Hurricane Dora — strongest Tropical Cyclone observed by new GOES-16.
29 October 2020
19 May 2017
Dora was the first hurricane of the 2017 eastern Pacific hurricane season when it formed off the coast of Mexico on 26 June. It was the strongest Tropical Cyclone that GOES-16 had so far observed (GOES-16 loop).
Like many other Tropical Cyclones, as a Category 1 hurricane Dora produced a large cirrus outflow at high levels, which can be observed in GOES-16 imagery using the IR split window bands (or related RGB products like the Dust RGB) and/or the new 1.3 micron band (band 4) that is dedicated to thin cirrus detection.
The GOES-16 Dust RGB on 26 June at 18:45 UTC (Figure 1, left) shows thick, cold ice clouds in dark red (see the eye of Hurricane Dora) and thin cirrus clouds in dark blue to black. Mid level water clouds are brown.
The new 1.3 micron band (a solar band), which is positioned in a water vapour absorption band, doesn't see either the surface features or the low level clouds, but nicely depicts the structure of the high level clouds of Dora, the thick clouds around the eye and the thin clouds in the high level outflow area of Dora.
Interestingly, as regards high level clouds around the eye of Dora, the NIR1.3 band seems to be very useful for analysing the structure of Dora — it sees the banding/spiral structure of the highest clouds better than the other solar bands.
This corresponds with what can be seen in the IR band 13 (IR10.4) — the spiralling of the coldest clouds, although not exactly the same.
This poses the question of whether this new band has other applications, rather than only the detection of thin cirrus clouds. For example, it could possibly be used for analysing the structure of tropical cyclones or major thunderstorm systems. Currently, we don't know if there is already research on this topic.
Compared to the Dust RGB, the NIR1.3 band (as shown here, with a standard enhancement) misses some of the very thin cirrus clouds that can be seen on the Dust RGB product. A special enhancement is needed for this band to detect very thin cirrus clouds, namely a stretching of the low end of the reflectance range.
According to our experience, a range of 0 to 2%, with a Gamma of 2.5, works well to detect the very thin cirrus clouds (at the cost of over-enhancing thick clouds and detecting some mid-level clouds).
An example of such a stretching/enhancement is given in this GOES-16 case from 19 May 2017 over the Yucatan Peninsula.
In the first image comparison (Figure 2), one can see the benefit of the new NIR1.3 band over the standard solar bands (Natural Colours RGB that uses VIS0.6, VIS0.8 and NIR1.6 bands) for thin cirrus detection. The Natural Colours RGB completely misses the thin cirrus clouds over land (Yucatan Peninsula), because of a lack of contrast (over the sea it is a bit better), while the 1.3 micron band shows a cirrus shield over the eastern part of the Peninsula.
In the second image comparison (Figure 3), one can see that the enhancement of the low end of the range of the NIR1.3 band, as discussed above (range from 0 to 2%), strongly improves the thin cirrus detection. In particular, the very thin "cirrus jet" to the west of the main cirrus shield becomes visible in this enhancement. However, the cost of this enhancement is that other features (in this case lower level clouds over Mexico mainland) get enhanced too and could be confused with thin cirrus. This is why a combined view, Dust RGB, NIR1.3 band plus derived products (if available) gives the best view on the cloud situation.
For interest users, the infrared characteristics of the thin cirrus over Yucatan is shown in a scatterplot of band 13 (IR10.4) against the difference of band 13–band 15 (IR10.4–IR12.3)
The very thin cirrus (magenta box) has higher BT 10.4 (more transparent so clouds are warmer) and has lower difference IR10.4–IR12.3 as the thin cirrus (blue box). In this case, the cirrus clouds are so high and cold that the BTD 10.4–12.3 reaches very large values (up to +15 K for thin cirrus and around 5-10 K for very thin cirrus) so that the red component of the Dust RGB stays low (no red for both).
In Europe, where cirrus clouds are not so high and cold, the split window BTD of very thin cirrus is usually lower such that the red component of the Dust RGB is higher resulting in a magenta-type colour.
Note: NOAA's GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.
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