Role of the wind shear


So far, we have discussed some of the non-meteorological factors, which can impact on the appearance of convective storms when viewed from space. However, even more important for the interpretation of satellite imagery of storms are the meteorological factors which can affect the general storm appearance. These include - its shape, size, cloud top temperature, storm-top microphysics, etc. In the next part let's focus on the role of the wind shear and its influence on shape of convective storms.

The impact of (vertical) wind shear is straightforward - the stronger the wind shear, the more elongated the shape of the storm anvil we can expect. More accurately, the distortion of the storm anvil is directly proportional to the difference between propagation speed of the storm core and the winds at the storm-top levels. Also, the persistence of storm core and strength of its updrafts play a significant role: long-lived storms with strong updrafts will create large anvils, and if these form in the presence of strong upper-level winds, the storm will produce a large, long anvil. In contrast, short-lived storms or storms forming at no-shear or low-shear environment will produce nearly circular or only slightly elongated anvils.

In mid-latitudes, most convective storms form in the presence of some amount of vertical wind shear; consequently most of the local storms form rather elongated anvils here. Cases such as the one shown below (Fig. 3) are rather exceptional ones. Though many storms may appear in their early stage as almost circular, they quickly loose their shape as they evolve, stretching their anvil downwind (storm-relative at upper levels).

 For a full size view, click on the image.
 Figure 3a. Example of a storm at mid-latitudes (North Italy), evolving in a weak-shear environment. 2006-06-25 13:00 UTC, Meteosat-8, HRV band, Mercator map projection. Such almost circular storms, maintaining their shape throughout most of their life cycle, indicating weak wind shear, are rather difficult to find over Europe.


Figure 3b. Same storm as above, in the IR10.8 band. Both of these images indicate that despite the weak shear environment, resulting in almost circular shape of the storm anvil, the core of the storm is shifted slightly asymmetrically southward (as indicated by location of storm's overshooting top area).
The most severe form of convective storms - supercells - typically form in a strongly sheared environment, with significantly elongated anvils, and with their cores being located at their (storm-relative) upwind side. Their anvil typically spreads far downwind, ahead of the storm core itself, or is shifted somewhat left or right from the storm core propagation (however, in many cases the actual layout may differ from this scheme).


For a full size view, click on the image.
Figure 4a. Example of mid-latitude storm, forming in a strongly sheared environment. 2013-06-20 15:37 UTC, Meteosat-8, HRV band, south Germany, Mercator map projection.



Figure 4b. The same storm as above, in the IR10.8 band. Spatial arrangement of the anvil with respect to the storm core, such as in this case, is not that common for Europe. Still the strongly elongated anvil, stretched to the north-east, indicates strong storm-relative winds at the upper levels.
Closer to the equator the situation is quite different from mid-latitudes especially in the tropics and, to some extent, the subtropics. In the tropics, the wind shear is usually much weaker than in higher latitudes (as there are no jet-streams close to the Equator), and thus the storms tend to form rather irregular anvils here. However, there are two different mechanisms which can also form elongated anvils even here. Some of the tropical and sub-tropical thunderstorms (at least over Africa) can propagate rather quickly, forming new cells on fast-moving outflow boundaries (generated by some of the earlier cells). As these new cells form, they leave the older dissipating anvil cirrus clouds behind, and as a result they can attain a seemingly similar elongated appearance as storms forming in a sheared environment in mid-latitudes. The main difference is that the cirrus material from the dissipating cells remains almost stationary, while the main body of the storm (storm core) moves quickly away. Beside the outflow boundaries, the low-level trade winds can produce a similar effect and impact on the storm appearance close to the Equator. For these storms, it is typical that their dissipating anvils remain at their original location and new storm cells, driven by low-level trade winds form to the west, giving the storms an elongated shape. Low level trade winds can produce a reverse wind shear profile from the mid-latitude conditions - the strongest winds are at lower troposphere levels, while in mid-latitudes it is just the opposite.

The images below show two limiting cases (as regards the amount of wind-shear) from the tropics of Africa. The first case  (Fig. 5) shows an early morning low-shear convective storm from the southern parts of Chad: 

 For a full size view, click on the image.

 Figure 5a. Very nice example of a storm developing above south Chad, in a weak-sheared environment, resulting in an almost perfect circular shape of the storm anvil. 2013-08-06  05:30 UTC, Meteosat-10, HRV band (original satellite projection). Thin lines show political borders between southern Chad and neighboring countries.


Figure 5b. The same storm as above, but in the IR10.8 band. In the IR-window band images such as this one, shown in black-and-white linear enhancement only, the thin semitransparent outer parts of the anvil seem more thick than in the HRV band,  further stressing the circular appearance of the storm.

The second case (Fig. 6, below) shows an example of strong-shear storm, when the elongated shape of its anvil is related to stronger low-level trade winds. This storm formed during the early morning above Nigeria, and persisted for many hours; the images below show the storm shortly after local noon. As the detailed images below lack the political borders, you can click the image left to see the location of the storm in a lower-resolution image with a map overlay.

 For a full size view, click on the image.

 Figure 6a. Example of a tropical convective storm with a long anvil, resulting from a strong wind shear (related to low-level trade winds). 2013-05-18, 12:30 UTC, Meteosat-10, HRV band, Nigeria (original satellite projection). The core of the storm is in the south-west of the cloud, manifested by overshooting tops.


 Figure 6b. The same storm and area as in Fig. 6a, but in the IR10.8 band. Such black-and-white non-enhanced images as this one show the overall anvil shape and size only, but reveal nothing about the detailed cloud-top structure. To reveal the details of the cloud-top brightness temperature, the images need to be color-enhanced, which is the subject of the next part of this material.
In reality, most of the convective storms ("thunderstorms") as seen in satellite imagery are much more complex than the examples above. Many storms interact with those which evolve nearby, occur in clusters, or form various mesoscale systems. This typically results in the formation of a large, irregular common anvil, making it impossible to distinguish the individual storms beneath. However, even in such complex cases, it is usually possible to infer from the individual thunderstorm appearance at their early stages on amount of the wind shear within which they form, and thus make the relevant assumptions about the possible accompanying weather  these.