本科氣象雙語標準課件第三章

1、Simplified one-cell global air circulation patterns,simplifying assumptions : The Earth is not rotating in space. The Earths surface is composed of similar materials. So, a temperature gradient of hotter air at the equator and colder air at the poles each hemisphere contains one three-dimensional ci

2、rculation cell,Surface air flow is from the poles to the equator. When the air reaches the equator, it is lifted vertically by the processes of convection and convergence. When it reaches the top of the troposphere, it begins to flow once again horizontally from the equator to the poles. At the pole

3、s, the air in the upper atmosphere then descends to the Earths surface to complete the cycle of flow.,Three cell model of global circulation,Planetary rotation cause the development of three circulation cells. Hadley cell; Ferrel cell; and Polar cell.,The equator air rises into the upper atmosphere

4、and then begins flowing horizontally to the Poles.,Coriolis force causes the deflection of this moving air in the upper atmosphere, and by about 30 of latitude the air begins to flow from west to east, which causes the accumulation of air in the upper atmosphere. To compensate for this accumulation,

5、 some of the air in the upper atmosphere sinks back to the surface creating the subtropical high pressure zone.,From this zone, a portion of the air moves back toward the equator completing the circulation system known as the Hadley cell. This moving air is also deflected creating the Northeast Trad

6、es and Southeast Trades.,The surface air moving towards the poles from the subtropical high zone is also deflected producing the Westerlies.,On the Earths surface at 60 latitude, the subtropical Westerlies collide with cold air traveling from the poles, which results in frontal uplift and the creati

7、on of the subpolar lows.,A small portion of this lifted air is sent back into the Ferrel cell after it reaches the top of the troposphere. Most of this lifted air is directed to the polar vortex where it moves downward to create the polar high.,Solar influence on the movement of the Earths global ci

8、rculation patterns,Actual Global Surface Circulation,Average circulation patterns differ somewhat from the three-cell mode. The two surface materials that dominate are water and land. Land heats and cools about four times faster than water, thus causing latitudinal pressure zones to be less uniform.

9、,Subtropical High Pressure-horse latitudes Each of the 5 basins has a large high-pressure centered at about 30 of latitude. Average diameter of perhaps 3200km, usu. elongated e-w. The ridges are significantly broken up over the continents, esp. in the summer when high temperatures produce lower air

10、pressure. Within the STHs, the weather is nearly always clear, warm, and calm.,Trade winds Between about 25N and 25S Persistent to direction (NE in the NH and SE in the SH) and velocity (force scale 3-4) Originate as warming, drying winds which are capable of holding an enormous amount of moisture.,

11、Intertropical Convergence Zone, doldrums, or intertropical front The ITCZ is found south of the equator In Jan. while in July, north of it. Bends in the line occur because of the different heating characteristics of land and water. The extreme movement helps to intensify the development of the Asian

12、 monsoon. Weak and erratic winds. It is a globe-girding zone of low pressure, associated with instability and rising air.,The Westerlies Between about 30and 60both NH and SH. Roaring Forties in the SH Experiences short-run variable weather,Subpolar Low-Pressure zone The meeting of the warm subtropic

13、al and cold polar air masses enhances frontal uplift and the formation of intense low pressure systems. in the SH at a latitude of between 50 and 70 , in the NH, 30 and 70 ,Polar Easterlies Polar High Pressure,Atmospheric center of action Aleutian Low/Hawaii high Icelandic Low/Azores High Asian (Sib

14、erian) High Asian (Indian)Low North America High/low South Pacific/Atlantic/Indian Ocean High Australia/South Africa/America High/Low,Atmospheric center of action Aleutian Low/Hawaii high Icelandic Low/Azores High Asian (Siberian) High Asian (Indian)Low North America High/low South Pacific/Atlantic/

15、Indian Ocean High Australia/South Africa/America High/Low,Monsoon,The seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that a

16、rrive with the southwest flow across southern India. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States. Mechanisms Heat Difference between Continents and OceansCoriolis Force by Rotation of the Earth.,E

17、ast Asian Nly winter monsoon,The Nly Winter Monsoon is a result of the strong high pressure cells which dominate Siberia in the winter. As high pressure cells migrate southeastward, they result in a strong Nly wind flow over eastern Asia and adjacent waters. The winter monsoon season dissipates as t

18、he cold highs weaken during the spring, resulting in the migration of the Polar Front north to its summer position. NW-N (30)-NE,East Asian Sly summer monsoon,Conversely, during summer months, thermal lows develop over Asia, creating the Sly Summer Monsoon. Warm, moist air is transported northward o

