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Thursday, 9th October 2008

The amazing story of Higos

Tropical cyclone Higos landed on western Guangdong last Saturday. Its surface circulation weakened and Higos was downgraded to a low pressure area. But its amazing story only started then.

Firstly, as we had expected, Higos made an abrupt 90-degree turn to the right after landing and edged closer to Hong Kong as it moved eastwards. Thus we maintained careful surveillance of its moves. Forecasters had a lot of worries during this time, rather like what it felt like on 7 July (see blog article dated 9 July).

Higos turned under the influence of a so-called "westerly trough" at the middle layer of the atmosphere. The other effect of this westerly trough was a rise in the surface pressure in China mainland, pushing cooler air southwards into Guangdong. This air ran into the remnant low pressure area of Higos and brought a sequence of subtle changes.

To the west of the Higos' centre, the southward march of cool air was aided by the northerly winds in the circulation of the remnant of Higos. It reached the coastal area of Guangdong as night fell. The front edge of the cool air may be treated as a cold front. To the west of Higos' centre, the opposite happened. The southward march of cool air was resisted by the warm air coming in from the ocean. Where the two air masses met, we had a quasi-stationary front, which may also be seen as a low-pressure trough. Figure 1 shows schematically the weather situation at 9 p.m. 5 October. Figure 2 shows the southward march of cooler air in the area north-west of the Pearl River estuary in the period before this time.

Figure 1
Figure 1 Schematic weather situation at 9 p.m. 5 October. The blue line represents the 22°C isotherm. The arrows indicate broadly the wind direction behind the cold front. "L" denotes the centre of the low pressure area.

Figure 2
Figure 2 Schematic chart illustrating the southward spread of the 22°C isotherm from noon to 9 p.m. on 5 October. The positions of the centre of the low-pressure area and the cold front are shown in grey.

Before the intrusion of cool air, the circulation of Higos' remnant consisted of a fairly uniform tropical air mass. However, after that intrusion, it effectively became a temperate depression involving a juxtaposition of cool and warm air masses. Such transformation normally occurs at higher latitudes, such as when typhoons move northwards and get close to Japan. To have it happening near Hong Kong is rare.

During the transformation, the source of energy sustaining the circulation of the low-pressure area also changed. A tropical cyclone derives its energy from the "latent heat" released when water vapour condenses to form raindrops. On the other hand, a temperate depression gets its energy when cool air sinks and warm air rises in different sectors of its circulation, during which "potential energy" is released to give "kinetic energy". This is in the realm of secondary school physics. Looking at figure 1 again, it was relatively cool to the west and warm to the east of the low-pressure centre of Higos. It is the kind of situation in which potential energy is released. Thus at this moment in time, the low-pressure area may be classified as a temperate depression. With the injection of energy, the circulation strengthened for a short while and higher winds speeds were also recorded in Hong Kong.

Figure 3 shows the path of the remnant low-pressure area of Higos in late Sunday and in the early morning of Monday. It shows an obvious northward shift which took place about the same time as the cool air invaded southward. It is another aspect reflecting the transformation of the nature of the low-pressure area. After midnight, the low-pressure area may be seen as a wave in the interface between the cool and warm air masses. Cool air spread southward and eastward on its rear side, that is, west of the centre. That explains why people noticed cooler weather setting in on Monday morning.

Figure 3
Figure 3 Movement of the centre of the low-pressure area from noon 5 October to the early morning of 6 October.

There are endless changes to weather. No single textbook could cover all the possibilities. Higos' abrupt change in direction and its transformation into a temperate depression near Hong Kong constitute a good example. The strengthening of winds in Hong Kong on Sunday arising from the intensification of a temperate depression which in turn came about through the superposition of the northeast monsoon on the remnant of a tropical cyclone is truly a special case. It looked odd to issue the strong monsoon signal in autumn with local winds blowing from the south or south-west. But considering the fundamentals of the weather situation, that is, a temperate depression coming about in the background of the monsoon, the strong monsoon signal was the only rational way of covering the event. It was a special case. It also would be a precedent case for the future.

The amazing story of Higos commenced with it being a tropical cyclone. The 90-degree turn changed its fate, leading eventually to its transformation into a temperate depression near Hong Kong. That was why we first had a number 1 tropical cyclone signal and later had the strong monsoon signal.

I love meteorology because weather is forever changing. What's more, you never know what is coming next.

C.Y. Lam

P.S. for serious weather fans: The above analysis was completed in very short time with limited data in hand. The purpose is to highlight the key aspects of the meteorological event. Figures 1 to 3 are schematic diagrams only. Please don't quibble about differences of a few kilometres here or a degree or so there. Such things belong to the work of future researchers with time to look into details.

Last revision date: <17 Jan 2013>