Other storm-top features in color-enhanced IR imagery

Despite the fact that most of the convective storms (deep convective clouds, thunderstorms) behave as indicated in the previous parts, with the highest parts being also the coldest ones, there are certain storm types for which the cloud top brightness temperature as depicted by satellites can not be interpreted in such a simple straightforward way.

In more complex cases, it is possible to observe embedded warmer areas just downwind of the overshooting tops, the size of which can range from few IR pixels only, up to an area covering substantial parts of the anvil top. The processes which can lead to formation of these warm embedded areas are rather complex (beyond the scope of this material), there are several proposed mechanisms which most likely can produce these. In reality, some of the observed patterns can result from combination of several mechanisms, and some of these mechanisms are still not quite unambiguously understood. However, all of these mechanisms seem to require the two following conditions to be met, which seem essential for formation of the observed features. First condition is that there has to be a thermal inversion just above the storm anvil top, without it the warm embedded area will not form. Second requirement is the presence of wind shear, which affects the form of the embedded warm area and the resulting type of the storm as observed in the color enhanced IR BT imagery (more on this below).

The first example (Figure 12 below) shows the "weakest" form of the embedded warm areas, when it covers several pixels only, just downwind of an overshooting top:

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Figure 12. Example of cold overshooting top and warm spot couplet (arrowed). 20101106, 15:30 UTC, Meteosat-9,  IR10.8-BT (182K-222K),  Democratic Republic of the Congo.

In cases like this one, the lifetime of the warm spots can be rather short; however if the "parent" overshooting top lasts longer or is repeatedly re-generated at the same place, the small warm area may persist longer.

However, if the conditions are favorable, the initial warm spot may take a form of much larger area, forming a larger scale and longer-lived storm top feature, which is referred to as a cold ring. Its example and related terminology is shown below, in Figure 13a.

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Figure 13a. Example of a storm with a cold ring and "central warm spot" in its center. The feature can be seen in color-enhanced  IR BT images only, in visible bands (such as HRV, shown left) there is no trace of this BT feature.

Storms forming the cold rings are typical for a weaker sheared environment. The embedded central warm spot (CWS) can not be interpreted as being warmer because of lower cloud top height or lower optical thickness, but as mentioned above the possible explanations of this phenomenon are beyond the scope of this material. A large fraction of storms with longer-lived cold-ring feature is often associated with some form of severe weather, such as strong downbursts or large hail. Additional more detailed information about storms with cold rings can be found e.g. here.

As the wind shear gets stronger, a different type of storm with embedded warm areas may occur - storms with cold-U or cold-V shaped features at their tops, commonly referred to as cold-U/V feature. In older literature (namely from the U.S. authors), the same feature was called enhanced-V (as the V-like shape was revealed in enhanced IR imagery). An example and related terminology is shown below, in Figure 13b.

For a full size view, click on the image

Figure 13b. Example of a storm with a cold-U feature and related warm areas inside the cold-U. Similar to the cold ring, the feature can be seen in color-enhanced  IR BT images only, in visible bands (such as HRV, shown left) there is no trace of this BT feature.
Storms exhibiting these type of cold features (cold-U/V) are known to be frequently accompanied be severe weather, many of these can be supercells or even tornadic storms. While storms with cold-rings can be found also in the tropics, near the Equator, storms with cold-U/V are rather typical for higher latitudes, but can form occasionally even at sub-tropics. The factor which seems to discriminate between these two storm types is the wind shear - which also seems to be the reason for their geographical distribution. Occasionally, a transition of a storm with cold ring into a storm exhibiting cold-U/V can occur, but not vice versa.

Additional comments and list of references to scientific papers addressing the cold-ring or cold-U/V (enhanced-V) features can be found e.g. in the A-Train observations of deep convective storm tops.