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Transcript
Lecture 11 March 15, 2010, Monday Atmospheric Pressure & Wind: Part 2 Winds in the upper free (frictionless) atmosphere (1) geostrophic wind (straight isobars) (2) gradient wind (curved isobars) supergeostrophic wind & subgeostrophic wind Cyclones, anticyclones, troughs, ridges Winds near the surface (subject to friction) Measuring wind Geostrophic Wind Flow aloft : no friction, balance between PGF & Coriolis force Upper air moving from high to low pressure with Coriolis deflection. Flow parallel to straight isobars as PGF balances the Coriolis force. This geostrophic flow (wind) may only occur in the free atmosphere. Gradient Wind Gradient Wind Flow aloft: air flow remains parallel to curved isobars or height contours. Wind direction changes, thus there is an acceleration caused by a net force or an imbalance between PGF and the Coriolis force. Northern Hemisphere Supergeostrophic flow Subgeostrophic flow Gradient Wind Gradient wind also only occurs in the upper free atmosphere. Supergeostrophic flow is around center of high pressure. Subgeostrophic flow is around center of low pressure. Southern Hemisphere Supergeostrophic flow Subgeostrophic flow Cyclone and Anticyclone in Upper Free Atmosphere Cyclone is around center of low pressure in both NH & SH. Cyclone and Anticyclone in Upper Free Atmosphere Airflow (Clockwise) Airflow (Counterclockwise) Cyclonic flow (Southern hemisphere) Anticyclonic flow (Southern hemisphere) Anticyclone is around center of high pressure in both NH & SH. The apparent turn of flow to the left (NH) or right (SH) when airflow converges to center of low pressure is due to the change in the direction of the PGF. Coriolis force always points to the right in NH and to the left in SH. Open but curved isobars: trough (low, L) & ridge (high, H) Ridge Axis Trough Axis Surface cyclones (anticyclones) tend to gradually become troughs (ridges) in the upper atmosphere (note the offsets in horizontal locations of the centers of low and high pressures. Enclosed area of low pressure, roughly circular. Upper free air: counter-clockwise (clockwise) in the NH (SH), inward PGF is greater than outward Coriolis force which is to the right (left) in NH (SH), subgeostrophic Near the surface: air converges to the center of low pressure, crossing isobars at the left (right) in NH (SH). Cyclones Enclosed area of high pressure, roughly circular. Upper free air: clockwise (counterclockwise) in the NH (SH), outward PGF is smaller than inward Coriolis force which is to the right (left) in NH (SH), Supergeostrophic Near the surface: air diverges from the center of high pressure, crossing isobars at the right (left) in NH (SH). Anti-cyclones Cyclones (low, L) and anticyclones (high, H) defined by closed isobars in a surface weather map. Vertical motions above surface cyclones anticyclones Air moves into the center of surface cyclone (low, L) has to rise upward (clouds, precipitation, storms) Air moves out the center of surface anticyclone (high, H) is compensated by sinking air from above (clear and fair weather) An azimuth In meteorology, wind direction is defined as the direction from which wind comes. N An azimuth W indicates the degree of angle from due north moving clockwise from 0o to 360o. E S Wind Vane (wind direction) Cup Anemometer (wind speed) Three-Dimensional Propeller Anemometer (3-D velocity) (air pollution application) Right: 3-D sonic anemometer is based on Doppler principle. Below: an aerovane (1-piece) measures both wind speed and direction. ECU Geography Weather Station