergence exceeds surface convergence, column mass decreases, lowering surface pressure.

Cyclones vs. Anticyclones

• Surface anticyclones feature upper-level convergence and generally exist in patterns opposite to cyclones.
• The identification of upper-level divergence impacts the behavior of cyclonic storms significantly.

Understanding Vorticity

• Vorticity is a fundamental measurement in fluid dynamics reflecting the rotation of air parcels.
• Understanding vorticity is essential to evaluate the structure and development of cyclones and anticyclones:
• Clockwise rotation signifies negative vorticity, while counterclockwise indicates positive vorticity.
• Changes in vorticity cause corresponding changes in divergence, affecting storm intensity.

Examples of Fluid Flow

• When analyzing bends in a river:
• A paddle wheel in strong flow experiences changes in direction of rotation due to differences in flow speed between the inner and outer parts of the bend.
• Clockwise bends yield negative vorticity; counterclockwise bends yield positive vorticity.

Additional Observations

• When air parcels experience changes in vorticity, they can start to spin cyclonically (counterclockwise), which relates to upper-level divergence.
• Striking a balance between upper divergence and surface convergence is critical to define storm behaviors, enhancing predictive capacities of weather events.