Friday, February 21, 2014

What does a tornado and an Olympic medalist have in common?

No, this isn't a joke. Here's a hint: every tornado needs it and every Olympic medalists control it. The answer: conservation of angular momentum.
The United States won Bronze during the team skate competition in the 2014 winter games.

In physics, angular momentum (L) is a measure of rotation. It is equal to the product of an objects mass (m), velocity (v), and radius of rotation (r).

L = mvr

Angular momentum is a conserved property meaning it doesn't change (as long as no external forces are applied to the system). Thus, if we change any variable in the equation, the other variables will change to a degree in which the angular momentum remains unchanged. Thus, we may write a change in angular momentum as follows where the "i" indicates a variable at an initial time and "f" indicates the variable at a final time. (We assume the mass does not change, but this doesn't have to be true in every case.)

m * v_i * r_i = m * v_f * r_f

A figure skater or freestyle skier can change their radius by hold out their arms or bringing their arms close to their body. When an Olympian holds his or her arms out, they increase their radius of rotation and their spinning velocity decrease. Conversely, when they bring their arms close to their body, their radius decreases and their rotation velocity must increase.

Tornadoes are subject to the same law. When a column of air is stretched, the radius of rotation is reduced causing the velocity to increase. Lots of stretching means strong winds. The figure below illustrates the conservation of angular momentum in large convective storms.

Wave Cloud

Looking south form University of Utah -- February 21, 2014
(c) kbkb

Ripples in water
The atmosphere is a fluid and can have waves. Wave clouds look like ripples in water that oscillate up and down.

Wave clouds are a sign that the atmosphere is stable, meaning air wants to stay at the level it's at. Sure enough, this morning's sounding shows the air is dry and stable.

The location of the clouds show where air is moving up, which cools the air and causes condensation. Cloud-free areas indicate descending air, which warming and evaporates the cloud.
For these waves to appear something must happen upstream to cause the air to oscillate up and down. The culprit here, as far as I can tell, is the mountains interacting with the west, north west wind.

The wave cloud shown a top evaporated within twenty minutes and now there are a bunch of contrails and cirrus clouds to the west.

Thursday, February 13, 2014

February Thunderstorm and Radar

In the middle of February one might expect snowstorms. Not today! What started out as a relatively warm afternoon turned into a thunderstorm in Salt Lake City. The source of this moisture is an atmospheric river, or pineapple express, coming from Hawaii. You can read more on Wikipedia here. An animation from NOAA shows the water vapor measured by satellite revealing the atmospheric river.

At 1:10 our instrumentation class launched a weather balloon. From the roof of the William Browning Building at the University of Utah, it looked to be a really nice day. See for yourself...
Looking West towards Oquirrh Mountains

 Looking West towards downtown Salt Lake

Looking North West towards Capital Building and Airport

Class members help prepare a weather balloon. The WBB weather station is in the background.

Letting it go! 

There was a persistent west wind, so the balloon floated almost in a straight line towards the east and over the mountains.

Here is the data we collected plotted on a skew-t chart:

Two hours later I was walking to the Institute building and got caught by a downpour. Lightning flashing in the corner of my eye and thunder roaring above my head. At an Olympic speed-walking pace I made it inside. On my way home around 5:50 PM, waiting at the Murray Frontrunner station, I looked back towards Salt Lake There was another large thunderstorm passing over downtown Salt and the University of Utah campus.

 Looking North from the Murray Frontrunner station.
 Looking North towards downtown Salt Lake City

Below are the radar images for the time the above picture was taken. Warmer colors represent stronger precipitation.

Radar Base Scan

Radar Composite

To understand the difference between a radar base scan and a composite you have to understand how a weather radar works. First, an electromagnetic wave is transmitted from the radar. If the radar beam hits rain or snow some of that energy is reflected back to the radar where a detector receives and measures the signal. Large rain or hail drops reflect a lot of energy while light rain and snow reflect less energy (sometimes dust and birds can be seen on radar scans). Since the speed of light is known, we can calculate how far that energy traveled to tell us how far away the rain is from the radar. 
To get a view of rain in all directions a radar beam rotates like a lighthouse light. However, a radar is a little different than a lighthouse. After each scan the angle it sends a pulse changes. Below shows different angles and the area covered by each tilted scan. (Note: the beam appears to curve up with distance. Doesn't radiation propagate in a straight line? Yes, the light is traveling straight, but remember that the earth is curved.)
National Weather Service
The base scan, as is shown in the first radar image above, only includes the reflectivity measured in the lowest scan angle--the 0.5 degree angle (darkest shade of blue). Rain shown in this layer could possibly be reaching the ground.
A composite scan shows the highest reflectivity value from the collection of scans (thus, a composite). The precipitation in the composite scan from above seems to be more widespread. This is because some of the rain in some areas is evaporating before is reaches the ground or the base scan.

Radars are helpful tools in a lot of ways. We can use radars to estimate total rain fall or the motion of storms. Below shows the direction this storm is moving at the given time.

And finally, a look at MesoWest plots from the weather station on WBB at the University of Utah with crude labels to indicate the time of the balloon launch, the time I got stuck in the rain, and the time I looked back to see another storm pass through Salt Lake.

Sunday, February 2, 2014

Drought from Space

This image from NASA satellites foreshadows just how severe our drought will be. There will certainly be a water shortage this year as snow pack is far below normal in most places and reservoirs are at record lows (news story link).