GEU 0047: Meteorology Lecture 9 Wind: Small-Scale and Local Systems

Earth and Sky Sand Dunes Snow Drifts Snow Roller For more on wind shaping land forms, visit your nearest geology class. Sand dune and sand ripple

Wind Speed Vectors Vectors are arrows representing wind direction and speed.

Wind Data Wind Vane Direction* *Direction indicates wind origin NOT direction it is blowing. Wind anemometer Speed Vane:風向標 Anemometer: 風速計

Figure 9.19 The aerovane (skyvane).

Wind Direction Compass Clockwise Convention North = 0o East = 90o South = 180o West = 270o Compass:指南針

Onshore/Offshore Other direction indicators based on geology and geography. (valley breeze, mountain breeze)

Wind Rose Average Wind Direction in a given location. An indicator of “prevailing winds.” Winds in this representation are mostly from the West, predominantly from the NW (from due East only 10%). Wind rose:風花圖

Wind Scales

Small-scale Winds Microscale: turbulence, gusts, wind devils, dust devils Mesoscale: sea breeze, valley/mountain breezes, tornadoes Macroscale: Westerlies, Jets, Fronts,Cyclones (L), Anticyclones (H)

Topographical Turbulence Small scale turbulence wind devils, eddies Large scale turbulence mountain range interference strong Vertical mixing Figure 9.2 Winds flowing past an obstacle. (a) In stable air, light winds produce small eddies and little vertical mixing. (b) Greater winds in unstable air create deep, vertically mixing eddies that produce strong, gusty surface winds.

Wind Reversal A wind gust caused by ground topography. Jeffreys’ Sheltering model: The rear face of the topography against which the wind blows experiences a higher pressure than the front face, which is sheltered from the force of the wind. Air eddies formed in front of it lead to pressure differences that push the wind reversal. Figure 9.6 Strong winds flowing past an obstruction, such as these hills, can produce a reverse flow of air that strikes an object from the side opposite the general wind direction.

Wind Break Near the surface, friction plays a major role in wind speed “Flag” trees created in an area with winds coming from one predominant direction (prevailing winds).

Surface Friction The amount of surface friction depends upon the stability of air (vertical mixing). (b) More vertical motion, friction effects are felt at higher altitudes. Within the region of frictional influence, vertical mixing increases the wind speed near the ground and decreases it aloft.

Mountain Peak Eddies The size and shape of eddies depends upon wind speed and the size and shape of the obstacle that created the eddies. The eddies are created down wind of the obstacle.

Figure 9.4 Satellite image of eddies forming on the leeward (downwind) side of the Cape Verde Islands during April, 2004. As the air moves past the islands, it breaks into a variety of swirls as indicated by the cloud pattern. (The islands are situated in the Atlantic Ocean, off Africa’s western coast.)

Island Wakes (Hawaii Islands) Filaments:細絲,細線 Phytoplankton:浮游植物

Mountain Waves Wave clouds created downwind of a mountain range.

Kelvin-Helmholtz Instability (e.g., CAT) Wind Shear Turbulence creating “billow” clouds. Billow:翻騰

Thermal Circulation Zero Temp. difference => Zero Pressure Gradient => Zero Wind Temperature Differences Create PGF and wind Air piles up and descends to create high pressure near the surface, flowing to low pressure only to rise at a surface low

Thermal Highs and Lows Thermal pressure systems: Are near the surface Weaken with altitude Maintained by uneven heating in local areas

Convergence/Divergence Surface Lows and Surface Highs need the support of upper level divergence and convergence to grow and intensify.

Sea Breeze During Day Differential Heating of Land and Sea. Cool wet marine air with warm dry continental air. Daytime heat (convection), land develops a low, ocean a high (subsidence).

Land Breeze at Night Nighttime land IR cooling, land high, ocean low. Reversal of daytime highs and lows and winds.

Florida Convergence Low Daytime sea breezes from both coasts yield frequent afternoon thunderstorms over central Florida in summer.

Lake Winds

Lake Winds Lake Breeze can also be observed much akin to the Florida convergence. (Land between the Great Lakes for example) Friction differences between land and water cause changes in wind speed and direction which can create low and high pressures.

Lake breezes and lake effects Heavy lake-effect snow hits Great Lakes states Moisture from the lakes produced lake-effect snow on the eastern and southern shores of the lakes

Mountain and Valley Breeze Diurnal cycle: Air on slopes is heated and is hotter than the air in the valley at the same altitude. It rises, creating “valley breeze.” Radiative cooling in the evening causes a wind reversal, and “mountain breeze.” *purple lines are isobars

Orographic Uplift Mountain Breeze forming cumulus clouds along the Grand Tetons. As mountain slopes warm during the day, air rises and often condenses into cumuliform clouds, 大提頓國家公園

Hawaii Sea Breeze Hawaii Sea Breeze and Orographic Uplift.

Katabatic Wind Cold dense air descending from elevated plateau regions through canyons and valley gaps in mountains in the arctic.

Chinook Wind Warm dry air associated with mountains. Rockies, Alps, Andes Adiabatic compression and heating of air as it descends mountains. Formed in response to low pressure systems moving air across the mountains.

Santa Ana Winds Warm dry air associated with desert mountains. Adiabatic compression and heating of air as it descends mountains. Formed in response to high pressure systems moving air across the mountains.

A blustery, dry and warm (often hot) wind that blows out of the desert A blustery, dry and warm (often hot) wind that blows out of the desert. In Raymond Chandler's story Red Wind, the title being one of the offshore wind's many nicknames, the Santa Anas were introduced as "those hot dry [winds] that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. Figure 9.35 Strong northeasterly Santa Ana winds on October 28, 2003, blew the smoke from massive wild fires across southern California out over the Pacific Ocean. The fires damaged more than 740,000 acres, destroyed over 2800 homes, and took 20 lives.

Figure 9.36 Surface weather map showing Santa Ana conditions in January. Maximum temperatures for this particular day are given in °F. Observe that the downslope winds blowing into southern California raised temperatures into the upper 80s, while elsewhere temperature readings were much lower. Fig. 9-36, p. 247

Haboob Dust Storm caused by the down draft associated with strong approaching thunderstorms.

Dust Devil Heated air rises and is funneled by circulation around obstacles that create eddies. Unstable convection at the surface

Figure 7 A huge dust storm covers Mars during July, 2001.

Nor’Easter Conditions Almost hurricane-like circulation around strong low pressure cyclones can create gale force winds and lots of precipitation (blizzards in the winter)

Science behind The storm forming off the coast of Nova Scotia, Canada, was nothing remarkable in itself. It was following the typical pattern of a "northeaster," a weather system that often affects the eastern coast of the United States and Canada during the fall and winter. However, Hurricane Grace had been collecting large quantities of energy—in the form of moisture and warm air—as it spun past the island of Bermuda. While Hurricane Grace's behavior was typical for the fall hurricane season, what was not typical was its collision with the northeaster off of Nova Scotia and the cold air behind it. When that collision took place on Tuesday, October 29, Grace donated its massive supply of prepackaged energy to the northeaster.

Local winds that originate over North Africa Figure 9.40 Local winds that originate over North Africa.