Wind: Global Systems Chapter 7

Overview of Global Circulation and Jet Streams

  • Global Circulations: The atmosphere consists of three primary cells that dictate wind patterns globally — the Hadley Cell located between the equator and 30° latitude, the Ferrell Cell situated between 30° and 60° latitude, and the Polar Cell extending from 60° latitude to the poles. These cells are critical in shaping the direction and intensity of jet streams, which are fast flowing air currents at high altitudes that significantly influence weather systems and climate patterns.

  • Relation to Jet Streams: The functional aspects of the atmosphere's three cells lead to the development of two major jet streams: the subtropical jet stream, which typically occurs at around 30° latitude, and the polar jet stream, which resides at higher latitudes. The interactions between these cells generate varying weather conditions across different regions, influencing front systems, storm tracks, and seasonal changes.

Thermal Circulations

  • Monsoons: Monsoons are significant seasonal winds characterized by a distinct shift in direction, leading to periods of substantial rainfall followed by dry spells. This phenomenon is crucial for agriculture in affected regions, particularly in South Asia.

  • Mountain and Valley Breeze: These breezes arise from differential heating; during the daytime, the sun heats up valley areas, causing warm air to rise (valley breeze), whereas at night, cooler mountain air sinks down (mountain breeze), creating localized wind patterns.

  • Sea Breeze: The sea breeze is generated due to temperature differences between land and sea, where cool air flows from the sea to the land, providing a refreshing effect on hot days, particularly along coastal regions.

  • Santa Ana Wind: A unique warm and dry wind that descends from the inland deserts to Southern California, the Santa Ana winds can exacerbate fire risks by drying out the landscape and creating favorable conditions for wildfire propagation.

  • Haboob: These intense dust storms are common in arid regions and are characterized by strong winds blowing dust over vast areas, creating visibility hazards and impacting air quality significantly.

ENSO (El Niño Southern Oscillation)

  • Impact on Texas: The ENSO cycle profoundly influences the climate of Texas, notably altering rainfall patterns. El Niño and La Niña phases lead to wetter or drier conditions across the region, drastically affecting agricultural output and water supply.

Upwelling and Downwelling Circulation

  • Upwelling: Upwelling occurs when nutrient-rich waters from the ocean floor rise to the surface, supporting diverse marine ecosystems and fisheries by providing essential nutrients necessary for phytoplankton growth.

  • Downwelling: This process involves cold, dense water that sinks, facilitating the transport of oxygen-rich water to deeper ocean layers, affecting marine life and ocean health.

Air Masses and Fronts

  • Basic Air Masses: Air masses are categorized by their source regions based on temperature and humidity characteristics. The main types include Polar (P), Tropical (T), Arctic (A), Maritime (m), and Continental (c). Each type plays unique roles in influencing weather patterns.

  • Source Regions: Certain areas, characterized by stable atmospheric conditions, are ideal for the formation of air masses, allowing for consistent temperature and moisture properties.

  • Weather Impact: Among the various air masses, maritime polar (mP) air masses introduce moisture and cooler temperatures, while continental tropical (cT) air masses bring hot, dry conditions, significantly influencing precipitation and temperature patterns across the U.S.

  • Movement: Air masses generally flow from high-pressure areas to low-pressure zones, often moving from west to east across the interior of the United States.

Synoptic Scale vs. Mesoscale

  • Differences: The synoptic scale encompasses larger weather systems such as cyclones and anticyclones, whereas the mesoscale involves smaller, more localized phenomena like thunderstorms and tornadoes.

  • Fronts: Identifying various front types — cold fronts, warm fronts, occluded fronts, and stationary fronts — is essential as they each lead to distinct weather patterns and conditions. Occluded fronts often result in complex weather scenarios where cold air overtakes warm air, while drylines are significant in delineating moisture content, crucial for thunderstorm development.

Mid-Latitude Cyclones and Cyclogenesis

  • Upper Levels: In the context of cyclogenesis, the upper levels of the atmosphere exhibit divergence, which promotes the formation and intensification of mid-latitude cyclones. The positioning and strength of jet streams are pivotal in influencing these cyclones' trajectories and associated weather events.

Thunderstorms

  • Ingredients: Thunderstorm formation depends on three crucial elements: adequate moisture, atmospheric instability, and mechanisms of lift. Without one of these components, the development of thunderstorms is not possible.

  • Severe Thunderstorm Criteria: Thunderstorms are classified as severe if they exhibit organized structure, potential for hail production, and strong winds exceeding 58 mph.

  • Supercells: These are highly organized thunderstorms characterized by a persistent updraft along with a rotating structure, often producing severe weather such as tornadoes.

  • Squall Lines: formations associated with cold fronts characterized by a line of severe thunderstorms that can lead to widespread severe weather.

  • Thunderstorm Dissipation: The end of a thunderstorm typically occurs when the updraft weakens or when rainfall drags the downdraft down, resulting in a cessation of storm activity.

  • Structure of a Supercell: Key components include the updraft at the center, a downdraft on one side, and a rotating column of air known as a mesocyclone, which can lead to tornado formation under certain conditions.

Updrafts and Downdrafts

  • Causes: Updrafts occur primarily from surface heating which creates instability in the atmosphere; conversely, downdrafts develop when precipitation falls and drags cooler air downward.

  • Doppler Radar: Doppler radar systems are critical tools in meteorology for monitoring storms, enabling forecasters to detect wind patterns, precipitation, and potential severe weather events effectively.

Thunder & Lightning

  • Charge Separation: The process of charge separation within clouds is vital for lightning formation, where negative charges accumulate at the cloud base, and positive charges are concentrated near the top.

  • Lightning Types: Various types of lightning include cloud-to-ground, which is most dangerous, and in-cloud lightning, which helps to balance charge differences within a storm. The rapid discharge of electricity generates visible flashes and subsequent thunder due to the thermal expansion of air along the lightning channel.