Storm cloud-top reflectivity in "microphysical" bands
In the previous parts we have utilized two MSG bands only - HRV and IR10.8 SEVIRI bands. However, there are also other spectral bands and their various combinations (such as RGB composite images or "sandwich" products), which can also be useful for monitoring of convective storm and their nowcasting. In this part let's briefly focus on some of the other bands, their interpretation and products based on them.
In the previous parts we have utilized two MSG bands only - HRV and IR10.8 SEVIRI bands. However, there are also other spectral bands and their various combinations (such as RGB composite images or "sandwich" products), which can also be useful for monitoring of convective storm and their nowcasting. In this part let's briefly focus on some of the other bands, their interpretation and products based on them.
Besides the information about storm-top "morphology", delivered to us in the visible bands (such as the HRV band), and storms' cloud top temperature and height derived from the thermal IR bands (such as the IR10.8 band), there is one more important source of information about the processes taking place inside convective storms - their cloud top microphysics (water/ice discrimination, particle size, type of ice crystals, particle concentrations, etc). Information about the cloud composition is contained in all of the spectral bands, however some of these are more sensitive to the cloud-top microphysics - namely bands located at or near 1.6, 2.2 and 3.7-3.9 microns. For this reason, these near-IR bands are occasionally referred to as "microphysical" bands. Below are several examples of storm-top appearance in some of these bands, and in their RGB combination.
As tops of convective storms are typically at very low temperatures, their cloud tops are composed of ice particles only (all super-cooled water droplets freeze if the ambient temperature drops below about -40°C, 233 K). As most of the ice particles have very low reflectivity in the "microphysical" bands, storms in general appear as very dark in these bands, when compared to clouds tops which are composed of water droplets (which have much higher reflectivity in these bands). This principle is used for discrimination of ice clouds from the water ones, both subjectively (looking at images by a human eye) and objectively (in various cloud classification algorithms). However, not all of the ice clouds look dark - if their tops contain higher concentrations of very small particles (with their size being somewhere between 1 to 10 microns), these can appear somewhat brighter than those storm top parts composed of more common large particles (typical size of the ice particles in storm tops is of the order of tens of microns (~ 20 to 100 microns). In the example above, the 1.6 micron band, we can see as very dark the parts of the anvil far downwind are very dark (top right part of the image), and also the overshooting top itself (dark spot at the south-west part of the storm). In contrast, parts of the storm top (approximately in the central parts of the anvil) are noticeably brighter, indicating the about presence of smaller ice particles. As this 1.6 micron spectral band is composed of reflected solar radiance only, it is not influenced by storm top temperature.
The situation is somewhat different in the 3.9 micron SEVIRI band (and all similar bands of other satellites or instruments, which are located between 3.5 - 4.0 microns) - this band is composed of both, thermal emitted and solar reflected radiance (during the daytime), thus its interpretation is somewhat less straightforward. If we display this image as other thermal IR bands (with dark shades representing higher radiances, warmer temperatures, and white indicating very low radiances), most of convective storms will appear in this band as almost white, as shown in the image below (Figure 15b). In this image the areas with smaller particles will reflect more solar radiance, thus the overall radiance will be higher and the area will appear as "warmer", darker as the darker area near the central part of the image.
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| Figure 15a. Appearance of the storm from 2013-05-11 (12:30 UTC, Meteosat-10, Nigeria, shown in Figures 6, 8 and 11) in the 1.6 micron band. |
The situation is somewhat different in the 3.9 micron SEVIRI band (and all similar bands of other satellites or instruments, which are located between 3.5 - 4.0 microns) - this band is composed of both, thermal emitted and solar reflected radiance (during the daytime), thus its interpretation is somewhat less straightforward. If we display this image as other thermal IR bands (with dark shades representing higher radiances, warmer temperatures, and white indicating very low radiances), most of convective storms will appear in this band as almost white, as shown in the image below (Figure 15b). In this image the areas with smaller particles will reflect more solar radiance, thus the overall radiance will be higher and the area will appear as "warmer", darker as the darker area near the central part of the image.
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| Figure 15b. The same storm as above, shown in the 3.9 micron SEVIRI band (displayed as thermal IR images). |
The principle of higher reflectivity of smaller ice particles (and related lower emissivity in the 3.9 micron band) is in the background of various "microphysical" RGB composite images, combining several SEVIRI bands. For an overview of these go to this EUMETSAT website or to this PowerPoint presentation. One of these microphysical RGBs was tuned to emphasize storm tops and their properties, for this reason it is sometimes called as "Convective Storm RGB" or just simply "Storm RGB" (for its description and definition go here). The next image (Figure 15c) is an example of this product:
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| Figure 15c. The same storm as above, shown as "Convective Storm RGB" (or "Storm RGB") composite image. |
Given the way ("recipe", or formula) this product is made, the are two reasons for the stronger yellow colour: either very low IR10.8-BT value (the pixels being very cold), or the microphysical cloud-top reflectivity (in the 1.6 and 3.9 micron bands ) being much higher than elsewhere within the storm top. In this case, comparing the image above with those in colour-enhanced IR10.8-BT images (e.g. those in Figure 8), we can instantly exclude low BT being the reason for the bright yellow area in the centre of the image, thus as the only explanation remains the area's high (microphysical) reflectivity. This can be confirmed comparing the Storm RGB image with those in Figure 15a and 15b - the storm indeed has a higher cloud-top reflectivity here.
And what is the practical use of all of this? The small particles (which are responsible for the higher 1.6 - 3.9 micron cloud-top reflectivity) can occur in the cloud top due to several mechanisms; the major two of these are:
And what is the practical use of all of this? The small particles (which are responsible for the higher 1.6 - 3.9 micron cloud-top reflectivity) can occur in the cloud top due to several mechanisms; the major two of these are:
- the small particles can be brought up into the storm top by very strong updrafts, which don't give the particles the necessary time to grow to larger ones;
- the small particles form above the anvil top in the dryer layers above the storm top, thus lack environmental moisture to grow into larger ones.
From the nowcasting perspective, it is the first mechanism which can be important for inferring storm severity; in such cases we can expect the increased cloud-top reflectivity in the area of overshooting tops, or nearby. However, unfortunately there is also a possibility (for various reasons) that strong updrafts will not generate small particles (or will generate these in small concentrations only), which will result in low (1.6 or 3.9 micron) cloud-top reflectivity thus one should not rely on this product thoughtlessly - it is only one of the pieces of a mosaic of information on possible storm severity.
