and Winds General Circulation
EAS 211 Spring 2005 01/24/05
Geostrophic Winds
Geostrophic Wind Wind flowing in a straight path, parallel to the isobars at a constant speed. Above the influence of friction (the Boundary Layer) a given parcel will accelerate toward the lower pressure due to the PGF. Coriolis force will deflect this parcel to the right (NH) The parcel will continue to accelerate until the PGF balances the Coriolis force and the parcel is said to be in geostrophic balance. Assuming geostrophic flow can usually give us a good approximation of the speed and direction of the flow in the upper atmosphere. Speed from spacing of the isobars. Closer together the PGF increases--wind speed increases and the Coriolis force will increase to maintain geostrophic balance.
Geostrophic Wind
Gradient Wind
Wind flowing at a constant speed, parallel to isobars in a curved path. Winds flow counterclockwise around a low pressure system. This is called cyclonic flow. Winds flow clockwise around a high pressure system. This is known as anti-cyclonic flow. These flows are both examples of gradient flows. The reasoning behind the nature of the flow of thesse pressure systems can be seen in the following
Upper Level Low Pressure System
PGF is greater than Cor. The difference b/ w the PGF and Cor is "net force" Centripetal force. Centripetal force keeps the wind blowing in a circular pattern.
Upper Level High Pressure System
Cor. > PGF Net force is outward Winds around a low pressure system are usually much stronger due to the increased PGF resulting from the gradient wind.
Gradient vs. Geostrophic
Geostrophic wind features no accelerations (ie. No changes in magnitude or direction.) Gradient wind does feature an acceleration This change in direction is described by the aforementioned centripetal accelearation.
Surface Winds
Winds in the BL are strongly influenced by friction Friction reduces the wind speed, which in turn reduces the Coriolis force This creates an imbalance b/w the Cor and PGF, so the wind blows across the isobras toward lower pressure. The winds near the sfc are not geostrophic due to friction Winds near sfc blow into a low and out of a high A way to remembering and applying the sfc and aloft balances we have discussed is through the Buys-Ballot's Law Stand with wind aloft to our backs, lower pressure will be to our left and higher pressure to our right in the NH Stand with our backs to the wind, then turn clockwise 30, lower pressure will be to our left.
Winds and Vertical Motion
Low Pressure System Low level convergence and upper-level divergence leads to upward vertical motion (UVM). UVM leads to unsettled wx and cloudiness. High Pressure System Reversed leads to DVM or subsidence The hydrostatic approximation:
P = - g z
Holds true except for in thunderstorms and tornadoes, b/c the UVM are many times stronger than a typical L or H, thus destroying the balance.
Wind Instruments
Cup anemometer--consists of three or more hemispheric cups mounted on a vertical shaft. Measures wind speed. The difference in wind pressure from one side of the cup to the other causes the cups to rotate around the shaft. The rate of rotation is directly proportional to the wind speed. Wind speed is received via an electrical current generated by the rotation of the cups or through a system of gears. Wind vane--measures wind direction Aerovane--measures wind speed and direction When the wind changes direction, it pushes against the tail and point the aerovane toward the wind.
Scales for Atmospheric Motions
Local Winds
Differential Heating Land and Sea Breeze Mountain Valley Breezes Country Breezes Chinook (Foehn) Winds Katabatic (Fall) Winds
Land Sea Breeze
Land heats more rapidly, air above expands and becomes less dense and rises At about 1 km AGL the air diverges , blows toward sea, sinks and return to land at low levels.
Land cools more rapidly, contracts air above. High Pressure develops over land.
Since the strongest temp gradients occur right along the ocean land boundary, the strongest winds occur at this time right along the edge of the beach and diminish inland. T contrast at night < T contrast day--stronger sea breeze Rising motion--clouds Day--land the Night--ocean
At peak of the sea breeze (late afternoon), the cool ocean air can reach depths of >300 m and extend inland 12 miles, often droping temps as much as 9 F in an hour. Warm air rising and cold air sinking--Direct Thermal Circulation (DTC)
Valley Breeze
Daytime Valley Breeze Mountain slopes oriented toward the sun (N-S) heat more rapidly than flat land or slopes oriented away from the sun. The air above the warmer slopes warms and expands upward, then diverges at higher altitudes
Mountain Breeze
Nighttime Mountain Breeze Mountain slopes cool more quickly and the cooler dense air flows from the slopes into the valleys.
General Circulation
Zonal (E-W) and Meridional (N-S) In 1735, George Hadley described a model call the single cell model in which Earth's sfc is uniformly covered The sun is always directly over the equator. (no seasonal shift of wind) The Earth does not rotate (no Cor, only PGF) Strong heating at the Equator causes air to expand upward, diverge toward both poles at upper-levels and sink back to the sfc before returning to the equator. This scenario is far too simplified to describe the earth's flow, but it does describe the "Thermally Direct Circulations" of the tropics DTC warm air rises and cool air sinks ITC cold air rises and warm air sinks
3 Cell model
Hadley Cell Solar heating along equator creates a zone of low pressure called the Intertropical Convergence Zone (ITCZ). Zone of strong UVM favoring heavy rainshowers, especially in the afternoon. Easy to identify on Satellite imagery Migrates N or S about 10 through seasons Also called the Doldrums, b/c of the hot and humid conditions. 20 -30 lat, air begins to sink forming the Subtropical Highs Descending air warms adiabatically (10C/km), suppressing cloud formation and producing desert conditions in the subtropics. Weak pressure gradients and light winds "Horse Latitudes" Strongest in the winter hemisphere, when the Temp gradient is strongest
Ferrel Cell Located b/w 30 and 60 latitude An ITC results from the circulation of the Hadley and Polar Cells Sfc air moves toward the sub-polar lows at ~ 60 latitude air rises, returns to subtropics and sinks This creates the Westerlies in the NH, the would be southerly winds are deflected by Cor. Causing winds to become westerly Polar Cell b/w 60 and 90 (the poles) DTC since warm air rises Sfc air moves from the Polar highs to sub-polar lows These are called the Polar Easterlies.
Global Atmospheric Circulation Model
Seasonal Pressure an...