Wind Systems. Concepts. Scale. Sea breeze: air coming from the sea Northwest wind: wind blowing from the northwest

Transcription

1 G109: Weather and Climate Wind Systems Readings A&B: Ch.8 (p ) CD Tutorial: El Niño Southern Oscillation Topics 1. Concepts 1. Scale 2. Wind Direction 3. Differential Heating 2. Microscale Winds 3. Mesoscale Winds 1. Land & sea breezes 2. Mountain or valley winds 3. Chinook 4. Santa Ana winds 5. Katabatic winds 4. Macroscale Winds 1. Global Circulation 1. Single-cell Model 2. Three-cell Model 3. Zonal Precipitation Patterns 4. Semi-Permanent Pressure Cells 2. Asian Monsoon 3. Jet Stream 4. Rossby Waves 5. El Niño Southern Oscillation Concepts Three major divisions Scale Scale Space Micro Meters Meso Kilometers Macro Synoptic km Planetary >1000 km (global) Time Seconds Minutes Seconds Hours Days Days Weeks Wind Direction Based on where the wind is Sea breeze: air coming from the sea Northwest wind: wind blowing from the northwest

2 Concepts Differential Heating Spatially - get differences in surface heating Some areas are warmer than others Occurs across the range of scales e.g. Micro: grass - concrete (Lab 5) Meso: land - lake Macro: equator - poles Heating rate and T differences winds Examples Turbulent eddies Microscale Winds Small whirls of air Dust devils Gusts

3 Mesoscale Winds: Land-Sea Breeze Land-Sea (or Land-Lake) Breeze Daily T differences between land and sea Daytime: land heated more intensely than water Air above land heats more, expands vertically Air aloft starts to flow Near Surface: Pressure Gradient Force Cool air blown onto land Mesoscale Winds: Land-Sea Breeze Nighttime: reverse Land cooled more rapidly than water Warmer over the water Air blown from the land to the ocean Sea breeze can have a significant modifying effect on the temperature in coastal areas E.g., Chicago lake breeze Size of breeze

4 Mesoscale Winds: Mountain/Valley Wind Daytime Slopes of mountains get more intense heating than air at the same elevation over the valley floor May see cumulus clouds over peaks thunderstorms in the afternoons Most common in Mesoscale Winds: Mountain/Valley Wind Sunset & Nighttime Rapid cooling of slopes Cool air drainage Most common in. Lowest areas are first to experience radiation fog, frost damage Note: seasonal preference Valley breezes are most common in Mountain breezes are most common in.

5 Mesoscale Winds: Chinook Winds Chinook / Foehn Different names in different places Chinook Rockies (Montana, Wyoming, Alberta) Foehn - Alps, N.Z. Low pressure system on the of a mountain barrier pulls the air across as it comes down mountain T can rise by 20 o C Usually occur Mesoscale Winds: Santa Ana Winds Santa Ana Winds California High pressure system over the Rocky Mountains Air flows away from high, down western slopes as it comes down mountain T can rise by 30 o C Usually occur Often contributes to spread of forest fires in CA

6 Mesoscale Winds: Katabatic Winds Katabatic Winds Cold downslope wind Cold air sinks because more dense but still than lower elevation air it displaces If channeled into narrow valleys high velocities Frequently occur at edges of Greenland and Antarctic ice sheets Different names in different places Bora: Balkans Adriatic sea Mistral: Alps France Macroscale Winds: Global Circulation Synoptic and planetary (macroscale) winds influence the smaller scale (mesoscale and microscale) winds Global Circulation Differential heating between equator and poles Global scale pressure differences Persistent large-scale motion

7 Macroscale Winds: Global Circulation Single Cell Model Differential heating Assumptions: Earth is uniformly covered with water Sun is directly over equator Single-cell pattern of flow Hadley Cell Warm air rises at Cold air sinks at Macroscale Winds: Global Circulation Single Cell Model Hadley Cell Earth s rotation Coriolis force: winds deflected to right in Northern hemisphere, to left in Southern hemisphere Winds: winds from poles to equator Single-cell pattern is not what we observe Breaks down due to:

