Thursday Oct. 3, 2013

Music from the Fleet Foxes this morning because the harmonies sound so good on the sound system in ILC 150.  You heard "Tiger Mountain Peasant Song", "Montezuma", and "Bedouin Dress" and "Lorelai" which is probably my favorite of the bunch.

The Experiment #1 reports have been graded and were available for pickup in class.  You can revise your report and improve your grade if you want to (a revision isn't required if you're happy with your current grade).  The revised reports are due in 2 weeks: on or before Thu., Oct. 17.  You only need to change sections where you want to earn additional credit.  Please return your original report with your revised report.

A new 1S1P Bonus Assignment dealing with Surface Weather Map Analysis is now available.  The completed analysis is due by Thu., Oct. 10. 

There's also a new Optional Assignment. 

Finally something new is being tried out for the first time in ATMO 170.  In addition to the regular online Lecture Notes, you'll find a second link with just the figures from each class (starting with last Tuesday's class) without any words (no explanation or discussion).  My hope is that you amy find that adding figure titles or captions and listing some of the key points being illustrated by each figure will be a quick and effective way of reviewing the material covered in class.

We'll be looking at the weather changes that you might expect to see ahead and behind warm and cold fronts.  We'll also be learning about the 3-D structure of warm fronts, something we haven't covered yet.

Fronts are boundaries between air masses with different temperatures (different densities) and are another way of causing rising air motions.



A 3-dimensional cross-sectional view of a cold front is shown below (we've jumped to p. 148a in the photocopied ClassNotes)


The person in the figure is positioned ahead of an approaching cold front.  Time wise, it might be the day before the front actually passes through. 

The warm air mass ahead of the front has just been sitting there and temperatures are pretty uniform throughout.  Cold fronts are found at the leading edge of a cold air mass.  The air behind the front might have originated in Canada.  It might have started out very cold but as it travels to a place like Arizona it can change (warm) considerably.  The air right behind the front will have traveled the furthest and warmed the most.  That's the reason for the cool, cold, and colder temperature bands (temperature gradient) behind the front.  The temperature behind the front should be colder than ahead of the front, but the air behind the front may not of uniform temperature.

Here are some of the specific weather changes that might precede and follow a cold front
Weather variable
 Behind
 Passing
 Ahead
Temperature
 cool, cold, colder*
warm
Dew Point
 usually much drier
may be moist (though that is often
not the case here in the desert southwest)
Winds
 northwest
 gusty winds (dusty)
 from the southwest
Clouds, Weather
 clearing
 rain clouds, thunderstorms in a
narrow band along the front
(if the warm air mass is moist)
 might see some high clouds
Pressure
 rising
 reaches a minimum
 falling

*  the coldest air might follow passage of a cold front by a day or two.  It can cool off very quickly in the cold dry cloud-free air behind a cold front. 

A temperature drop is probably the most obvious change associated with a cold front.  Here is southern Arizona, gusty winds and a wind shift are also often noticeable when a cold front passes.

The pressure changes that precede and follow a cold front are not something we would observe or feel but are very useful when trying to locate a front on a weather map.

In the next figure we started with some weather data plotted on a surface map using the station model notation.  We'll try to make a little more sense of this data.  We'll work through this example carefully because you will be doing the same thing if you decide to do the 1S1P Bonus Assignment.


In some respects fronts are like spokes on a wheel - they rotate counterclockwise around centers of low pressure.  It makes sense to first determine the location of the low pressure center.  Before trying to locate a cold front, we needed to draw in a few isobars and map out the pressure pattern.
 
Isobars are drawn at 4 mb increments above and below a starting value of 1000 mb.  Some of the allowed values are shown on the right side of the figure (992, 996, 1000, 1004, 1008 etc).  The highest pressure on the map is 1003.0 mb, the lowest is 994.9 mb.  You must choose from the allowed list of isobar values and pick only the values that fall between the high and low pressure values on the map.  Thus we need to draw in  996 mb and 1000 mb isobars.

In the figure below stations with pressures lower than 996 mb have been colored in purple.  These will be enclosed by the 996 mb contour.  Pressures between 996 and 1000 mb have been colored blue.  These stations will lie outside the 996 mb contour but inside the 1000 mb isobar.  Finally stations with pressures greater than 1000 mb have been colored green.  The 1000 mb isobar will separate the blue stations from the green stations.

The map below shows the same picture with the 996 mb and 1000 mb contours drawn in.



