CHAPTER 8

WIND SYSTEMS

CHAPTER 8

WIND SYSTEMS

General refers to the average air flow, actual winds will vary considerably

Average conditions help identify driving forces

The basic cause of the general circulation is unequal heating of the earth’s surface◦ Warm air is transferred from the tropics to the

poles◦ Cool air is transferred from the poles to the

tropics

Single Cell Model◦ Assume

1. Uniform water surface2. Sun always directly overhead the equator3. Earth does not rotateResult: huge thermally direct convection cell (Hadley)

Three Cell Model◦ Allow earth to spin = three cells (Hadley,

Ferrell, Polar)◦ Alternating belts of pressure starting with L at

equator◦ Alternating belts of wind with NE just north of

equator

First, consider a nonrotating earth that’s completely covered with ocean

The PGF is really the only force driving the winds

http://veimages.gsfc.nasa.gov/643/itcz_goes11_lrg.jpg

Adding some realism◦ Semi-permanent high and lows◦ Northern vs. Southern Hemisphere◦ Major features (pressure systems, wind belts,

ITCZ) shift seasonally with the high sun Towards the warm pole

Fig. 8.3, p. 212© Brooks Cole/Cengage Learning

Fig. 8.3, p. 212© Brooks Cole/Cengage Learning

Fig. 8.4, p. 213© Brooks Cole/Cengage Learning

Average Wind Flow and Pressure Patterns Aloft◦ North-south temperature and pressure gradient at

high altitudes creates west-east winds, particularly at mid to high latitudes.

General Circulation and Precipitation Patterns◦ Rain where air rises (low pressure)◦ Less rain where air sinks (high pressure)

Fig. 8.9, p. 216© Brooks Cole/Cengage Learning

Fig. 8.9, p. 216© Brooks Cole/Cengage Learning

http://daphne.palomar.edu/pdeen/Animations/23_WeatherPat.swf

Fig. 8.5, p. 213© Brooks Cole/Cengage Learning

Fig. 8.6, p. 214© Brooks Cole/Cengage Learning

Fig. 8.7, p. 214

© Brooks Cole/Cengage Learning

Fig. 8.8, p. 215© Brooks Cole/Cengage Learning

Where the circulation cells meet, we observe jet streams: narrow regions of very strong winds aloft

The polar jet usually provides a good estimate for the dividing line between warm and cold air

The subtropical jet affects Texas weather in the winter

The jet streams tend to be wavy and aren’t constant in time

100-200 kt winds at 10-15 km, thousands of km long, several 100 km wide and a few km thick (polar and subtropical)

Established by steep temperature and pressure gradients between circulation cells◦ Gradients greatest at polar jet

Jet Streaks are areas of stronger wind within the jet stream

Fig. 8.12, p. 218

Arabic for “seasonal” Winds that change drastically from season

to season Have some similarities to land/sea breeze,

but on a much larger scale

Cooling over land in winter causes sinking air, high pressure, dry conditions

Land heats up in summer (much more than ocean), causes rising air, low pressure, very wet conditions

Also a monsoon, though weaker, in the southwestern United States

Douglas et al. (1993)

Fig. 8.15, p. 222

Fig. 8.16, p. 222

Sea breezes and land breezes are mesoscale circulations near coastlines (for example, Texas Gulf coast)

Land and water heat and cool at different rates during the day and at night (remember heat capacity/specific heat?), causing gradients in temperature and pressure

Sea breezes:◦ Bring cooler air to coastal areas◦ Bring more humid air as well◦ Can cause thunderstorms inland from the coast

Land Water

Land Water

At night, land cools faster than water---the opposite processes take place

Offshore flow, sinking air over land, rising air over water

Stepped Art

Fig. 9-25, p. 241

Mountain and Valley Breeze◦ On mountain slopes, warm air rises during the

day creating a valley breeze; during night nocturnal drainage of cool air creating a mountain breeze; gravity winds

◦ Associated with cumulus clouds in the afternoon

Fig. 8.19, p. 226

Fig. 8.20, p. 226

Windy Afternoons◦ Afternoon convection◦ If the air begins to sink as part of a convective circulation, it

may pull some of the stronger winds aloft downward with it◦ If this sinking air should reach the surface, it produces a

momentary gust of strong wind◦ In addition, this exchange of air increases the average wind

speed at the surface

Fig. 8.21, p. 227

Katabatic (fall) windsCold wind rushes down elevated slopes, usually 10 mi/hr or less but can reach hurricane strength

Chinook/Foehn Winds◦ Dry warm descending on the leeward side of a

orographic barrier◦ Eastern slope of Rockies (chinook), Europe

(foehn), Argentina (zonda)◦ Snow-eater

Fig. 8.23, p. 228

Fig. 8.24, p. 229

Fig. 8.25, p. 229

+49° F in seven minutes (Great Falls, MT) Spearfish, SD: -4°F to 54, back to 11, up to 55!

Santa Anna Winds◦ Warm dry that blows from east or northeast

downslope into Southern California◦ Very fast, desiccates vegetation, providing fuel for

fires◦ Canyons can funnel and enhance

Fig. 8.26, p. 230

Fig. 8.27, p. 230

Fig. 8.28, p. 230

Fig. 8.29, p. 231

Other extreme winds◦ Texas or blue norther◦ Norte◦ Bora, Mistral (famous katabatic winds)◦ Blizzard, burga, purga◦ Duststorm, sandstorm

Haboob, Shamal◦ Dust devil, whirlwind◦ Leste, levete, sirocco, khamsin, simoom

Fig. 8.30, p. 231

Fig. 8.31, p. 232

Fig. 8.32, p. 232

Fig. 8.33, p. 233

Fig. 8.34, p. 233

Aircraft Turbulence◦ CAT (clear air turbulence)◦ Increasing wind speed shear◦ Billow clouds