Wednesday, Mar. 27, 2019

Ravel's "Bolero" played by the Copenhagen Philharmonic Orchestra (Sjællands Symfoniorkester) at Copenhagen's Central Railway Station (4:52),  Gustav Holst "The Planets: Jupiter The Bringer of Jollity" Berklee Contemporary Symphony Orchestra at The Prudential Center (4:22), Edvard Grieg's "In the Hall of the Mountain King" Centre Commercial Auchan Noyelles, Lille, France (6:00)

We'll be using page 103a, page 103b, page 104a, page 99, and page 100 from the ClassNotes today


This Snow Crystal Morphology Diagram (from SnowCrystals.com) illustrates how the shapes of snow crystals depends on temperature and the amount of moisture in the air.

Inside a cold cloud, once the ice crystal process is underway

A variety of things can happen once a snow crystal forms.

First it can break into pieces, then each of the pieces can grow into a new snow crystal.  Because snow crystals are otherwise in rather short supply, ice crystal multiplication is a way of increasing the amount of precipitation that ultimately falls from the cloud.

Snowflakes

 

Several snow crystals can collide and stick together to form a snowflake.  Snow crystals are small, a few tenths of a millimeter across.  Snowflakes can be much larger and are made up of many snow crystals stuck together.  The sticking together or clumping together of snow crystals is called aggregation (I frequently forget the name of this process and don't expect you to remember it either).

Riming (accretion) and graupel (aka snow pellets & soft hail)
The next process and particle are something that I hope you will remember.

Snow crystals can collide with supercooled water droplets.  The water droplets may stick and freeze to the snow crystal.  This process is called riming or accretion (note this isn't called collision coalescence even though it is the same idea).  If a snow crystal collides with enough water droplets it can be completely covered with ice.  The resulting particle is called graupel.  Graupel is sometimes mistaken for hail and is called soft hail or snow pellets.  Rime ice has a frosty milky white appearance.  A graupel particle resembles a miniature snow ball.  Or smaller finer grained version of the shaved ice in a "snow cone."   Graupel particles often serve as the nucleus for a hailstone.  You'll find lots of pictures on the internet (here is a nice side by side comparison of graupel and hail from South New Jersey Today ).

Graupel is made of milky white frosty rime ice.  Sleet, we will find, is made of clear ice.  Here are some pictures to help you better appreciate the differences in appearance. 





Here's a snowball.  It's white and you can't see through it.  It's made up of lots of smaller crystals of ice.  Graupel is just a small snowball.  source
The ice in a snow cone is basically the same.  Lots of smaller chunks of ice.  The ice is frosty white (before you added the flavored syrup).   source

Graupel vs sleet, rime ice vs clear ice
Graupel is sometimes referred as snow pellets.  Sleet is sometimes called ice pellets.

clear transparent sugar crystals
source of this photograph



frosty white sugar cubes
are made up of many much smaller grains of sugar

Appreciating the differences in the appearance of clear ice and rime ice.

Formation of hail
This figure (page 103b in the ClassNotes) gives you an idea of how hail forms.

In the figure above a hailstone starts with a graupel particle (Pt. 1, colored green to represent rime ice).  The graupel falls or gets carried into a part of the cloud where it collides with a large number of supercooled water droplets which stick to the graupel but don't immediately freeze.  The graupel gets coated with a layer of water (blue) at Pt. 2.  The particle then moves into a colder part of the cloud and the water layer freeze producing a layer of clear ice (the clear ice, colored violet, has a distinctly different appearance from the milky white rime ice), Pt. 3.  In Tucson this is often the only example of hail that you will see: a graupel particle core with a single layer of clear ice (you can look through the clear ice layer and see the graupel inside, here is a link to some photographs from a Arizona Daily Star June, 2015 article).  You'll also find photographs if you search "hail crossection photograph"



Here's a photograph of a hail stone that fell from a thunderstorm cloud in midtown Tucson during the Fall 2018 semester (photo credit: Jim Dugan).  It clearly shows the frosty white graupel particle surrounded by a layer of clear ice.  It's pretty unusual to get hailstones this large in Tucson.
In the severe thunderstorms in the Central Plains, the hailstone can pick up additional layers of rime ice and clear ice and hailstones can be composed of many alternating layers of rime and clear ice.  An  unusually large hailstone (around 3 inches in diameter) has been cut in half to show (below) the different layers of ice.  The picture below is close to actual size.  If something like this were to hit you in the head it would split your skull open.  Here's some pretty good video of a hailstorm in Phoenix.