The next step was to try to locate the warm air mass in the picture.  We've left the isobars in the figure below but now have used colors to identify air masses with different temperatures.


Temperatures are in the 60s in the lower right portion of the map; this area has been circled in orange.  Cooler air to the west of the Low pressure center has also been identified.  Based on just the temperatures just should have a pretty good idea where a cold front would be found.


The cold front on the map seems to be properly positioned.  Note how the cold front is positioned at the leading edge of the cold air mass, not necessarily in front of the coldest air in the cold air mass.   3 of the stations from the bottom center portion of the map have been redrawn below.


The air ahead of the front (Pts. B & C) is warm, moist, has winds blowing from the S or SW, and the pressure is falling.  These are all things you would expect to find ahead of a cold front.

Overcast skies are found at Pt. B. very near the front. 

The air behind the front at Pt. A is colder, drier, winds are blowing from the NW, and the pressure is rising.  That is just what you would expect behind a cold front.  So our location of the front looks pretty good.

We've learned a fair amount about cold fronts: cross-sectional structure, weather changes that precede and follow passage of a cold front, and how to locate a cold front on a surface weather map.  We really haven't covered warm fronts yet.


We used the automobile analogy above to understand or visualize how or why an advancing mass of cold dense air is able to lift lighter low density air in front of it.
Warm fronts are more like a fleet of Volkswagens overtaking a Cadillac



The VWs are still lighter than the Cadillac.  What will happen when the VWs catch the Cadillac?





They'll run up and over (overrun) the Cadillac.



The same happens along a warm front.  The approaching warm air is still less dense than the cold air and will overrun the cold air mass. 

The back edge of a retreating cold mass has a much different shape than the advancing edge.  The advancing edge bunches up and is blunt.  The back edge gets stretched out and has a ramp like shape.  The warm air rises more slowly and rises over a much larger area out ahead of the warm front.  This is an important difference between warm and cold fronts.

Here's the 3-dimensional view again that's in the ClassNotes.


Here's a map view of a warm front.




Here are the kinds of weather changes that would precede and follow passage of a warm front.
Weather Variable
 Behind (after)
 Passing
 Ahead (before)
Temperature
 warmer
cool
Dew point
 may be moister
drier
Winds
 SW, S, SE
from the East or SE, maybe even the S
Clouds, Weather
 clearing
wide variety of clouds that may precede arrival of the front by a day or two
clouds may produce a wide variety of types of precipitation also
(snow, sleet, freezing rain, and rain)
Pressure
 rising
 minimum
 falling

Probably the key difference between warm and cold fronts (other than a cold-to-warm rather than a warm-to-cold change) is the wide variety of clouds that a warm front cause to form cover a much larger area out ahead of the front.  That's why it's highlighted in yellow above.  Clouds associated with a cold front are usually found in a fairly narrow band along the front.

Now we'll follow the same procedure that we did last Friday and try to locate a warm front on a surface weather map.


We'll start by drawing some isobars to map out the pressure pattern.  A partial list of allowed isobars is shown at the right side of the map above (increments of 4 mb starting at 1000 mb).  We've located located the highest and lowest pressure values on the map.  Then we choose allowed isobar values that fall between these limits.  In this case we'll need to draw 992 mb and 996 mb isobars.

Here's the map with color coded pressures.  Pressures less than 992 mb are purple, pressures between 992 and 996 mb are blue, and pressures greater than 996 mb are green.


Note that station B has a pressure of exactly 992.0 mb, the 992 mb isobar will go through that station.  The 996 mb isobar will go through station A because it has a pressure of exactly 996.0 mb.


Here's the map with the isobars drawn in.  On the map below we use colors to locate the warm and cooler air masses.



The warm air mass has been colored in orange.  Cooler air east of the low pressure center is blue.  Can you see where the warm front should go?


Here's the map with a warm front drawn in (the map was redrawn so that the edge of the warm (orange) air mass would coincide with the warm front).  The change in wind directions was probably more noticeable that the temperature change.  Most of the cloud outlined in green are probably being produced by the warm front.  You can see how more extensive cloud coverage is with a warm front.  Two of the stations near the right edge of the picture and on opposite sides of the front are redrawn below.

The station north of the front has cooler and drier air, winds are from the east, skies are overcast and light rain is falling.  The pressure is falling as the warm front approaches.  These are all things you'd expect to find ahead of a warm front.  Behind the front at the southern station pressure is rising, the air is warmer and moister, winds have shifted to the south and the skies are starting to clear.

