Typhoon In-fa (storm No. 27 of the 2015 typhoon season) developed late in 2015, over the still warm waters of the central Northern Pacific (Figure 1).
09 March 2021
20 November 2015
By Jochen Kerkmann
According to the typhoon forecast track (source: CIMSS), In-fa quickly strengthened to a category 4 tropical cyclone with a pronounced eye (on 21 November), but then weakened under increasing vertical wind shear each day to tropical storm status on 25 November.
Figure 2 shows In-fa on 24 November when it was a category 1 typhoon with a cloud-filled eye surrounded by a large band of thunderstorms wrapping into the low-level centre in the northern quadrant, and stretching north-east.
Forecasters at the Joint Typhoon Warning Center (JTWC) noted that the system continued to be elongated to the northeast along the leading edge of an approaching mid-latitude trough (elongated area of low pressure).
The standard Airmass RGB in Figure 2 shows cold, high-level clouds in a strong white colour, which is a result of selected ranges that over-enhance high, cold clouds, thus not allowing viewers to easily distinguish features like overshooting tops, radial cirrus or gravity waves.
The 'tropical' Airmass RGB (also shown for comparison in Figure 2) alleviates this problem by using ranges more appropriate for cold, high clouds. In particular, for the green range (IR9.6–IR10.4) it uses a range from -25 to +25 K (instead of -40 to +5 K). This makes this RGB very suitable for detecting overshooting tops (white). Also see the large overshooting 'dome' in the centre of typhoon In-fa.
The detailed development of this "dome" can be seen in the animation (rapid scans at 2.5 minutes intervals) shown in the animated gif (Figure 3). This animation shows Band 03 of the Advanced Himawari Imager (AHI) (VIS0.64 channel), which is the only band that has a horizontal resolution (sampling distance) of 500 m.
At around 3:19 UTC two convective towers (close to each other) can be observed that penetrate the high cloud anvil of typhoon In-fa. A few minutes later, these two towers merge into a quickly expanding convective "dome" with lots of gravity waves spreading around.
On the 25 November animation (Figure 4) In-fa (then downgraded to tropical storm status) continued to produce huge convective blobs in its centre. The animation shows one of these fascinating systems for a period of one hour (2:32 to 3:32 UTC) seen in the high-resolution band (VIS0.64).
It looks like a giant octopus with arms (radial cirrus) growing in all directions.
It would be very interesting to simulate such a system with a high-resolution cloud model, and try to understand the features seen in the animation.
When Prof. Pao Wang from the Academia Sinica looked at the Himawari-08 loop he replied that "the radial cirrus feature at the top of the clouds has been studied by a few before, my feeling is that it is similar to the radial cirrus, namely, it is associated with interference of gravity waves. But here the motion is more complicated as the whole system is strongly rotating, and hence the waves are rotating with it. Since the waves are originally generated by cumulonimbus clouds (Cbs) in the centre of the tropical storm, they also propagate outward and you can perceive such outward propagation in this loop as well."
Finally, the shear structure of In-fa on 25 November can be well seen on the RGB images shown in Figures 5 and 6. The centre of the deep convection is displaced to the north-east of the LLCC (Low Level Circulation Centre). This displacement is best seen in the animations.
Figure 5 shows the standard Day Natural Colour RGB using the bands NIR1.6 (on red), VIS0.8 (on green) and VIS0.4 (on blue).
Ice clouds are shown in cyan colour and water clouds in pink colours. However, it has often been noted that the separation of ice and water clouds is far from being perfect in this RGB product. In particular, water clouds with large droplets have often been confused with ice clouds — see scatter plot of water and ice clouds in VIIRS bands (source Météo France).
When we use the NIR2.3 (AHI band 06) instead of the VIS0.8 (AHI band 04) on the green beam, we can devise a new RGB product (let us call it 'Day Cloud Phase RGB') that has similar cloud colours than the Natural Colour RGB, but with improved separation of ice and water clouds. This new RGB product is shown in Figure 6. Ice clouds are shown in cyan (small ice) or bluish colours (large ice) and water clouds in magenta colours.
Looking at the clouds in the Day Natural Colour RGB, some water clouds have a slightly cyan colour (could be ice clouds, see e.g. the circled area on the enlarged image), but when you look at the new Day Cloud Phase RGB it becomes clear that these clouds are water clouds. This becomes more obvious when you toggle between them (Figure 7).
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