Lecture 23 - Satellite photographs and precipitation producing processes pt. 1

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

1. An infrared satellite photograph detects the 10 micrometer (um) IR radiation actually emitted by the ground, the ocean and by clouds. You don't depend on seeing reflected sunlight, so clouds can be photographed during the day and at night. You may recall that 10 um 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 um radiation and then emit IR radiation. 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.

4.   Two very different clouds (a thunderstorm and a cirrostratus cloud) would both appear white on the satellite photograph and would be difficult to distinquish. Meteorologists are interested in locating tall thunderstorms because they can produce severe weather.

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 (afternoon when the ground is warmest) to lighter grey (early morning when the ground is cold) during the course of a 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. Here's a link to an IR satellite photograph loop.

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 distinquish.

Here's a summary of what we have learned so far.

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

The next topic we will cover is precipitation formation and types of precipitation. Only two of the 10 main cloud types are able to produce significant amounts of precipitation. Apparently it isn't as easy for clouds to make precipitation as you might think.

This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50 um thick for comparison). It is relatively easy to make cloud droplets (the cloud that forms around a piece of dry ice or when you exhale on a cold day contains water droplets or ice crystals). You cool moist air to the dew point and raise the RH to 100%. Water vapor condenses pretty much instantaneously onto a cloud condensation nucleus to form a cloud droplet. It would take much longer for condensation to turn a cloud droplet into a raindrop. You know from personal experience that once a cloud forms you don't have to wait very long for precipitation to begin to fall.

Part of the problem is that it takes quite a few 20 um diameter cloud droplets to make one 2000 um diameter raindrop. How many exactly? Before answering that question we will look at a cube (rather than a sphere).

It would take 64 individual sugar cubes to make a 4 cube x 4 cube x 4 cube cube. That is because the bigger cube is 4 times wider, 4 times deeper, and 4 times taller. Volume is 3 dimensions.

The raindrop is 100 times wider, 100 times deeper, and 100 times taller than the cloud droplet. The raindrop has a volume that is 100 x 100 x 100 = 1,000,000 (one million) times larger than the volume of the cloud droplets.

Fortunately there are two processes capable of quickly turning small cloud droplets (or ice crystals) into much larger precipitation particles in a cloud.

The collision coalescence process works in clouds that are composed of water droplets only. Clouds like this are only found in the tropics. We'll see that this is a pretty easy process to understand. This process will only produce rain, drizzle, and something called virga (rain that evaporates before reaching the ground).

The ice crystal process produces precipitation everywhere else. This is the process that makes rain in Tucson, even on the hottest day in the summer (summer thunderstorm clouds are tall and reach into cold parts of the atmosphere where temperatures drop well below freezing). Hail and graupel (a precipitation particle that is often mistaken for hail) frequently fall from these storms; proof that the precipitation started out as an ice particle. This process can also produce other kinds of precipitation such as snow and sleet.

Here's what you might see if you looked inside a warm cloud that contained just water droplets:

The collision coalescence process works best in a cloud filled with cloud droplets of different sizes. A larger than average cloud droplet will fall faster, overtake, and collide with smaller slower moving ones. The colliding droplets then often merge to form a larger droplet.

This is an acclerating growth process. As the falling droplet grows it falls faster, and sweeps out an increasingly larger volume inside the cloud. The rate of growth accelerates as the droplet gets bigger.

The figure below shows the collision-coalescence process at work in the two precipitation producing clouds: nimbostratus (Ns) and cumulonimbus (Cb).

Ns clouds are thinner and have weaker updrafts than Cb clouds. The largest raindrops fall from Cb clouds because the droplets spend more time in the cloud growing. In a Cb cloud raindrops can grow while being carried upward by the updraft and also when falling in the downdraft.

Raindrops grow up to about 1/4 inch in diameter. When drops get larger than that, wind resistance flattens out the drop as it falls toward the ground. The drop begins to "flop" around and breaks apart into several smaller droplets. Solid precipitation particles such as hail can get much larger (an inch or two or three in diameter).

Most clouds or at least portions of most clouds will contain ice crystals. In these cases the ice crystal process will be the dominant precipitation producing process. Here are a couple of carefully drawn pictures of the internal structures of cold clouds

The bottom of the thunderstorm, Point 1, is warm enough (warmer than freezing) to just contain water droplets. The top of the thunderstorm, Point 2, is colder than -40 F (equal to -40 C) and just contains ice crystals. The interesting part of the thunderstorm and the nimbostratus cloud is the middle part, Point 3, that contains both supercooled water droplets (water that has been cooled to below freezing but hasn't frozen) and ice crystals. This is called the mixed phase region. This is where the ice crystal process will be able to produce precipitation. This is also where the electrical charge that results in lightning is generated.

The supercooled water droplets aren't able to freeze even though they have been cooled below freezing. At Point 4 we see this is because it is much easier for small droplets of water to freeze onto an ice crystal nucleus or for water vapor to be deposited onto an ice crystal nucleus (just like it is easier for water vapor to condense onto condensation nuclei rather than condensing and forming a small droplet of pure water). Not just any material will work as an ice nucleus however. The material must have a crystalline structure that is like that of ice. There just aren't very many materials with this property and as a result ice crystal nuclei are rather scarce.