Chapter 7 Atmospheric Circulations Scales of atmospheric motions

Chapter 7: Atmospheric Circulations Scales of atmospheric motions n Eddies - big and small n Local wind systems n Global wind patterns and the oceans n

Scales of Atmospheric Motions scales of motion n microscale n mesoscale n synoptic scale n planetary scale n • Lots of important weather events occur on microscales, like evaporation of liquid water molecules from the earth’s surface.

Fig. 7 -2, p. 171

Eddies - Big and Small eddy n rotor n wind shear n turbulence n • Wind shear can sometimes be observed by watching the movement of clouds at different altitudes.

• Kelvin Helmholtz waves • Clear-air turbulence • Billow clouds Figure 1, p. 173

Local Wind Systems Thermal Circulations isobars and density differences n thermal circulations n

Sea and Land Breezes sea breeze n land breeze n sea breeze front n Florida sea breezes n • Sea and land breezes also occur near the shores of large lakes, such as the Great Lakes.

Fig. 7 -6, p. 175

Seasonally Changing Winds the Monsoon wind system n India and eastern Asian monsoon n other monsoons n

Fig. 7 -10, p. 178

Mountain and Valley Breezes valley breeze n mountain breeze n • The nighttime mountain breeze is sometimes called gravity winds or drainage winds, because gravity causes the cold air to ‘drain’ downhill.

Katabatic Winds n Strong drainage winds: steep slope • Katabatic winds are quite fierce in parts of Antarctica, with hurricane-force wind speeds. • Bora: a cold, gusty northeasterly wind along the Adriatic coast in the former Yugoslavia

Chinook (Foehn) Winds Chinook winds—warm and dry n compressional heating n chinook wall cloud n • In Boulder, Colorado, along the eastern flank of the Rocky Mountains, chinook winds are so common that many houses have sliding wooden shutters to protect their windows from windblown debris. • It is called a Foehn along the leeward slopes of Alps.

Fig. 7 -14, p. 180

Santa Ana Winds Santa Ana wind n compressional heating n wildfires n • Many Southern California residents regularly hose down their roofs to prevent fires during Santa Ana wind season. • Difference between this and Chinook wind Santa Ana: from elevated desert plateau; Chinook: from cold plateau

Desert Winds n n dust and storms dust devils – from surface usually with a diameter of a few meters and a height of <100 m

General Circulation of the Atmosphere cause: unequal heating of the earth’s surface n effect: atmospheric heat transport n • Ocean currents also transport heat from the equator to the poles and back.

Single-cell Model basic assumptions: no rotation n Hadley cell n why is the single-cell model wrong? n • One of the world’s premier atmospheric science research facilities, the Hadley Centre for Climate Research, is named after George Hadley.

Three-cell Model model for a rotating earth n Hadley cell • Many global circulation terms, n doldrums including ‘trade winds’ and n subtropical highs ‘doldrums’, were named by n trade winds mariners who were well acquainted with wind patterns. n intertropical convergence zone • Upper troposphere easterly is n westerlies inconsistent with the observed westerly n polar front n polar easterlies n

Fig. 7 -21, p. 185

Average Surface Winds and Pressure: The Real World semipermanent highs and lows n Bermuda high & Pacific high n Icelandic low & Aleutian low n Siberian high n • The Bermuda High frequently brings hot, muggy weather to the eastern US. in summer • The ITCZ shifts toward the north in July (from January)

Fig. 7 -22 a, p. 188

Fig. 7 -22 b, p. 189

The General Circulation and Precipitation Patterns n ITCZ, midlatitude storms, polar front • Most of the world’s thunderstorms are found along the ITCZ. • Low rainfall over the subtropical regions

Westerly Winds and the Jet Stream jet streams n subtropical jet stream n polar front jet stream n Low-level jet stream over the Central plains of the U. S. (within 2 km above surface), bringing moist and warm air to form nighttime thunderstorms

Global wind-driven ocean current Fig. 7 -29, p. 193

Winds and Upwelling n Upwelling is strongest when wind is parallel to the coastline

El Niño and the Southern Oscillation El Niño events n Southern Oscillation n La Niña n teleconnections n • ENSO is an example of a global-scale weather phenomenon. • Often Arizona has a wetter winter during El Nino and a drier winter during La Nina; • Usually northwestern U. S. has a drier winter during El Nino and a wetter winter during La Nina.

Fig. 7 -32, p. 196

Other Atmosphere-Ocean Interactions North Atlantic Oscillation n Arctic Oscillation: pressure difference between n Arctic and regions to its south n Pacific Decadal Oscillation

Fig. 7 -36, p. 199