Plumes above storm tops

As already shown in the previous section, an area of the increased 3.7 µm cloud top reflectivity sometimes resembles a "plume like” shape (hereafter referenced as "plume"), distinctly different from all the other forms of the 3.7 µm features found at (or above) tops of deep convective storms. It should be emphasized here that this form of the increased 3.7 µm cloud top reflectivity (plume) is observed much less frequently than all the other possible forms.

Before proceeding to a detailed description of plumes, as an example here is a "classic” case, one of the first ones recorded and probably the most spectacular one:

(however, this case is not quite typical - this storm exhibits a "split” plume while majority of plumes has a single form).

European observations, summarized in (Levizzani and Setvák, 1996), though based on "snapshots" of the AVHRR instrument only, have shown several important characteristics of the plumes:

  • plumes can be found either in the 3.7 µm daytime imagery, or in visible/near infrared data; they can not always be identified in all of these channels simultaneously;

  • plumes are in most cases vertically separated from the top of the anvil (in visible/near IR band they cast shadows on anvil tops);

  • typically, plumes are rather thin as the details of underlying anvil structures are discernible through them in visible/near infrared imagery;

  • once revealed in the above-mentioned spectral bands, it is usually difficult or impossible to find a trace of the plume in the enhanced thermal IR imagery (AVHRR channels 4 and 5);

  • the source of the plumes is often (however not always!) very small. If the plume can be followed back to its source (which means that the source is still active at the time of observation), its size is comparable to AVHRR pixel size (or a few of them);

  • the source of plumes is always shifted downwind from the coldest tops, and the magnitude of this shift ranges from a few to 10 - 20 km; if an embedded IR warm spot is present, the source of the plume is located within (or above) it;

  • since the length of a plume often exceeds several tens of km while maintaining uniform structure along its length, the source of such plume has to be stable over a period of time on the order of from tens of minutes to a few hours.

(Please note that the "plumes" we are speaking about on these pages are a different phenomenon than those which you can find occasionally elsewhere and which are called "anvil plume" or "storm plume". Those terms refer to storm anvils which are significantly elongated by upper level winds and which are built up by storm updrafts. "Our" plumes typically originate in the lee of the updraft region and are vertically separated from the anvil top.)

Now seems to be the right time for more (European) cases. When looking at these, please notice how they match the above summarized characteristics - these will not be repeated in the notes accompanying the individual cases.

References and related papers
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