19、ver the coastal areas of Asia. The Sly Monsoon of Asia is less intense than that of the Indian Ocean. Gale force winds seldom occur and then only when associated with strong flow into a tropical cyclone or with channeling through constricted waters such as in the Taiwan-Luzon Straits. SE-SW,Indian m

20、onsoon-NE,In the winter (Nov.-Apr.), the relative cooling of the Indian plateau and Middle East with respect to the fairly stable ocean temperature establishes a low level pressure gradient between the land and water along the south coast of Asia with resultant off-shore winds.,The Himalayan- Hindu

21、Kush and Iranian mountain barriers prevent the extremely cold air of Central Asia from penetrating into the area. Thus, the Northeast Monsoon is relatively weak (scale 3-4).,Indian Monsoon-SW,In May, when the transition from the winter to Summer Monsoon is taking place, the pre-monsoon southeast flo

22、w is deflected to the right as it crosses the equator. This additive effect of cross equatorial flow combined with the initial acceleration of the flow along the deserts of Africa results in the Summer Monsoon being much stronger, with winds often reaching gale force during the peak of the Southwest

23、 Monsoon.,At peak of monsoon, July and August, average swell heights in the Arabian Sea often exceed 18 ft (5.5 m) off the Somalia coast.,Jan.,July,Localized Winds-Sea and Land Breezes,Sea and land breezes are types of thermal circulation systems that develop at the interface of land and ocean. Sea

24、breeze usually begins in midmorning and reaches its maximum strength in the later afternoon when the greatest temperature and pressure contrasts exist. It dies down at sunset. As night wind flow moves from the land to the open ocean.,Localized Winds-Mountain and Valley Breezes,Common in regions with

25、 great topographic relief. A valley breeze develops during the day.,As the air heats it begins to flow gently up the valley sides. Vertical ascent of the air rising along the sides of the mountain is usually limited by the presence of a temperature inversion layer. When the ascending air currents en

26、counter the inversion they are forced to move horizontally and then back down to the valley floor. This creates a self-contained circulation system. If conditions are right, the rising air can condense and form into cumuliform clouds.,Localized Winds-Mountain and Valley Breezes,Convergence of the dr

27、aining air occurs at the valley floor and forces the air to move vertically upward. The upward movement is usually limited by the presence of a temperature inversion which forces the air to begin moving horizontally. This horizontal movement completes the circulation cell system.,During the night, t

28、he air along the mountain slopes begins to cool quickly because of long wave radiation loss . As the air cools, it becomes more dense and begins to flow down slope causing a mountain breeze.,Google “wind names” (more than 70 local winds),Bora: A cold, northerly wind blowing from the Hungarian basin

29、into the Adriatic Sea. Cape Doctor: The strong southeast wind which blows on the South African coast. Also called the DOCTOR. Elephanta: A strong southerly or southeasterly wind which blows on the Malabar coast of India during the months of September and October and marks the end of the southwest mo

30、nsoon. Gregale: A strong northeast wind of the central Mediterranean. Norther: A cold strong northerly wind in the Southern Plains of the United States, especially in Texas, which results in a drastic drop in air temperatures. Also called a Blue Norther. Papagayo: A violet northeasterly fall wind on

31、 the Pacific coast of Nicaragua and Guatemala. It consists of the cold air mass of a norte (north) which has overridden the mountains of Central America. ,Areas and seasons of strong winds at sea,Gale wind (7 Bft scale) frequency in January,In the winter, the Aleutian Low and the Icelandic low devel

32、op in the North Pacific ocean and North Atlantic ocean and the most favorable conditions for vigorous frontal activity are concentrated along the east coasts of North America and Asia. Therefore, gale wind frequency is high and it is widely distributed in the ocean. In the waters north of 30N, the f

33、requency of winds force7 scale is 10%20%. The gale wind frequency is higher in North Atlantic than that of the North Pacific.,Areas and Seasons of Strong Winds at Sea,Gale wind (7 Bft scale) frequency in July,In the summer, the south-west monsoon is so strong that the wind frequently reaches Beaufor

34、t scale 8 to 9 in the North Indian Ocean. In the southern hemisphere oceans, for waters south of 30S, gale winds may be encountered through the year. The scope of high frequency of gale wind is larger in winter. For waters around Cape of Good Hope, the winds are even stronger due to the terrain effect.,Number of days of fog per year,General distribution of sea fog,Advection fog mainly occurs in the cold water areas where the warm current meets th