Notes

Outline

Lecture 3: ATMOSPHERE (Outline)  Basic Structures and Dynamics  General Circulation in the Troposphere  General Circulation in the Stratosphere  Jetstreams

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Air Pressure and Air Density Weight = mass x gravity Density = mass / volume Pressure = force / area                    = weight / area

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Units of Atmospheric Pressure  Pascal (Pa): a SI (Systeme Internationale) unit for air pressure.      1 Pa = a force of 1 newton acting on a surface of one square                  meter       1 hectopascal (hPa) = 1 millibar (mb)  [hecto = one hundred =100] Bar: a more popular unit for air pressure.      1 bar = a force of 100,000 newtons acting on a surface of one                   square meter               = 100,000 Pa               = 1,000 hPa               = 1,000 mb One atmospheric pressure = standard value of atmospheric pressure at lea level = 1013.25 mb = 1013.25 hPa.

Air Mass and Pressure Atmospheric pressure tells you how much atmospheric mass is above a particular altitude.  Atmospheric pressure decreases by about 10mb for every 100 meters increase in elevation.

Hydrostatic Balance in the Vertical

What Does Hydrostatic Balance Tell Us?  The hydrostatic equation tells us how quickly air pressure drops wit height. èThe rate at which air pressure decreases with height (DP/ Dz) is equal to the air density (r) times the acceleration of gravity (g)

The Ideal Gas Law An equation of state describes the relationship among pressure, temperature, and density of any material. All gases are found to follow approximately the same equation of state, which is referred to as the “ideal gas law (equation)”. Atmospheric gases, whether considered individually or as a mixture, obey the following ideal gas equation:

Hydrostatic Balance and Atmospheric Vertical Structure Since P= rRT (the ideal gas law), the hydrostatic equation becomes:            dP =  -P/RT x gdz è   dP/P =  -g/RT x dz    P =  Ps exp(-gz/RT)    P = Ps exp(-z/H) The atmospheric pressure decreases exponentially with height

Temperature and Pressure

The Scale Height of the Atmosphere One way to measure how soon the air runs out in the atmosphere is to calculate the scale height, which is about 10 km. Over this vertical distance, air pressure and density decrease by 37% of its surface values. If pressure at the surface is 1 atmosphere, then it is 0.37 atmospheres at a height of 10 km, 0.14 (0.37x0.37) at 20 km, 0.05 (0.37x0.37x0.37) at 30 km, and so on. Different atmospheric gases have different values of scale height.

Thermal Energy to Kinetic Energy

Pressure Gradient Force  PG = (pressure difference) / distance Pressure gradient force force goes from high pressure to low pressure.  Closely spaced isobars on a weather map indicate steep pressure gradient.

Single-Cell Model: Explains Why There are Tropical Easterlies

Coriolis Force

Coriolis Force Change with latitudes

Coriolis Force  Coriolis force causes the wind to deflect to the right of its intent path in the Northern Hemisphere and to the left in the Southern Hemisphere. The magnitude of Coriolis force depends on (1) the rotation of the Earth, (2) the speed of the moving object,  and (3) its latitudinal location. The stronger the speed (such as wind speed), the stronger the Coriolis force. The higher the latitude, the stronger the Coriolis force. The Corioils force is zero at the equator. Coriolis force is one major factor that determine weather pattern.

How Does Coriolis Force Affect Wind Motion?

Geostrophic Balance

Another Kind of Coriolis Force The Coriolis force also causes the east-west wind to deflect to the right of its intent path in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflections are caused by the centrifugal force associated with the east-west motion, and , therefore, related to rotation of the Earth, and are also considered as a kind of Coriolis force. Although the description of the deflection effect for north-south and east-west motions are very different, their mathematical expressions are the same.

Surface Friction  Friction Force =  c * V       c = friction coefficient      V = wind speed

Frictional Effect on Surface Flow

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Balance of Force in the Horizontal

Surface Geostrophic Flow

Centrifugal Force

Gradient Wind Balance  The three-way balance of horizontal pressure gradient, Coriolis force, and the centrifugal force is call the gradient wind balance.  The gradient wind is an excellent approximation to the actual wind observed above the Earth’s surface, especially at  the middle latitudes.

