Tuesday Nov. 13, 2012

Three  songs before class after a nice 3-day weekend (if I were in charge there'd be one 3-day weekend every month).  You heard "Marching Bands of Manhattan" (the organ at the start sounds very nice on the audio system in ILC150) and "I Will Follow You Into the Dark" from Death Cab for Cutie and "Reckoner" from Radiohead.

I spend a lot of time reading 1S1P reports over the weekend.  The "Causes of the Seasons" and the "1983 Flood" reports are done and were returned in class today.  As students reach 45 points (the maximum number allowed) I'll add their names to this list.

The "Foucault Pendulum" reports were collected today.  And there are three additional 1S1P topics available with various due dates.  These may well be the last topics of the semester.

The Scientific Paper reports should be graded by Thursday. 

Here's a map that shows all of the 3-cell model surface features.  Some of what is predicted here is actually found on the earth.  That's what we looked at briefly at the start of class.



This is the same figure included on the Quiz #4 Study Guide.  With a little practice you should be able to start with a blank sheet of paper and reproduce this figure.  Concentrate on the features found between 45 S and 45 N latitude (everything inside the dotted lines).



The 3-cell model assumes that the earth is of uniform composition and not tilted toward or away from the sun.  It predicts belts of high pressure at 30 N and 30 S latitude as shown above at left.  Because the real world has oceans and continents we find centers of high pressure, not belts, located near 30 latitude.  They move north and south of 30 degrees during the year as the N. Pole tilts toward and away from the sun.

There is a pretty nearly continuous belt of low pressure, the intertropical convergence found in the real world near the equator (it moves a little north and south of the equator depending on the time of year).  It's also usually npretty apparent on a satellite photograph (look for the band of clouds near the center of the  picture).

The H pressure centers are shown a little more clearly on the figure below (on a handout distributed in class)


The high pressure center off the East Coast of the US is called the Bermuda High.  The Pacific High is found off the west coast.  Don't worry about the names of the Highs off the east and west coasts of South America.

Winds blowing around these centers of high pressure create some of the world's major ocean currents. The California current is a cold southward flowing current found off the west coast of the US.  The Gulf Stream is the warm northward flowing current along the east coast.

We briefly mentioned the El Nino phenomenon in class last week.  Ocean water in the tropical Eastern Pacific is normally cold.  The water warms as it moves westward.  This temperature pattern is shown above.  You can now better understand why this is true.  Two cold ocean currents, the California current north of the equator and its analog in the southern hemisphere meet at the equator in the eastern Pacific.  During an El Nino event the two cold ocean currents stop short of the equator.  Warm water is able to make it way into the eastern Pacific.  The ocean water temperature pattern basically reverses.  This has a profound effect on weather around the globe.

The following two pictures show how the 3-cell models features move during the year.



This is a winter picture (northern hemisphere winter, the North Pole is tilted away from the sun).  All of the 3-cell model features have moved south of their nominal locations.  The intertropical convergence zone (ITCZ) which is normally at the equator has moved south of the equator.


Here's the summer picture (North Pole tilted toward the sun).  The ITCZ has moved north of the equator.

The movement of the Pacific High north and south of its position near 30 degrees latitude is part of what causes our summer monsoon in Arizona.



In the winter the Pacific High is found south of 30 N latitude (the bottom of the figure above).  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean is moist (though the coastal water is cold so this air isn't as moist as it would be if it came off warmer water).  Before reaching Arizona the air must travel over high mountains in California.  The air loses much of its moisture as it does this (remember the rain shadow effect).  The air is pretty dry by the time it reaches Arizona.  Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest Pacific ocean into Arizona.

 During the summer, the Pacific High moves north of 30 N latitude.  Winds on the southern side of the subtropical high have an easterly component.   Moist air originating in Mexico and from over warm water in the Gulf of Mexico blows into Arizona.  The sun heats the ground during the day, warm moist air in contact with the ground rises and produces convective thunderstorms.

Tucson gets about 12 inches of rain in a normal year.  About half of this comes during the "summer monsoon" season (July, August, and September).  Remember the word monsoon refers to a seasonal change in wind direction.

We're next going to spend a class or two on thunderstorms.  Here's a little bit of an introduction (found on p. 150 in the ClassNotes)


Thunderstorms come in different sizes and levels of severity.  We will mostly be concerned with ordinary single-cell thunderstorms (also referred to as air mass thunderstorms).  They form in the middle of warm moist air, away from front.  Most summer thunderstorms in Tucson are this type.   An air mass thunderstorm has a vertical updraft.  A cell is just a thunderstorm "unit."

Tilted updrafts are found in severe and supercell thunderstorms.  As we shall see this allows those storms to get bigger, stronger, and last longer.  The tilted updraft will sometimes begin to rotate.  We'll see this produces an interesting cloud feature called a wall cloud and tornadoes.  Supercell thunderstorms have a complex internal structure;  we'll watch a short video at some point that shows a computer simulation of the complex air motions inside a supercell thunderstorm.

