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.

Stability

?

VERTICAL VERTICAL+

small-scaleHORIZONTAL

log (P)

Z

ocean

land ocean

low P

low Phigh P

high P

log (P)

Z

land

hot cold

log (P)

Z

land

ocean

Fig 5.3

Fig 5.2

VERTICAL+

synoptic-scaleHORIZONTAL

We need Coriolis tounderstand larger

circulations!

The Atmosphere in Motion: Simple to ComplexScience A-30—Course Framework

Road map to Science A-30 (Lectures 8-11): The Atmosphere in Motion

GeostrophyCyclonic flow

Highs and Lows

Friction, pressure gradients, and Coriolis force lead to circulations that give us typical weather.

Hadley Circulation—Global climate patterns

Global scale circulations arise by analogous processes, leading to climate zones, deserts, monsoons.

low pressure

high pressure

Pressure gradient forceN

S

Motion of an air parcel subjected to a north/south pressure gradient. Pt. A1, initially at rest; Pt. A3, geostrophic flow. The atmospheric mass will be redistributed to establish a pressure force balanced by the Coriolis force, and motion parallel to the isobars.

The geostrophic approximation is a simplification of complicated atmospheric motions. This approximation is applied to synoptic scalesystems 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. )

Geostrophy

For air in motion, not on the equator, not near the surface

•Coriolis Force ≈ Pressure gradient force

•Air motion is parallel to isobars

Vgeostrophic= DP∆

Ω )sin(21

λρ

Vg geostrophic wind (m/s)Ω 7.29 10-5 radian/sλ latitudeD distance (m)∆P pressure diff. (N/m2)

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 low-pressure 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

cold

warm

heat transport by a meandering jet stream

http://twister.sfsu.edu/courses/metr200/handouts/jetstream.html

cold

Warm moist

Occlusion

Structure of a midlatitude cyclone

log (P)

Z

ocean

land ocean

low P

low Phigh P

high P

log (P)

Z

land

hot cold

log (P)

Z

land

ocean

Fig 5.3

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 Hthan 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.

warmcool

log (P)

Z

ocean

land ocean

low P

low Phigh P

high P

log (P)

Z

land

hot cold

log (P)

Z

ocean

land

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 Hthan 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.

coolwarm

Z

ln(P)

Pressure anomaly scale (mb)

land

sea

warmwarm coldcold

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.

Infrared composite/global moll

ITCZ

global upwelling

hurricanes (2)

6 Sep 96

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.

http://www.geog.ouc.bc.ca/physgeog/contents/7q.html

driven by heating

driven by cooling

"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.

Walker Circulation

Z

ln(P)

Pressure anomaly scale (mb)

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

Infrared composite/global moll

ITCZ

global upwelling

hurricanes (2)

6 Sep 96

polar cold front

http://vortex.plymouth.edu/mollsat_ir_an.gif