Single satellite images
HRV
Monitoring small scale events the high resolution visible imagery can provide excellent information.
The MSG HRV channel operates on a horizontal resolution of 1 km (sub satellite area) and its ability to discriminate small scale features can also be beneficial when observing volcanic ash clouds. However it should be noted that it can be difficult to monitor very thin ash clouds with HRV imageries. The detection depends on the reflectivity of the underlying surface and is easier over oceans. Moreover the sun angle plays an important rule, whereas during the morning hours there is a better chance to detect ash clouds due to strong forward scattering of the solar radiation on the ash particles. Thus, the detection of volcanic ash cloud with HRV images is only possible under certain conditions and should be considered as an addition tool.
Figure 3.2: HRV loop 20100415 09UTC-15UTC
As you can see the high resolution visible images cannot provide a clear discrimination of the ash cloud. Of course the viewer gets a vague impression about its location, but for preciser information different products should be consulted.
IR3.9 - Reflected Component
The IR3.9 channel consists of 2 different parts: a solar component (during day) and a thermal component (both during day and night). For the detection of volcanic ash the reflected part of the solar component can be very useful, since is particularly sensitive to cloud phase and particle size. This characteristic is based on a notably different emissivity of the IR3.9 channel compared to other channels such as the IR10.8 channel. Considering the detection of volcanic ash there will be differences in the reflection of ash particles and cloud particles. This difference can finally be seen in the image. However using the IR3.9r images one should be aware of limb cooling. Thus, for larger viewing angles the optical path through the atmosphere is greater. As a consequence features at higher latitudes appear a little blurred.
Figure 3.3: IR3.9r loop 20100415 10UTC-16UTC
The analysis of this case shows that the detection of the ash cloud with the IR3.9 reflected component did not work well. It only gives a hint where the ash cloud could be found. One of the reason might be the fact, that Iceland is situated in higher latitudes and limb cooling has a blurring effect on the satellite images.