11/1/99
PRECIPITATION DETECTION BY RADAR
Thick towering cumulus clouds appear white because they strongly reflect (or scatter) visible light at all wavelengths [recall that visible light is just one form of electromagnetic radiation, with wavelengths between 0.4 and 0.7 micrometers]. However, the small water droplets that make up a typical cloud (having a diameter of 10-30 micrometers) do not reflect electromagnetic radiation having longer wavelengths. Radar (the name comes from the acronym for radio detection and ranging) is electromagnetic radiation at wavelenghts much longer than visible light. A typical weather radar operates at a wavelength of 10 centimeters (roughly 100,000 times longer than visible light). Radar beams are not reflected or scattered strongly by the liquid wer droplets, and so one can use them to "see" into clouds.
Precipitation particles within a cloud, as we know, are much bigger than a typical cloud droplet. These particles (a few millimeters in diameter at most) DO strongly reflect radar. By sending radar pulses into a cloud, one can see through the cloud droplets to detect any precipitation particles within the cloud.
In class I gave a brief discussion of the Doppler effect (see page 192 in Danielson) and how a radar pulse sent out from a transmitter will be reflected by precipitation particles within a cloud. The time it takes for this pulse to go out and come back to the radar antenna gives a measure of how far away the precipitation is. The strength of the reflected pulse gives information on the amount and type of precipitation present in the cloud. The "Doppler shift" in the frequency of the reflected radar pulse provides information on whether the precipitation is moving towards or away from the radar as well as the speed of this motion. For a more detailed description, as well as a picture of the Weather Service's Doppler radar, check out the Tucson NWS radar page.
PRECIPITATION TYPES
It was mentioned above that radar can give some information on different precipitation types within a cloud. But we haven't yet discussed just what those different types are, and what formation processes determine the type of precipitation that actually make it to the ground. As discussed in the text, the type of precipitation depends not only on the temperatures within the cloud where the precipitation particles are forming (via collision/coalescence and/or Bergeron process), but on the temperature of the air between the cloud base and the surface (the air precipitation falls through), and on the temperature of the surface. The handout from class is reproduced below; it tries to summarize how all these effects work together. For example, freezing rain is formed when frozen precipitation falls through a layer of warm air (i.e., temperatures are above freezing below the cloud) and melts; then it encounters a very shallow layer of cold air at the surface where it freezes, and everything is coated with a thick layer of clear ice. Compare this to sleet, where the basic process is the same, except that the layer of warm aloft air is very shallow, and the layer of cold surface air is deep. In this case, the precipitation will freeze prior to contact with the ground.
Type
Cloud Temp.
Falling thru
Surface Temp.
Remarks
"Warm" rain
or drizzle
Above freezing
Above freezing
Above freezing
Collision, coalescence;
Droplets < 0.5 mm in diameter
"Cold" rain
Below freezing
Above freezing
Above freezing
Bergeron process; droplets > 0.5 mm in diameter
Freezing rain
Either above or
below freezing
Above freezing
Below freezing
Rain falls through shallow freezing layer at surface
Snow
Below freezing
Below freezing
Below Freezing
Water vapor depositing on ice nuclei forms crystal structure.
Sleet or
ice pellets
Below freezing
Above freezing
Below freezing
"Frozen raindrops"; ice crystals fall through shallow warm layer, then deep cold layer
Graupel or snow pellets
Below freezing
Above = graupel
Below = s.p.
Above = graupel
Below = s.p.
Ice crystal falling through supercooled water in cloud; a.k.a "soft hail"
Hail
Below freezing
Above freezing
Above freezing
Ice chunks < 5 mm in diameter; growth same as graupel but particle rides updrafts. Each cycle adds layer of ice
Precipitation falling from a cloud encounters air resistance (a form of friction). The speed at which a precipitation particle falls is determined both by the force of gravity and by this frictional force - this is called the particle's "terminal veolcity".
Terminal Velocity:
A falling object accelerates due to gravity (this is called "free fall"). Air resistance (i.e., friction) will act to slow falling object. Eventually, frictional force equals gravitational force, net acceleration of object equals zero. Object continues to fall at constant speed, which we call terminal velocity. For typical raindrop, terminal velocity is 20 mph (see Table 5.2 in Danielson)Air resistance
causes rain drops to flatten as they fall. Typical rain drop shape = "hamburger bun", not "tear drop"
11/5/99
THUNDERSTORMS
We now consider what happens when cumumlus clouds grow into thunderstorms, producing not only thunder and lightning, but also strong winds, flash flooding, hail, and possibly tornadoes. For thunderstorms to form, you need lifting (i.e. rising air), and you need moisture. You also need the environmental lapse rate to be conditionally unstable (which is fairly common) or absolutely unstable (not very common).
Some lifting mechanisms that can cause thunderstorms:
- buoyant lifting (accomplished by surface heating)
- orographic lifting (flow over mountains)
- frontal lifting (cold front: at boundary between warm, humid air and advancing cold air, warm (less dense) air is forced to rise)
- convergence (because the atmosphere is continuous, converging air rises; see Figure 8.17 in Danielson
Remember that as air is rising it cools adiabatically. Any water vapor in the air can condense into liquid form is the rising motion is strong enough to cool the air to below its dew point temperature. As the water vapor condenses and a cloud forms, latent heat is released. Remember that for an air parcel to remain bouyant and keep rising, it must be warmer than its environment. This relese of latent heat is important because it warms the rising air, and feeds the growth of a thunderstorm. That's why water vapor is sometimes referred to as "fuel" for thunderstorms.
To discuss the structure and duration of a thunderstorm, we begin with the most common (and simplest) type - the ordinary or "air mass" thunderstorm. This refers to a thunderstorm that grows due the process of buoyant lifting, far from orography or fronts, within a warm moist mass of air that is heated, most likely by the sun during a hot hazy day in summer. An ordinary thunderstorm has three stages, and each lasts about 15-30 minutes. These are:
- Cumulus or growing stage: In this stage we have a cumulus cloud forming, characterized by rising air motions (i.e. updrafts) throughout. In the top of the cloud, temperatures will be below freezing, so the Bergeron process can start producing big precipitation partcles.
- Mature stage: The cloud reaches its highest point, often encountering a stable temperature inversion (e.g., the tropopause) and spreading out in classic anvil shape. Precipitation particles have grown large enough that their terminal velocity is larger than the speed of the updraft, and they begin to fall. As the precipitation falls, it can "drag" the surrounding air with it. As it falls into warmer regions of the cloud, and if dry environmental air gets entrained into the cloud, this precipitation may evaporate. Remember that evaporation cools the air, making it denser and thus "heavier". All this cold dense air falling through the cloud with the precipitation is the downdraft. When the downdraft hits the surface, it can spread out in a "foot" shape and displace the warmer surface air. This is called the gust front. In this stage, the storm produces high winds, lightning, thunder, and heavy precipitation. Note in Figure 12.1 of Danielson that two adjacent regions of the cloud can have temperatures above freezing (in the updraft) and below freezing (in the downdraft) - so that there will be a mixture of both frozen and liquid precipitation particles. This will be important later when we examine lightning in more detail.
- Dissipating stage: Eventually, the cold downdraft can replace all the warm, buoyant air at the surface, which effectively "turns off" the rising motions within the storm. In this stage, only weak downdrafts are present, and light precipitation falls. All that's left is a lot of alto- and cirrostratus clouds (these can block out the sun and also put an end to surface heating, at least temporarily).