Lecture 13 March 24, 2010, Wednesday. Atmospheric Pressure & Wind: Part 4

Transcription

1 Lecture 13 March 24, 2010, Wednesday Atmospheric Pressure & Wind: Part 4 Synoptic scale winds Mesoscale winds Microscale winds Air-sea interactions

2 The largest synoptic scale wind. Monsoon: Arabic for season. Monsoons Often incorrectly referred to only as heavy rainy summer season. Actually refer to the climatic pattern of seasonal reversal of winds due to seasonal thermal differences between large continents and large water bodies (high and low pressure cells), and heavy precipitation alternates with dry conditions on an annual basis. Best exemplified by the Asian monsoon which is characterized by alternating offshore (onshore) flow with dry (wet) conditions during cool (warm) months

3 Asian Monsoons Winter: northeast winds from Himalayas are compressed, warmed and dry, these offshore winds are maintained by the subsidence due to convergence of subtropical jet streams in the upper air. Summer: heating of continent leads to convergence at the surface and onshore winds which draw moist air northward, with strong orographic lifting, heavy rain falls over south of the continent. Monsoon Low (Depression)

4

5 Arizona (Southwest, Mexican) Monsoon Little like the Asian Monsoon: much less rainfall amount due to scattered thunderstorms (dessert climate), no west-east tall mountain barrier, smaller continent of North America

6 Chinook (Rockies), Foehn (Alps), and Santa Ana (California) winds: Synoptic Scale Winds Leeward side Sinking air is heated by compression Foehn: mid-latitude cyclones pass the Alps from southwest Chinooks: similar winds on the eastern side of the Rocky Mountains, form when low pressure systems occur east of the mountains Both Foehn and Chinook winds are most common in winter Santa Ana winds: occur during transitional seasons (most common in the Fall, but also in spring), when high pressure develops over the Rockies, descending air down the western slopes, often spread wildfires

7 A foehn wind or föhn wind is a type of dry down slope wind which occurs in the lee of a mountain range. It is a rain shadow wind which results from the subsequent adiabatic warming of air which has dropped most of its moisture on windward slopes. As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes. Föhn winds can raise temperatures by as much as 30 C (54 F) in just a matter of hours. Central Europe enjoys a warmer climate due to the Föhn.

8 Adiabatic warming of downward moving air produces the warm Chinook wind

9

10 The Santa Ana winds in Southern California sweep down across the deserts and across the Los Angeles Basin pushing dust and smoke from wildfires far out into the Pacific Ocean.

11 (Synoptic to Mesoscale) Locally chilled cold dense air in highland areas (such as Antarctic and Greenland ice sheets) falls to the lower elevations

12 Katabatic wind spilling off an ice shelf Katabatic wind in Antarctica Coastal polynyas are produced in the Antarctic by katabatic winds

13 Sea and Land Breezes Daily scale temperature gradient Sea breeze land warms: thermal low Land breeze land cools: thermal high In principle, lake breeze is similar to sea breeze

14 A sea-breeze front is a weather front created by a sea-breeze, as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, cumulonimbus clouds, the front can sometimes trigger thunderstorms.

15

16 Sea and Land Breezes Sea breezes moderate coastal temperatures Sea breezes begin around noon and reach kph in mid-afternoon Sea breezes have the largest influence in the tropics near cold ocean currents

17 Mountain and Valley Breezes (Mesoscale) Daytime: air is heated more over mountain slopes than valley floor (solar angle), air glides up mountain slope: valley breeze Nighttime: air is cooled more on the slopes than over valley floor, dense cold air drains down the slope: mountain breeze

18 Country Breeze (Mesoscale) Heat Island Cool country Cool country

19 Major Wind Systems: Microscales Dust Devils

20

21 A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small (half a meter wide and a few meters tall) to large (over 10 meters wide and over 1000 meters tall). The primary vertical motion is upward. Dust devils are usually harmless, but rare ones can grow large enough to threaten both people and property. They are comparable to tornadoes in that both are a weather phenomenon of a vertically oriented rotating column of air. Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a supercell thunderstorm. Dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.

