Summary of Lecture 10, 04 March 2008 Introduce the Hadley circulation and examine global weather patterns. Discuss jet stream dynamics jet streams arise because the Coriolis force prevents Hadley-type circulations from transporting heat poleward of about 30 degrees of latitude. Steep temperature gradients develop, and the geostrophic balance sets up very strong winds and shear. These "jet stream" winds are not confined, they meander. They transport heat as they do this. They also bring us much of the weather system activity in the middle latitudes. Examine the "Walker Circulation", a global-scale set of upwelling and subsidence regions along the equator that look like giant "sea breeze" circulations, since the Coriolis force does not come into play. The El Niño oscillation is an ocean-atmosphere phenomenon involving the shifting of the Walker circulation, with global climatic effects. Teleconnections : A close look at weather patterns shows the connections between tropics, middle latitudes, jet streams, Hadley, and Walker circulations.
VERTICAL Stability The Atmosphere in Motion: Simple to Complex Science A-30 Course Framework VERTICAL + small-scale HORIZONTAL VERTICAL + synoptic-scale HORIZONTAL? Z land Z ocean log (P) log (P) high P low P low P hot land high P ocean cold Fig 5.2 land Z ocean log (P) Fig 5.3 We need Coriolis to understand larger circulations!
Road map to Science A-30 (Lectures 8-11): The Atmosphere in Motion Geostrophy Cyclonic flow Highs and Lows Hadley Circulation Global climate patterns Friction, pressure gradients, and Coriolis force lead to circulations that give us typical weather. Global scale circulations arise by analogous processes, leading to climate zones, deserts, monsoons.
low pressure N Pressure gradient force high pressure Motion of an air parcel subjected to a north/south pressure gradient. Pt. A 1, initially at rest; Pt. A 3, geostrophic flow. The atmospheric mass will be redistributed to establish a pressure force balanced by the Coriolis force, and motion parallel to the isobars. S
Geostrophy For air in motion, not on the equator, not near the surface Coriolis Force Pressure gradient force Air motion is parallel to isobars The geostrophic approximation is a simplification of complicated atmospheric motions. This approximation is applied to synoptic scale systems and circulations, roughly 1000 km. (It is easiest to think about measuring the pressure gradient at a constant altitude, although other definitions are more rigorous. ) V geostrophic = 1 2Ω ρ sin( λ) P D V g geostrophic wind (m/s) Ω 7.29 10-5 radian/s λ latitude D distance (m) P pressure diff. (N/m 2 )
Circulation of air around regions of high and low pressures in the Northern Hemisphere. Upper panel: A region of high pressure produces a pressure force directed away from the high. Air starting to move in response to this force is deflected to the right (in the Northern Hemisphere), giving a clockwise circulation pattern. Lower panel: A region of low pressure produces a pressure force directed from the outside towards the low. Air starting to move in response to this force is also deflected to the right, rotating counter-clockwise. Directions of rotation of the wind about high or low centers are reversed in the Southern Hemisphere, as explained earlier in this chapter.
The effect of friction around a high pressure region is to slow the wind relative to its geostrophic velocity. This causes the pressure force to slightly exceed the Coriolis force. The three forces add together as shown in the figure. Air parcels gradually drift from higher to lower pressure, in the case shown here, from the center of a high pressure region outward. An analogous flow (inward) occurs in a lowpressure region.
Air converges near the surface in low pressure centers, due to the modification of geostrophic flow under the influence of friction. Air diverges from high pressure centers. At altitude, the flows are reversed: divergence and convergence are associated with lows and highs respectively, closing the circulation through analogous processes noted in the sea breeze example
October 8, 1996
winds, pressure field, and weather
Near surface circulation around a low pressure area March 7, 2006. Jet Stream
March 6, 2008
http://twister.sfsu.edu/courses/metr200/handouts/jetstream.html cold warm heat transport by a meandering jet stream
Structure of a midlatitude cyclone Occlusion Warm moist cold
Z high P low P land hot land log (P) warm Z Z land ocean ocean log (P) low P high P ocean cool cold Fig 5.2 Illustration of the sea breeze, showing the circulation and the relative pressures in the horizontal direction, near the ground and aloft. The land heats up during the daytime, but the sea does not. Due to the higher temperature, the atmosphere over the land has lower density and a larger scale height H than over the sea. The lower density makes air over the land buoyant relative to air over the sea, and it rises. The larger scale height makes the pressure aloft higher over the land than over the sea, causing mass to be transferred from land to the sea at altitude. The associated addition of mass to the air column over the sea raises the pressure at the sea surface, setting up the distribution of high and low pressure and giving rise to the circulation shown. log (P) Fig 5.3
Z high P low P land hot land log (P) cool Z land Z ocean ocean log (P) low P high P ocean warm cold Illustration of the land breeze, showing the circulation and the relative pressures in the horizontal direction, near the ground and aloft. The land cools off by radiation of heat to space during the nighttime, but the sea cools much less. Due to the lower temperature, the atmosphere over the land has higher density and a smaller scale height H than over the sea. The lower density air over the sea becomes buoyant relative to air from the land, and it rises. The larger scale height makes the pressure aloft higher over the r the sea than over land, causing mass to be transferred from sea to the land at altitude. The associated addition of mass to the air column raises the pressure at the land surface, setting up the distribution of high and low pressure and giving rise to the circulation shown. log (P)
cold warm cold warm land Pressure anomaly scale (mb) Z ln(p) sea
cold warm
The general circulation of the atmosphere as envisioned by Hadley in 1735: a vast "sea breeze", with rising motion over the equator and sinking motions over the poles. Hadley wanted to explain why sailors encountered westerly winds at midlatitudes and easterly ("trade winds") near the equator. He deduced that this trend was caused by rotation of the earth.
hurricanes (2) 6 Sep 96 Infrared composite/global moll ITCZ global upwelling polar cold front
Schematic picture of the General Circulation of the atmosphere, showing the Hadley circulation between ±30 latitude and the effect of the Coriolis force on the return flow at the surface, giving rise to the easterly trade winds in the tropics. The westerlies at middle latitudes are depicted as arising from the high-latitude branches of the subtropical high pressure regions. The mid-latitude or polar jet streams are shown occupying the regions of strongest temperature and pressure gradients at the mid-latitude/polar latitude boundaries.
driven by cooling http://www.geog.ouc.bc.ca/physgeog/contents/7q.html driven by heating
Walker Circulation "Normal" Walker circulation The warmest water in the Pacific surrounds Indonesia at the equator. Buoyancy drives rising motion there, sinking over cold water at the equator near Central and South America. Trade winds are strong, tending to push warm water towards Indonesia. "El Niño conditions" Warm water moves to the West Pacific, and the circulation reverses. Drought occurs over Indonesia and Eastern South America. Weak or reversed trade winds push warm water east.
Pressure anomaly scale (mb) Z ln(p)
Schematic picture of the General Circulation of the atmosphere, showing the Hadley circulation between ±30 latitude and the effect of the Coriolis force on the return flow at the surface, giving rise to the easterly trade winds in the tropics. The westerlies at middle latitudes are depicted as arising from the high-latitude branches of the subtropical high pressure regions. The mid-latitude or polar jet streams are shown occupying the regions of strongest temperature and pressure gradients at the mid-latitude/polar latitude boundaries.
winds, pressure field, and weather
hurricanes (2) 6 Sep 96 Infrared composite/global moll ITCZ global upwelling polar cold front
http://vortex.plymouth.edu/mollsat_ir_an.gif