Hail is produced in strong thunderstorms with tilted updrafts.  You would never see hail (or graupel) falling from a nimbostratus cloud. Here is a photo of a record setting 8" diameter hailstone collected in South Dakota.  It is currently the national record holder.  Here's another hailstone that is almost as big.  It holds the record for Oklahoma.   Click here to see a gallery of images showing hail damage to automobiles.


The growing hailstone can fall back into the updraft (rather than falling out of the cloud) and be carried back up toward the top of the cloud.  In this way the hailstone can complete several cycles through the interior of the cloud.  The article above mentions a supercell thunderstorm.  We will discuss these later in the semester.

Types of precipitation
Finally on page 104a in the ClassNotes are illustrations of some of the things that can happen once a precipitation particle falls from a cloud.  I've split this into two groups for clarity.



Essentially all the rain that falls in Tucson is produced by the ice crystal process.  The left figure above shows how this happens.  A falling graupel particle or a snow flake moves into warmer air and melts.  The resulting drops of water fall the rest of the way to the ground and would be called RAIN. 

In the middle picture graupel particles can survive the trip to the ground without melting even in the summer.  Many people on the ground would call this hail but that wouldn't be quite right.  Graupel is less common in the winter because it comes from thunderstorms and they don't form very often in the winter.  Snow can survive the trip to the ground in the winter but not the summer.  Snow does occasionally make it to the valley floor in Tucson.

Sometimes the falling raindrops will evaporate before reaching the ground.  This is called VIRGA and is pretty common early in the summer thunderstorm season in Arizona when the air is still pretty dry.  Lightning that comes from thunderstorms that aren't producing much precipitation is called "dry lightning" and often starts brush fires.





Rain will sometimes freeze before reaching the ground.  The resulting particle of clear ice is called SLEET.  FREEZING RAIN by contrast only freezes once it reaches the ground. Everything on the ground can get coated with a thick layer of ice.  It is nearly impossible to drive during one of these "ice storms."  Sometimes the coating of ice is heavy enough that branches on trees are broken and power lines are brought down (either by the weight of ice or falling tree limbs).   It sometimes takes several days for power to be restored.  Here's a gallery of images taken after ice storms.

Satellite photographs of clouds
Now that we've finished the section on cloud identification this is a good time to learn a little bit about the 2 most common types of satellite photographs.

IR photographs
When you see satellite photographs of clouds on the TV weather report you are probably seeing infrared satellite photographs.


1. An infrared satellite photograph detects the 10 micrometer (μm) IR radiation actually emitted by the ground, the ocean and by clouds.  You don't depend on seeing reflected sunlight, so the earth can be photographed during the day and at night.  You may recall that 10 μm radiation is in the middle of the atmospheric window, so this type of radiation is able to pass through air without being absorbed.  If clouds don't get in the way, you can see the ground on an IR photograph.

2.   Clouds absorb 10 μm radiation and then emit 10 μm IR radiation of their own.  The intensity of the cloud radiation will depends on the cloud's temperature.  The top surface of a low altitude cloud will be relatively warm.  Warmer objects emit IR radiation at a greater rate or at higher intensity (the Stefan Boltzmann law).  This is shown as grey on an IR satellite photograph.  A unimpressive grey looking cloud on an IR satellite photograph may actually be a thick nimbostratus cloud that is producing a lot of rain or snow.

3.   Cloud tops found at high altitude are cold and emit IR radiation at a lower rate or lower intensity.  This shows up white on an IR photograph. 