Here's the picture again with an additional front drawn in.

There's pretty good evidence of a cold front on the left portion of the map.

One last picture, we go back to the map on p. 39 in the ClassNotes.


This is one of the maps that we looked at at the start of this section of material.  One of the questions we had was what might be causing the clouds, rain, and drizzle in the northeastern part of the country, and the rain shower along the Gulf Coast.  The cold front is almost certainly the cause of the rain shower and much of the wet weather in the NE is probably being caused by the warm front.

Up to this point we've been learning about surface weather maps.  Maps showing conditions at various altitudes above the ground are also drawn.  Upper level conditions can affect the development and movement of surface features (and vice versa). 

Here we'll mostly just learn 3 basic facts about upper level charts.  The Optional Assignment that I mentioned in class will go much further than we went in class.

First the overall appearance is somewhat different from a surface weather map.  The pattern on a surface map can be complex and you generally find circular (more or less) centers of high and low pressure (see the bottom portion of the figure below).  You can also find closed high and low pressure centers at upper levels, but mostly you find a relatively simple wavy pattern like is shown on the upper portion of the figure below (sort of a 3-dimensional view).  You'll find this basic picture on p. 41 in the ClassNotes.
 

A simple upper level chart pattern is sketched below (a map view).  There are two basic features: wavy lines that dip southward and have a "u-shape" and lines that bend northward and have an "n-shape".

The u-shaped portion of the pattern is called a trough.  The n-shaped portion is called a ridge.

Troughs are produced by large volumes of cool or cold air (the cold air is found between the ground and the upper level that the map depicts).  The western half of the country in the map above would probably be experiencing colder than average temperatures.  Large volumes of warm or hot air produce ridges.  You can find out why this is true by reading "Upper level charts pt. 2".


The winds on upper level charts blow parallel to the contour lines generally from west to east.  This is a little different from surface winds which blow across the isobars toward low pressure.  An example of surface winds is shown below.


That's it for this first section.  Really all you need to be able to do is
1. identify troughs and ridges,
2. remember that troughs are associated with cold air & ridges with warm air, and
3. remember that upper level winds blow parallel to the contour lines from west to east.

The next two figures weren't discussed in class.  I've included them here just to give you some idea of the interaction between weather at ground level and conditions above the ground.

Here's the earlier picture again overlaying surface and upper-level maps.



On the surface map above you see centers of HIGH and LOW pressure.  The surface low pressure center, together with the cold and warm fronts, is a middle latitude storm.

Note how the counterclockwise winds spinning around the LOW move warm air northward (behind the warm front on the eastern side of the LOW) and cold air southward (behind the cold front on the western side of the LOW).  Clockwise winds spinning around the HIGH also move warm and cold air.  The surface winds are shown with thin brown arrows on the surface map.

Note the ridge and trough features on the upper level chart.  We learned that warm air is found below an upper level ridge.  Now you can begin to see where this warm air comes from.  Warm air is found west of the HIGH and to the east of the LOW.   This is where the two ridges on the upper level chart are also found.  You expect to find cold air below an upper level trough.  This cold air is being moved into the middle of the US by the northerly winds that are found between the HIGH and the LOW. 

Note the yellow X marked on the upper level chart directly above the surface LOW.  This is a good location for a surface LOW to form, develop, and strengthen (strengthening means the pressure in the surface low will get even lower than it is now.  This is also called "deepening").  The reason for this is that the yellow X is a location where there is often upper level divergence.  Similary the pink X is where you often find upper level convergence.  This could cause the pressure in the center of the surface high pressure to get even higher.  You can read more about this in Upper level charts pt. 3.  The upper level winds could also cause the surface storm to weaken (the low pressure would get higher).
.



One of the things we have learned about surface LOW pressure is that the converging surface winds create rising air motions.  The figure above gives you an idea of what can happen to this rising air (it has to go somewhere).  Note the two arrows of air coming into the point "DIV" and three arrows of air leaving (more air going out than coming in), this is upper level divergence).  The rising air can, in effect, supply the extra arrow's worth of air.

Three arrows of air come into the point marked "CONV" on the upper level chart and two leave (more air coming in than going out = upper level convergence).  What happens to the extra arrow?  It sinks, it is the source of the sinking air found above surface high pressure.