Tue., Apr. 17 notes

Here's a view of the surface features found between 30 S and 30 N latitude from above. This is the material you should become familiar with before next week's quiz. Here's a link to the satellite photograph that I showed in class. To show the whole globe images from several weather satellites are blended together. You can usually see the ITCZ pretty clearly on the satellite photo, it is the band of clouds found near the center of the picture. Clouds are mostly absent to the north and south of the ITCZ. This is the region of the subtropical highs, the sinking air keeps clouds from forming.

Here's a sketch of surface features found from about 15 N to 75 N laitude. We didn't cover this in class but I'll insert it here anyways. Other than the prevailing westerlies you don't need to worry about these high latitude features for the quiz.

Winds blowing north from H pressure at 30 N toward Low pressure at 60 N turn to the right and blow from the SW. These are the "prevailing westerlies." The polar easterlies are cold winds coming down from high pressure at the north pole. The subpolar low pressure belt is found at 60 latitude. This is also a convergence zone where the cold polar easterly winds and the warmer prevailing westerly winds meet. Because the air masses south and north of 60 latitude are so different, the boundary between these two different kinds of air is called the polar front and is often drawn as a stationary front on weather maps. A strong current of winds called the polar jet stream is found overhead. Strong middle latitude storms will often form along the polar front.

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.

The following picture might show this more clearly (this was on a class handout distributed in class)

The high pressure center found off the west coast of the US is called the Pacific High. The Bermuda High is found off the east coast. Don't worry about the names of the high pressure centers found in the southern hemisphere.

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.

The 3-cell model predicts a belt of low pressure near 60 latitude. In the real world we find centers of low pressure: the Aleutian Low off the west coast of Canada and the Icelandic Low off the east coast. These weren't mentioned in class. A true belt of low pressure is found near 60 latitude in the southern hemisphere. This is because that part of the globe is mostly just ocean

The figure above shows the intertropical convergence zone (colored pink) south of the Equator. This happens during the northern hemisphere winter. The ITCZ and the other features move northward in summer. This can be seen by comparing the figure above with the one below.

The movement of the Pacific High north and south of its nominal 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. 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 southhern 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. Remember the word monsoon refers to a seasonal change in wind direction.

The close proximity of the Pacific high, with its sinking air motions, is what gives California, Oregon, and Washington dry summers.

Speaking of thunderstorms, that's where we're going next. 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 fraction 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.

"Mother Nature" lifts 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.

The picture below shows some of the features at the base of a thunderstorm.

The cold downdraft air spilling out of a thunderstorm hits the ground and begins to move outward from underneather the thunderstorm. The leading edge of this outward moving air is called a gust front. You can think of it as a dust front because the gust front winds often stir up a lot of dust here in the desert southwest (see below).

The gust front in this picture (taken near Winslow, Az) is moving from the right to the left. Visibility in the dust cloud can drop to near zero which makes this a serious hazard to automobile traffic. Dust storms like this are sometimes called "haboobs".

There's lots of video on YouTube of an impressive dust storm this past summer. Here's an example from Gilbert Arizona. Another from South Mountain.

A narrow intense downdraft is called a microburst. At the ground microburst winds will sometimes reach 100 MPH (over a limited area); most tornadoes have winds of 100 MPH or less. Microburst winds can damage homes (especially mobile homes that aren't tied to the ground), uproot trees, and seem to blow over a line of electric power poles at some point every summer in Tucson.

Microbursts are a serious threat to aircraft especially when they are close to the ground during landing or takeoff. An inattentive pilot encountering headwinds at Point 1 might cut back on the power. Very quickly the plane would lose the headwinds (Point 2) and then encounter tailwinds (Point 3). The plane might lose altitude so quickly that it would crash into the ground before corrective action could be taken. Microburst associated wind shear was largely responsible for the crash of Delta Airlines Flight 191 while landing at the Dallas Fort Worth airport on Aug. 2, 1985 (click here to watch a simulation of the final approach into the airport, I don't show it in class because it contains some of the actual cockpit communications).

Falling rain could warn of a wet microburst (see photo below). In other cases, dangerous dry microburst winds might be invisible (the virga, evaporating rain, will cool the air, make the air more dense, and strengthen the downdraft winds).

This is as far as we got in class on Tuesday. There were just a couple of things remaining and I have added them below. I'll go over these briefly at the start of class on Thursday.

Just to hammer home the idea of what a gust front might look like I showed another (the last) of my homemade videos (not shown in class on Tuesday)

A large plastic tank was filled with water, the water represents air in the atmosphere. Then a colored mixture of water and glycerin, which is a little denser than water, is poured into the tank. This represents the cold dense air in a thunderstorm downdraft. The colored liquid sinks to the bottom of the tank and then spreads out horizontally. In the atmosphere the cold downdraft air hits the ground and spreads out horizontally. These are the strong winds that can reach 100 MPH. The video is on VHS tape so I can't put it online (you aren't missing much).

Here are three microburst videos from YouTube (the second two were shown in class). The first video shows a microburst from some distance away. The second video was taken in the heavy rain and strong winds under a thunderstorm in the microburst. You'll see a power pole snapped in half by the microburst winds at about 2:26 in the video. Here's a third video of a microburst that hit Princeton KS in July 2009. Someone watching the storm estimated the winds were at least 90 MPH. Try to imagine being caught outdoors during the last video, you would have difficulty walking. And if there were any debris being blown around by the winds you'd be at some risk of serious injury.

The following picture shows a shelf cloud (a thunderstorm cloud feature mentioned earlier in the semester when we were learning to name clouds).

Warm moist air if lifted by the cold air behind the gust front which is moving from left to right in this picture. The shelf cloud is very close to the ground, so the warm air must have been very moist because it didn't have to rise and cool much before it became saturated and a cloud formed. Here are a couple of pretty good videos (Grand Haven, MI and Massillon, OH)

We'll spend most of the day on tornadoes in class on Thursday though we will learn a little bit more about thunderstorms (tornadic thunderstorms) in the process.