RGB compare: (HRV; HRV; 10.8) vs. (IR10.8 Enhanced) Time Sequence: 27 July 2006     

This chapter presents a visual comparison between the high resolution (HRV) RGB (left) and the IR10.8 Enhanced (IRE) image (right).

Two problems for detecting highly active convective zones (indicated by overshooting tops and very low cloud top temperatures) will be discussed in these images:

• The spatial resolution as a limiting factor for detection of small scale overshootings.
• The observation time (sun angle) as a determinant factor to detect overshootings in the HRV channel.

For interpretation of colours in the IR Enhanced image klick upon the key-symbol, see above.

The colours of the clouds in the HRV; HRV; 10.8 RGB can be interpreted as:

• blueish white: high thick clouds (e.g. Cbs)
• blueish transparent: thin cirrus clouds
• yellowish: low-level clouds

The image below points out the various cloud features seen in the images.

Two convective cells growed rapidly during the last hour. From the IR Enhanced image it can be seen that CTT is partly below -64°C (purple colour). This indicates huge CBs and probably severe thunderstorms with hail or heavy rain. As mentioned earlier, no hail was reported though. The bigger southern cell has the typical structure of a super cell with the lowest CTT near its centre indicating the most probable updraft's position.

The cloud complex marked by the arrow has a small scale overshooting top which can be detected better by the IR Enhanced image than by the HRV composite at this time. The area of the overshooting is revealed by violet colour (low CCT) in the IR Enhanced image while the HRV RGB shows a very bright colour in this area.
Due to an high sun angle this overshooting top is not that clearly revealed by the composite image. However, overshooting tops at a later point in time will be detectable much better as a result of lower sun angle and cloud shadow.

In the HRV image at 14 UTC the typical eastward outblown cirrus anvil clouds can be seen. They cause an asymmetry of the upper cloud form (seen in satellite images) with a sharp contour at the windward side and a fibrous outblown structure at the leeward side.
These outblown cirrus clouds give a direct hint to the streaming direction in the upper troposphere. They are also visible from the ground as an indicator for an approaching thunderstorm.

The different convective cells merged and the formation of a squall line is about to start which can be followed in the next images.

At 15 UTC several overshooting tops develop and are clearly revealed by the High Resolution Visible RGB. The high reaching convective towers cause shadows on their eastern sides while the sunlit western edge appears in bright colours.

Most of these overshooting tops representing small convective cores are not detectable in the IR image. The spatial resolution of the IR10.8 channel does not allow to resolve these small-scaled overshooting. Therefore, this is a shortcoming of this channel compared to the HRV RGB.

The overshooting tops are concentrated along a north-south oriented stripe which coincides with the developing squall line.

One hour later the HRV image also reveals overshooting tops that cannot be identified with IR imagery. However, the IR image shows other small areas of low CTT where overshooting tops can be assumed. But the HRV images shows that no overshootings occur in these areas.

In contrast, vertical structures in the HRV imagery are not necessarily extremely cold as the comparison with IR image shows. This indicates growing convective clouds that did not yet extend to the top of troposphere.

Therefore, the combination of Enhanced IR imagery with the HRV composite is useful to study overshooting tops more accurately.

The convected dome has grown and now reached high levels of the troposphere with CTTs lower than -64°C.

The initial stage of this convective development was very well revealed by the HRV composite one hour before while the structure was not clearly visible in IR Enhanced image.

Structures are much more homogenous in both images at this time step, indicating the decaying phase of the convective development.