8 Macroscale Winds: Global Circulation Three-Cell Model more realistic model Macroscale Winds: Global Circulation Three-Cell Model more realistic model Hadley Cell: Inter-Tropical Convergence Zone (ITCZ) (0 o ) Very strong low pressure zone rising air Light winds: doldrums Sub-tropical High (30 o N/S) Sinking air Light winds: horse latitudes Trade winds (0-30 o N/S)

9 Macroscale Winds: Global Circulation Three-Cell Model more realistic model Ferrel cell Some of sinking air at subtropical high diverges poleward (mid-latitudes) Macroscale Winds: Global Circulation Three-Cell Model more realistic model Polar cell: high latitudes Thermally driven circulation Polar High (90 o ) Very cold conditions Sinking, diverging air Sub-polar Low (60 o N/S) Rising air Polar Flow from Very strong deflection by Coriolis force

10 Macroscale Winds: Global Circulation Zonal Precipitation Patterns Equatorial Low Rising air Sub-tropical High Sinking air Migrates N / S with seasons Sub-polar Low Rising air Polar High Sinking air Macroscale Winds: Global & Synoptic Three-cell model not quite true: doesn t include land/water differences Three-cell model breaks down in upper-level winds do not have the distinct structure of Ferrel cell and polar cell, although surface winds are correct there But it was a very useful starting point for considering global circulation In the real atmosphere, we instead find a number of semi-permanent High and Low pressure cells

11 Macroscale Winds: Global & Synoptic Semi-permanent Pressure Cells January Macroscale Winds: Global & Synoptic Semi-permanent Pressure Cells July

12 Macroscale Winds: Asian Monsoon Seasonal wind due to seasonal changes in mean pressure Winter: Sinking air from jet stream Summer: Strong heating over continent. Draw moisture from warm Indian Ocean toward India and Asia Himalayan Mountains cause strong orographic uplift Macroscale Winds: Jet stream An area of increased wind speeds Narrow band: km wide Speeds: km h -1 Height: 9-12 km ( ) Typically found above the largest horizontal T gradient e.g., at polar front Move north and south with the seasons Stronger in the the T gradients are largest Most powerful jet-stream: Weaker jet-stream: when

13 Macroscale Winds: Rossby Waves Recall: Upper air (zones of low pressure extending equator-ward) and. (zones of high pressure extending poleward) Wavelike flow around earth at mid-latitudes Rossby waves: long waves in flow Usually 3-7 Rossby waves encircling earth Migrate west to east Change in wavelength and amplitude Macroscale Winds: Rossby Waves Large amplitude Rossby waves (. flow) transport: Warm air from subtropics to high latitudes Cold polar air to low latitudes Small amplitude Rossby waves ( flow) Flow is more westerly, less equator-pole exchange of heat Changes in the flow along the wave lead to: Divergence aloft Draws air Leads to Convergence aloft Forces air Inhibits

14 El Niño Southern Oscillation El Niño weak warm current occurring along the west coast of South America (particularly Peru) Appears every 3-7 years around Christmas time Lasts about 1 year Warm current is not good for fishing industry was warmest event ever recorded Occurs due to a reversal in Walker Circulation the interaction between atmospheric circulation and ocean circulation in the equatorial Pacific El Niño Southern Oscillation During a normal (non-el Niño) year: Easterly trade winds drag warm surface water from East to West across Pacific Upwelling of cold water along the west coast of South America Low pressure area: High pressure:

15 El Niño Southern Oscillation A normal (non-el Niño) year El Niño Southern Oscillation During an El Niño year: Weakening or reversal of trade winds drag warm surface water from W to E across Pacific No upwelling of cold ocean water Sea Surface Temps (SST s) in Eastern Pacific become warmer than normal Low pressure area shifts to Eastern Pacific. along west coast of South America, Central America and even California High pressure shifts from to western Pacific The reversal in surface pressure is called the

16 El Niño Southern Oscillation During El Nino year: El Niño Southern Oscillation When El Niño dissipates: Normal (non-el Niño) conditions OR La Niña conditions During a La Niña year: Very strong easterly trade-winds in the Pacific Very strong upwelling of cold water along the west coast of South America SST s become colder than normal In Western Pacific: warm water promotes uplift, which intensifies surface low, and intensifies easterly trade winds Along west coast of America s: very High pressure