An example of an IR satellite photograph is shown below.  Slightly different shades of white or grey on IR satellite photographs are difficult to distinguish with the naked eye.  The satellite sensor on the other hand is able to precisely measure the intensity of the IR radiation it is photographing.  The images are often color enhanced to bring out very small differences in intensity that are due, ultimately, to differences in cloud temperature.
IR image
17 Z  Nov. 7, 2014
 color enhanced image
17 Z Nov. 7, 2014

The right image above is an enhanced version of the image above at left (both images are from the National Oceanic and Atmospheric Administration Geostationary Satellite Server site).  The intensity that corresponds to a particular color can be determined using the scale shown at the right edge of the image.  For example yellow appears to indicate an intensity of about 200 - 205.  This corresponds to a temperature of about 215 K (-58 C or -72 F)

4.   Two very different clouds (a thunderstorm and a cirrostratus cloud) would both appear white on the satellite photograph and would be difficult to distinguish.  Meteorologists are interested in locating tall thunderstorms because they can produce severe weather.  Fortunately, as we will see, these two cloud types have very different appearances of visible satellite photographs, so this ambiguity can be resolved.

5.   The ground changes temperature during the course of the day.  On an infrared satellite animation you can watch the ground change from dark grey or black (during the afternoon when the ground is warmest) to lighter grey (early morning when the ground is cold) during the course of a day.  In the sketch below the ground temperature varies between 80 F and 50 F during the day. Because of water's high specific heat, the ocean right alongside doesn't change temperature much during the day and remains grey throughout the day.  The ocean remains 65 F throughout the day in the figure below. 



 Morning when ground is cool
 Afternoon when the ground is warmer




early morning (14 Z = 7 am MST)
 afternoon  (21 Z = 2 pm MST) 


Early morning (ground is cool) and afternoon (ground has warmed) photographs are shown above.  Focus in on the center of the pictures (SW Arizona, S California, NW Mexico and the northern end of Baja California).  There don't appear to be any clouds there so we are able to see the ground and ocean.  Note how much darker the ground appears in the right (warm afternoon picture).  I don't see a change in the images of the ocean west of the California/Mexico border in the two images

Here's a link to an IR satellite photograph loop.  It is sometimes easier to see the changing appearance of the land surface as it warms and cools when the pictures are in motion. 

Visible photographs

A visible satellite photograph photographs sunlight that is reflected by clouds.  You won't see clouds on a visible satellite photograph at night.  Thick clouds are good reflectors and appear white.  Thinner clouds don't reflect as much light and appear grey.  The low altitude layer cloud and the thunderstorm above would both appear white on this photograph and would be difficult to distinguish.





The table above summarizes what we have learned so far. 

The figure below shows how if you combine both visible and IR photographs you can begin to distinguish between different types of clouds.




Low stratus or nimbostratus clouds appear grey and white on infrared and visible photographs, respectively.  Thunderstorms appear white on both types of satellite photographs.
1745 Z (10:45 am MST) Full Disk IR image
 1745 Z Full Disk Visible image

A comparison of full disk INFRARED and VISIBLE images is shown above.  Perhaps the most obvious difference is the area of clouds out over the Pacific Ocean and west of South America.  The clouds are bright white on the VIS image but barely discernible on the IR image.  These must be fairly thick low altitude clouds, stratocumulus perhaps. 

There's another even more striking difference between the two images - the appearance of outer space.  The earth appears on a white background on the IR image.  Remember that white on an IR image indicates weak intensity IR emissions.  In this case the satellite is not detecting any IR emissions coming from space - zero intensity IR radiation.  The earth appears on a black background in the VIS image.  The satellite is not seeing any visible light coming from space.

Geostationary orbit
The photographs above were taken from a satellite in geostationary orbit.  The satellite is positioned above the equator at an altitude of about 22,000 miles altitude (by comparison the International Space Station is about 260 miles above the earth).  At that altitude the satellite completes one orbit a day.  The motion of the satellite and the spin of the earth are in the same direction.  Thus the satellite remains above and is always viewing the same point on the ground. 

Newest GOES 16 and GOES 17 satellites

Here's a link to imagery from the GOES 16 (positioned over the eastern US) and GOES 17 (positioned over the western US)