22 Microscale Winds Turbulence, dust devils, gusts Dust Devil vs. Tornado (Mesoscale) Dust devils form on clear days Tornadoes form in thunderstorms Dust devils develop from ground up Tornadoes develop from cloud down Dust devils rarely reach wind speeds of 60 mph, and most are short-lived (few minutes) and reach heights of only 100 meters Tornadoes generate winds as high as 280 mph

23 U.S. Near-Surface Ocean Circulation Gyers Gulf stream Ekman spiral Upwelling SST: Sea Surface Temperature During El Nino Air-Sea Interactions El Nino La Nina ENSO

24 The Gulf Stream Satellite Image Reds-oranges: o C Yellows-greens: o C Blues: o C Purples: 2-9 o C

25 Near-Surface Ocean Currents Gyre Gyre cold currents - west coasts of continents warm currents - east coasts of continents

26 Northern hemisphere Ekman spiral Southern hemisphere 100 m Surface ocean currents driven by wind stress (drag force) which is deflected by Coriolis force, surface ocean water moves at a 45 o angle to the right (left) in the northern (southern) hemisphere of the surface wind Current speed decreases & direction turns increasingly towards right (left) with depth in N.H. (S.H.), 180 o at ~ 100 m depth, current dies out.

27 Upwelling Strong winds drag surface waters away from coastal locations. Colder, nutrient rich waters from the deep ocean rise, or upwell, to replace these waters. Most pronounced off western coast of South America as cold water upwelling ensures the driest desert on Earth, the Atacama. Southern California, hot & dry Santa Ana winds bring people to the beach but cold water from upwelling keeps people out of water. warm cold South America thermocline

28 Vertical profile of ocean water temperature Mixing zone: also called surface layer, driven by winds, warmest (by the Sun) Thermocline transition zone: thermocline layer, friction & viscosity dampens mixing effect temperature rapidly decreases with increasing depth greatest vertical temperature gradient Deep cold zone: or deep water, dark and high pressure make it hard to study it, freezes at ~ 2 o C

29 Upper Air Circulation at the Equator Easterly Westerly Pacific Ocean

30 El Nino

31 Air-Sea Interactions El Nino Spanish for boy child, it tends to occur near Christmas season, so named in reference to the Christ child. In normal condition, trade winds move warm ocean surface waters near the equator westward, as does the Walker Circulation at equator. western Pacific: higher temperature and sea level, lower surface pressure, increase in convective precipitation eastern Pacific: upwelling of cold ocean water, cooler air temperature, higher surface pressure, dry condition. El Nino develops when trade winds weaken or even reverse direction to flow eastward, and warm water moves eastward. unusual warm ocean surface water in eastern equatorial Pacific Ocean.

32 Normal Walker Circulation El Nino

33 La Nina Conditions Walker Circulation Sea surface skin temperature anomalies in November 2007 showing La Niña conditions El Nino

34 Air-Sea Interactions Southern Oscillation, ENSO, La Nina Southern Oscillation: pressure pattern seesaw in tropical Pacific winds shift from easterly to westerly irregular occurrence 2-5 years El Nino & Southern Oscillation are linked ENSO La Nina (Spanish for girl child): opposite to El Nino, strong easterly trade wind, unusually cold ocean surface water in eastern Pacific along the coast of South America (very dry winter).

35 During El Niño, increased precipitation in California due to a more southerly, zonal, storm track. Increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, polar jet stream. The subtropical jet stream across the deep tropics of the Northern Hemisphere is enhanced due to increased convection in the equatorial Pacific, which decreases tropical cyclogenesis within the Atlantic tropics below what is normal, and increases tropical cyclone activity across the eastern Pacific. Across North America during La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track and jet stream. The storm track shifts far enough northward to bring wetter than normal conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers. Across the North Atlantic, the jet stream is stronger than normal, which directs stronger systems with increased precipitation towards Europe. El Nino