EAS 308, Meeting 7

Cumulonimbus

We now study the most dangerous type of clouds, the cumulonimbus (cumulus in Greek means “heap” and nimbus means “rainy cloud”). Dangerous because these clouds can produce flash floods, extreme wind gusts and tornados, killer lightning and damaging hail. So, how do the “fair weather” cumuli become a dangerous cumulonimbus? To help answer that question, we return briefly to the last chapter.

Very large clouds:

Figure 78 (pg. 63) shows an environmental sounding with analysis of the sounding demonstrating the formation of a tall cumulus cloud called a cumulonimbus. The cumulus initially grow vertically from “fair” weather size to “congestus” size and dissipate in the direr air aloft. But, they leave moisture a loft. This “pumping” of moisture aloft paves the way for the Cb to form. Notice how the saturated parcel remains much warmer than the environment to great height permitting the tall Cb.

Now, we return the Chapter 7.

When clouds produce rain:

Earlier in our studies, we learned how the clouds associated with the warm front can produce a wide variety of precipitation all at the same time but at different locations (the sequence is as you move from the warm to the cold air): rain, freezing rain, sleet, snow. Now we study the Cb, the cloud that produces rain and hail.

There are two mechanisms that form rain in Cb’s: the warm- and cold-rain processes.

First, the warm-rain process. When tiny cloud droplets (too small to fall) in Cb’s that do not penetrate the freezing level collide and coalesce - because of their slightly different sizes - they form rain drop embryos. These embryonic drops collide with other embryos and grow large enough to form rain drops which fall from the cloud. The fall speed of a typical rain drop is about 10 knots. So, the updraft in which the drop formed must be less that 10 knots for the drop to fall from the cloud.

When the Cb penetrates the freezing level, some cloud droplets freeze to form ice crystals. The crystals grow at the expense of the sub-cooled, liquid cloud droplets similar to how the frost grows in your refrigerator. The crystals begin to fall and collide with the droplets and become small hail stones. These fall below the freezing level, melt and form rain drops. If the small stones continue to collect droplets, they can grow large enough to fall all the way to the surface as hail.

Where does the energy come from?:

The shaded region in Fig 80represents the latent heat of condensation which is released in the rising saturated parcel. It can be seen much more energy is released in tropical Cb’s than in polar Cb’s because warm air can “hold” more water vapor than cold air..

Multi-cells and super-cells:

In a non-wind sheared environment (Fig. 81), the classic evolution of a multi-celled Cb occurs: cumulus stage, mature stage and dissipation stage (each stage taking about 30 min). The stages explain the showery nature of precipitation from Cb’s. This is in contrast to the more uniform rain from the warm front which is composed of primarily stratiform clouds rather than cumuliform clouds.

In a wind sheared environment (Figs. 82 and 84), the precipitation-formed downdraft does not fall into the updraft allowing the storm to have a long life. The essential feature of a super-cell is the continued separation of the updraft and downdraft. I now will show and award-winning animation of the super-cell (Wilhelmson video).

In-class exercise leading to Assignment 7:

The web-site you visited in Assignment 5 () is a rich source of cloud images, especially the formation, maturation and dissipation ofcumulonimbus clouds. To illustrate this sequence, I downloaded a sequence of hourly visible and infrared images for 20 September 2006. Let’s have a look focusing on Florida:

If I had done this exercise real-time, I could have found a radar image that proved this was a Cb, that is raining. The site archive hourly radar images but it take four days for them to appear. When I prepared this exercise, (21 Sept) there were satellite images for 16 and 17 Sept on the UCAR-RAP site and radar images for 16 and 17 Sept on the NOAA-NCDC site. This leads to Assignment 7.

7-1