Chapter 7: Atmospheric Circulations

Atmospheric Motion

• Scales

  • Microscale- small scale smoke distribution

  • Mesoscale-circulation of city air

  • Macroscale

    • Synoptic scale-circulations around pressure areas

    • Global scale

Eddies-Big and Small

• Eddy: Whirls of air

  • When wind encounters a solid object, a whirl of air forms on the object’s downwind side

    • Size and shape often depend upon spize and shape pf the obstacle and on wind speed

    • Types: rotors, mountain wave eddy and turbulence

      • 1. Wind sear-turbulent eddies can cause wind changes in magnitude, direction or both abruptly

      • 2. CAT clear air turbulence

Local WInd Systems

• Thermal Circulation

  • Circulations brought on by changes in air temperature, in which warmer air rises and colder air sinks

• Thermal highs and lows

  • Regions of surface atmospheric pressure created as the atmosphere either cools or warms

Local WInd Systems Types

• Sea and land breeze

  • Daytime: cooler ocean air replaces rising inland warm air producing clouds and rainshowers

  • Evening: as the land cools, the cool air rushes out to the sea and causes that warm air to rize

  • The uneven heating of the land and water cause strong temperature and pressure gradients along the beach

Mountain and Valley Breezes

• Daytime heating forces less dense air gently upslope (valley) If well developed and contain sufficent moisturem cab get clouds and rain, sometimes severe ub the early afternoon

• Nighttime cooling causes less dense cool air to flow gently downslope (mountain)

• Katabatic winds

  • Technically any downslope wind but the name is usually reserved for wind that is stronger than a typical mountain breeze

  • Ideal condition is from a high plateau that accumulates snow and becomes very cold

  • The cold dense air will fall over the edge through gaps and saddles

• Chinook (foehn) winds

  • Warm, dry, downslope wind on the eastern slope of the Rocky Mountatians

  • Foehn wind name originated in the European alps

  • Known as the zonda in ARgentina. Down the Andean slopes

• Santa Ana WInds

  • Warm, dry wind that blows downhill from the east or northeast into southern California

  • Funneled through canyons in the San Gabriel and San Bernardino mountains that spread over the Los Angeles Basin and San Fernando Valley

  • Blows dust and sand and dries everything out

  • Dangerous fire hazard

  • Santa Ana science

• Desert WInds

  • Winds of all sizes develop over a deserthuge dust storms over dry regions

  • Can develop over he Sahara and continue miles out to sea

  • The exceptional drought in the 1930’s on the farmland of the United States great plains blew large dust clouds

  • The dust bowl

  • Haboobs

  • Dust devils

• Seasonally changing winds-the monsoon

  • Arabic mawsim (i.e., seasons)

  • Monsoon wind systems change direction seasonally

    • Blows from one direction in summer and from the opposite direction in winter

    • In eastern and southern Asia, teh winter monsoon, which blows from land to water, is dry, the summer monsoon, which blows from water to land, is wet

Global Winds

• General circulation of the atmosphere

  • Winds throughout the world are averaged over a long period of time

    • 1. Actual winds may vary considerably from this average

    • 2. Underlying cause of the general circulation is the unequal heating of the Earth’s Surface

Single Cell Model

• Assumption

  • Earth’s surface is uniformly covered with water

  • Sun is always directly overhead at the equator

  • Earth does not rotate

• Result: huge thermally direct convection cell (i.e, Hadley cell)

Three Cell Model

• Assumption:

  • Earth is allowed to spin

  • Simple convection system breaks into a series of cells

• Doldrums

  • Over equatorial waters, the air is warm, horizontal pressure gradients are weak, and winds are light

Additional Concepts

• Subtropical highs-belts of high pressure near 30 degrees

• Horse latitudes- weak pressure gradient near the center of the subtropical highs would “becalm” sailing vessels

• Trade winds- steady winds for sailing ships

• Intertropical convergence zone (ITCZ)-trade winds converge

• Westerlies- prevailing winds between the subtropical highs and subpolar lows

• Polar front-cold air moving away from the poles

• Subpolar low-area of convergence caused by the polar front

• Polar easterlies- deflection of the polar fronts by the orioles effect

Average surface wind and pressure: the real world

• Semi-permanent highs and lows move only slightly over the course of a year

• Northern and southern hemispheres contrast

• Major pressure systems, wind belts, and ITCZ shift seasonally with teh high sun

• North in July and south in January

Global WInds

• General circulation and precipitation patterns

  • Global scale: abundant rainfall where air rises and very little where the air sinks

• Westerly winds and the jet stream

  • Jet stream: rivers of fast-flowing air where upper-level winds tend to concentrate into narrow bends

Atmosphere-Ocean Interactions

• Global wind patterns and surface ocean currents

  • Wind vows over the oceans and causes the surface water to drift along with it

  • Creates pressure differences

  • Gulf stream: warm water current flowing northward along the east coast of the United States

• Winds and upwelling

  • Upwelling: rising of cold water from below

  • Wind must flow roughly parallel to the coastline

• El nino, la nina, and southern oscillation

  • Souther oscukkationL seesaw pattern of reversing surface air pressure at opposite ends of the pacific ocean

    • El nino: warm-water episode

    • La nina: cold-water episode

Wind and Ocean Currets

• Other atmosphere-ocean interactions

  • Pacific decadal oscillation-middle latitude pacific ocean variability

    • Influences weather along U. S west coast for long periods (20-30 years)

• North atlantic oscillation-has a substantial effect on the weather in Europpe and along the east coast

• Arctic oscillation-interaction at the poles closely related to the NAO. land masses around the north pole cause it to be irregular while at teh south pile it is nearly circular. Responsible for polar vortex weather events