Super- and Sub-Geostrophic Wind

Single-Cell Model: Explains Why There are Tropical Easterlies

Breakdown of the Single Cell è Three-Cell Model

Baroclinic Instability

Properties of the Three Cells

Atmospheric Circulation: Zonal-mean Views

The Three Cells

Thermally Direct/Indirect Cells Thermally Direct Cells (Hadley and Polar Cells)      Both cells have their rising branches over warm temperature zones and sinking braches over the cold temperature zone. Both cells directly convert thermal energy to kinetic energy. Thermally Indirect Cell (Ferrel Cell)      This cell rises over cold temperature zone and sinks over warm temperature zone. The cell is not driven by thermal forcing but driven by eddy (weather systems) forcing.

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Is the Three-Cell Model Realistic?  Yes and No!     (Due to sea-land contrast and topography)    Yes: the three-cell model explains reasonably well the surface wind distribution in the atmosphere.      No: the three-cell model can not explain the circulation pattern in the upper troposphere. (planetary wave motions are important here.)

Semi-Permanent Pressure Cells  The Aleutian, Icelandic, and Tibetan lows The oceanic (continental) lows achieve maximum strength during winter (summer) months The summertime Tibetan low is important to the east-Asia monsoon  Siberian, Hawaiian, and Bermuda-Azores highs The oceanic (continental) highs achieve maximum strength during summer (winter) months

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Sinking Branches and Deserts

Global Distribution of Deserts

Upper Tropospheric Circulation

Subtropical and Polar Jet Streams

Thermal Wind Relation

Temperature and Pressure

Thermal Wind Equation               ¶U/¶z  µ  ¶T/¶y  The vertical shear of zonal wind is related to the latitudinal gradient of temperature.  Jet streams usually are formed above baroclinic zone (such as the polar front).

Jet Streams Near the Western US

Parameters Determining Mid-latitude Weather Temperature differences between the equator and poles The rate of rotation of the Earth.

Rotating Annulus Experiment

How Cyclone Grows?

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Life Cycle of Mid-Latitude Cyclone  Cyclogenesis  Mature Cyclone  Occlusion

Cold and Warm Fronts

Tropical Hurricane The hurricane is characterized by a strong thermally direct circulation with the rising of warm air near the center of the storm and the sinking of cooler air outside.

They Are the Same Things… Hurricanes: extreme tropical storms over Atlantic and eastern Pacific Oceans. Typhoons: extreme tropical storms over western Pacific Ocean. Cyclones: extreme tropical storms over Indian Ocean and Australia.

East-West Circulation

Walker Circulation and Ocean Temperature

Walker Circulation and Ocean

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Monsoon: Another Sea/Land-Related Circulation of the Atmosphere

How Many Monsoons Worldwide?

Orbital-Scale Changes in Methane The Vostok ice record shows a series of cyclic variations in methane concentration, ranging between 350 to 700 ppb (part per billion). Each CH4 cycle takes about 23,000 years. This cycle length points to a likely connection with changes in orbital procession. The orbital procession dominates insolation changes at lower latitudes.

Trapping Gases in the Ice Air moves freely through snow and ice in the upper 15 m of an ice sheet. Flow is increasingly restricted below this level. Bubbles of old air are eventually sealed off completely in ice 50 to 100 m below the surface.

Monsoon and Methane On the 23,000-year cycle, methane variations closely resemble the variations of monsoon strength. The peak values of methane match the expected peaks in monsoon intensity not only in timing but also in amplitude. This match suggests a close connection between CH4 concentrations and the monsoon on the 23,000-year climate cycle. By why?

Earth’s Orbit and Its Variations First, Earth spins around on its axis once every day è The Tilt. Second,  Earth revolves around the Sun once a year è The shape of the Orbit. Both the tilt and the shape of the orbit have changed over time and produce three types of orbital variations:      (1) obliquity variations      (2) eccentricity variations      (3) precession of the spin axis.