We won't spend anytime discussing mesoscale convective systems except to say that they are a much larger storm system.  They can cover a large portion of a state.  They move slowly and often thunderstorm activity can persist for much of a day.  Occasionally in the summer in Tucson we'll have activity that lasts throughout the night.  This is often caused by an MCS.

The following somewhat tedious material was intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  I don't expect you to remember all of the details given below.  The figures below are more carefully drawn versions of what was done in class.


Refer back and forth between the lettered points in the figure above and the commentary below.

The numbers in Column A show the temperature of the air in the atmosphere at various altitudes above the ground (note the altitude scale on the right edge of the figure).  On this particular day the air temperature was decreasing at a rate of 8 C per kilometer.  This rate of decrease is referred to as the environmental lapse rate (lapse rate just means rate of decrease with altitude).  Temperature could decrease more quickly than shown here or less rapidly.  Temperature in the atmosphere can even increase with increasing altitude (a temperature inversion).

At Point B, some of the surface air is put into an imaginary container, a parcel.  Then a meterological process of some kind lifts the air to 1 km altitude (in Arizona in the summer, sunlight heats the ground and air in contact with the ground, the warm air becomes bouyant - that's called free convection).  The rising air will expand and cool as it is rising.  Unsaturated (RH is less than 100%) air cools at a rate of 10 C per kilometer.  So the 15 C surface air will have a temperature of 5 C once it arrives at 1 km altitude. 

Early in the morning "Mother Nature" is only able to lift the parcel to 1 km and "then lets go."  At Point C note that the air inside the parcel is slightly colder than the air outside (5 C inside versus 7 C outside).  The air inside the parcel will be denser than the air outside and the parcel will sink back to the ground. 

By 10:30 am the parcel is being lifted to 2 km as shown at Point D.  It is still cooling 10 C for every kilometer of altitude gain.  At 2 km, at Point E  the air has cooled to its dew point temperature and a cloud has formed.  Notice at Point F, the air in the parcel or in the cloud (-5 C) is still colder and denser than the surrounding air (-1 C), so the air will sink back to the ground and the cloud will disappear.  Still no thunderstorm at this point.


At noon, the air is lifted to 3 km.  Because the air became saturated at 2 km, it will cool at a different rate between  2 and 3 km altitude.  It cools at a rate of 6 C/km instead of 10 C/km.  The saturated air cools more slowly because release of latent heat during condensation offsets some of the cooling due to expansion.  The air that arrives at 3km, Point H, is again still colder than the surrounding air and will sink back down to the surface.

By 1:30 pm the air is getting high enough that it has become neutrally bouyant, it has the same temperature and density as the air around it (-17 C inside and -17 C outside).  This is called the level of free convection, Point J in the figure.

If you can, somehow or another,  lift air above the level of free convection it will find itself warmer and less dense than the surrounding air as shown at Point K and will float upward to the top of the troposphere on its own.  This is really the beginning of a thunderstorm.  The thunderstorm will grow upward until it reaches very stable air at the bottom of the stratosphere.

This was followed by a time lapse video tape of actual thunderstorm formation and growth.  I don't have a digital version of that tape, so here is a substitute time lapse of a day's worth of thunderstorm develop

The events leading up to the initiation of a summer air mass thunderstorm is summarized in the figure below.  It takes some effort and often a good part of the day before a thunderstorm forms.  The air must be lifted to just above the level of free convection (the dotted line at middle left in the picture).  Once air is lifted above the level of free convection it finds itself warmer and less dense that the air around it and floats upward on its own.  I've tried to show this with colors below.  Cool colors below the level of free convection because the air in the lifted parcel is colder and denser than its surroundings.  Warm colors above the dotted line indicate parcel air that is warmer and less dense than the surroudings.  Once the parcel is lifted above the level of free convection it becomes bouyant; this is the moment at which the air mass thunderstorm begins. 



Once a thunderstorm develops it then goes through 3 stages.


In the first stage you would only find updrafts inside the cloud (that's all you need to know about this stage, you don't even need to remember its name).

Once precipitation has formed and grown to a certain size, it will begin to fall and drag air downward with it.  This is the beginning of the mature stage where you find both an updraft and a downdraft inside the cloud.  The falling precipitation will also pull in dry air from outside the thunderstorm (this is called entrainment).  Precipitation will mix with this drier air and evaporate.  The evaporation will strengthen the downdraft (the evaporation cools the air and makes it more dense).  The thunderstorm is strongest in the mature stage.  This is when the heaviest rain, strongest winds, and most of the lightning occur.

Eventually the downdraft spreads horizontally throughout the inside of the cloud and begins to interfere with the updraft.  This marks the beginning of the end for this thunderstorm. 


The downdraft eventually fills the interior of the cloud.  In this dissipating stage you would only find weak downdrafts throughout the cloud.

Note how the winds from one thunderstorm can cause a region of convergence on one side of the original storm and can lead to the development of new storms.  Preexisting winds refers to winds that were blowing before the thunderstorm formed.  Convergence between the prexisting and the thunderstorm downdraft winds creates rising air that can initiate a new thunderstorm.