Friday Feb. 14, 2014

My attempt at some music for Valentine's Day: "Amado Mio" and "Let's Never Stop Falling in Love" from Pink Martini.

The Experiment #2 materials were available for checkout for the first time today.  There are perhaps a dozen sets of materials remaining that you will have a chance to checkout next Monday.

We'll be looking at the 3-dimensional structure and weather changes associated with cold and warm fronts.  We'll also take a stab at locating a cold front on a surface weather map.
Fronts are boundaries between air masses with different densities that are brought into contact by the air motions around surface centers of low pressure.  The density differences are usually caused by temperature differences.  Fronts are also another way of causing air to rise (actually the 3rd process mentioned so far this semester). 
Rising air is important because rising air expands and cools.  If you cool moist air enough clouds can form. 

The four processes that cause rising air motions are shown above.  This figure wasn't shown in class. 


The storm system above is referred to as a middle latitude storm or extratropical cyclone.  Cyclone just means winds spinning around a low pressure core.  Tornadoes are sometimes called cyclones.  Extratropical just means outside the tropics.  Hurricanes form in the tropics and are called tropical cyclones.

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.  There are 3 fairly important features to notice in this picture.

1.    The front edge of the advancing mass of cold air has a rounded shape.






A vertical slice through a cold front is shown below at left.  Friction with the ground causes the front edge to "bunch up" and gives it the blunt shape it has.  You'd see something similar if you were to pour something thick and gooey on an inclined surface and watch it roll downhill.

2. Because it is denser, the cold air lifts the warm air out of the way.




The cold dense air mass behind a cold front moves into a region occupied by warm air.  The warm air has lower density and will be displaced by the cold air mass.  In some ways its analogous to a big heavy Cadillac plowing into a bunch of Volkswagens.

You're in a better position to appreciate a video recording of the cold front passing through Tucson.  The first video was a time lapse movie of a cold front that came through Tucson on on Easter Sunday morning, April 4, 1999.  Click here to see the cold front video (it may take a minute or two to transfer the data from the server computer in the Atmospheric Sciences Dept., be patient).  Remember this is a time lapse movie of the frontal passage.  The front seems to race through Tucson in the video, it wasn't moving as fast as the video might lead you to believe.  Cold fronts typically move 15 to 25 MPH. 

The 2nd video was another cold front passage that occurred on February 12, 2012.

3.    Note the cool, cold, colder bands of air behind the cold front.


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.
**nighttime temperatures drop much more quickly in dry air than in moist or cloudy air.  This is part of the reason it can get very cold a day or two after passage of a cold front.

Gusty winds and a shift in wind direction are often one of the most obvious change associated with the passage of a cold front in Tucson.

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 and eventually locate a cold front.  Study this example carefully because you will be doing the same thing on an upcoming assignment.

Step #1
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.

Step #1 cont'd
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.

End of Step #1
The map below shows the same picture with the 996 mb and 1000 mb contours drawn in.




Step #2
The next step was to try to locate the warm air mass in the picture.  I'll start with a new map that keeps the isobars.  The colors now will represent different air 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.  Does the green, blue, purple (cool, cold, colder) band look familiar?  Based on just the temperatures just should have a pretty good idea where a cold front would be found.

Step #3
We'll go ahead and drawn in the cold front.


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.

Step #4
Check front location using some of the other weather changes (wind shift, dew point, pressure changes)


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.  Now we have to do the same for warm fronts.
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.



and a map view (below)


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.  This happens because the warm air rises more gradually and moves out over a wider area ahead of the warm front as it rises.

Clouds associated with a cold front are usually found in a fairly narrow band along the front.  The warm air is pushed up abruptly.  All of the lifting is confined to a fairly narrow band. 

There wasn't enough time at this point to try to locate a warm front on the surface weather map below (found on p. 149b in the ClassNotes).  But there's no reason you couldn't give it a try yourself.  In the off-chance that occurs, here's a link to the complete step-by-step analysis.