Precession of Ellipse The precession of the ellipse is known as the elliptical shape of Earth’s orbit rotates itself at a slower rate than the wobbling motion of the axial precession.

Time Scales of Precession The combined effects of these two precessions cause the solstices and equinoxes to move around Earth’s orbit, completing one full 360° orbit around the Sun every 23,000 years.

The Orbital Monsoon Hypothesis The 23,000-year cycle of orbital procession increases (decreases) summer insolation and at the same time decreases (increases) winter insolation at low and middle latitudes. Departures from the modern seasonal cycle of solar radiation have driven stronger monsoon circulation in the past. Greater summer radiation intensified the wet summer monsoon. Decreased winter insolation intensified the dry winter monsoon.

How Did Monsoon Affect Methane? Orbital procession affects solar radiation at low latitudes       è solar radiation affects the strength of low-latitude monsoons       è monsoon fluctuations changes the precipitation amounts in Southeast Asia       è heavy rainfalls increase the amount of standing water in bogs       è decaying vegetation used up any oxygen in the water and creates the oxygen-free conditions needed to generate methane       è the extent of these boggy area must have expanded during wet monsoon maximum and shrunk during dry monsoon minimum.

Sea/Land Breeze

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Temperatures in Stratosphere

Ozone Distribution

Circulation in Stratosphere

Stratosphere: Circulation and Temperature

Zonal-Mean Circulation in the Stratosphere

Ozone Production and Destruction

Ozone Distribution The greatest production of ozone occurs in the tropics, where the solar UV flux is the highest. However, the general circulation in the stratosphere transport ozone-rich air from the tropical upper stratosphere to mid-to-high latitudes. Ozone column depths are highest during springtime at mid-to-high latitudes. Ozone column depths are the lowest over the equator.

Climate Variations in Stratosphere  Quasi-Biennial Oscillation (QBO)  Sudden Warming: in Northern Pole  Ozone Hole: in Southern Pole

QBO

Why QBO?

Sudden Warming  Every other year or so the normal winter pattern of a cold polar stratosphere with a westerly vortex is interrupted in the middle winter.  The polar vortex can completely disappear for a period of a few weeks.  During the sudden warming period, the stratospheric temperatures can rise as much as 40°K in a few days!

Why Sudden Warming?  Planetary-scale waves propagating from the troposphere (produced by big mountains) into the stratosphere.  Those waves interact with the polar vortex to break down the polar vortex.  There are no big mountains in the Southern Hemisphere to produce planetary-scale waves.  Less (?) sudden warming in the southern polar vortex.

Antarctic Ozone Hole The decrease in ozone near the South Pole is most striking near the spring time (October). During the rest of the year, ozone levels have remained close to normal in the region.

Why No Ozone Hole in Artic?

Polar Stratospheric Clouds (PSCs) In winter the polar stratosphere is so cold (-80°C or below) that certain trace atmospheric constituents can condense. These clouds are called “polar stratospheric clouds” (PSCs). The particles that form typically consist of a mixture of water and nitric acid (HNO3). The PSCs alter the chemistry of the lower stratosphere in two ways:       (1) by coupling between the odd nitrogen and chlorine cycles       (2) by providing surfaces on which heterogeneous reactions can occur.

The 1997 Ozone Hole

Ozone Hole Depletion Long Antarctic winter (May through September) The stratosphere is cold enough to form PSCs PCSs deplete odd nitrogen (NO) Help convert unreactive forms of chlorine (ClONO2 and HCl) into more reactive forms (such as Cl2). The reactive chlorine remains bound to the surface of clouds particles. Sunlight returns in springtime (September) The sunlight releases reactive chlorine from the particle surface. The chlorine destroy ozone in October. Ozone hole appears. At the end of winter, the polar vortex breaks down. Allow fresh ozone and odd nitrogen to be brought in from low latitudes. The ozone hole recovers (disappears) until next October.