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  • Challenges in forecasting typhoons from the east

  • Monday, 27th July 2015

Hong Kong is situated on the south China coast, with mainland China to its north and the South China Sea to the south. When tropical cyclones come from the sea and make landfall over the south China coast, they will usually continue to move inland, weaken progressively and dissipate. Some tropical cyclones approach Hong Kong from the east and travel close to the coast of southern China. If the direction of their westward movement changes even slightly (Figure 1), not only will there be a difference in how they make landfall, but there can also be substantial difference in their intensity variations. How can such a small change lead to a large difference?

For a tropical cyclone over the sea to the east of Hong Kong, if it adopts a westerly track with a slight northerly component, it will hit the eastern part of the south China coast sooner (Track 1 in Figure 1; Figure 2), and travel inland for some distance before moving to the north of Hong Kong. That means, the tropical cyclone will weaken over the land for some time before approaching Hong Kong. By that time, it will have weakened considerably and its threat to Hong Kong will be relatively low. Conversely, if its westward track bears a slight southerly component, the tropical cyclone can stay very close to the coast without making landfall, or re-enter the sea after hitting the land (Track 2 in Figure 1). Either way, it can maintain its intensity while edging closer to Hong Kong, posing a greater threat to Hong Kong as demonstrated by Typhoon Maggie that hit the territory in 1999 (Figure 3). Back then, the Observatory issued the Tropical Cyclone Warning Signal No. 9 and Maggie made landfall over the Sai Kung peninsula, bringing gale force winds and heavy rain to Hong Kong. Due to sheltering by terrain, the northerly winds brought by tropical cyclone from the east will usually be relatively weak at first, but strengthen abruptly as the tropical cyclone comes very close to Hong Kong. The case of Maggie clearly illustrated that northerlies in Hong Kong strengthened from strong winds to gales within less than 3 hours (Figure 4). It can readily be seen that for tropical cyclones from the east, even if there is a tiny change in their direction of westward movement, their impact on the weather of Hong Kong can be drastically different.

Figure 1

Figure 1      A small change in the direction of westward movement of tropical cyclone can lead to a change in
how it makes landfall and a substantial difference in its intensity variations.

Figure 2

Figure 2      In September 2001, Tropical Cyclone Nari made landfall at Shantou and then weakened gradually.
It became a tropical depression before it was closest to Hong Kong.

Figure 3

Figure 3      Maggie moved westwards along the south China coast and maintained the intensity as a typhoon
prior to its landfall at Hong Kong.

Figure 4

Figure 4      Local winds strengthened from strong winds to gales within less than 3 hours
when Maggie came very close to Hong Kong.

Linfa that affected Hong Kong in early July 2015 is also an example of tropical cyclones coming from the east. In the morning of 9 July, Linfa was centred over the sea at more than 200 km to the east of Hong Kong. Being a typhoon with intact structure, it headed towards Hong Kong on a westward track. The fixed-wing aircraft jointly sent by the Observatory and the Government Flying Service recorded hurricane force winds in the vicinity of Linfa's eye, and gales of Linfa also extended out to about 100 km from its centre. As Linfa was forecast to traverse along the coast instead of moving inland, its intensity was anticipated to sustain. Local winds might also pick up quickly within a short period of time when Linfa moved further close to Hong Kong. It turned out that while Linfa skirted at just about 50 km to the north of the Observatory (Figure 5), it rapidly weakened and its gales did not affect Hong Kong generally. This formed a stark contrast to what happened with Tropical Cyclone Utor in 2001 (Figure 6). Although Utor weakened into a severe tropical storm following its landfall near Shanwei and skirted to the north of the Observatory at around 80 km inland, it was still able to bring gale force winds and heavy rain to Hong Kong for a prolonged period.

Figure 5

Figure 5      Linfa rapidly weakened before arrival and skirted at just about 50 km to the north of the Observatory.

Figure 6

Figure 6      Utor moved over the inland areas at about 80 km to the north of the Observatory and brought
gale force winds and heavy rain to Hong Kong.

Although the weather conditions and scientific factors in the above cases are different, all of them demonstrate that a small change can lead to a large difference. Therefore, forecasting typhoons from the east is full of challenges and their threat to Hong Kong should not be taken lightly. These cases deserve more research studies and they also serve as prime examples for considering the issuance of tropical cyclone warnings in the future.

LS Lee

  • Interactions between Tropical Cyclones

  • Tuesday, 7th July 2015

Entering July this year, tropical cyclone activities over the western North Pacific have been increasing. The satellite imagery in Figure 1 shows that there are a total of three tropical cyclones over the northeastern part of the South China Sea and the western North Pacific. You may wonder: If Chan-hom continues to edge closer to Linfa, what will be the impacts on the latter? Concurrent occurrences of three tropical cyclones happened in the past, but were not many. Some examples in recent years can be found in the HKO's educational material.

Figure 1

Figure 1      Visible imagery captured at 8 a.m. on 6 July 2015 showing tropical cyclone Linfa over the northeastern part
of the South China Sea as well as Chan-hom and Nangka over the western North Pacific.
(The image was originally captured by MTSAT-2 of the Japan Meteorological Agency)

Dating back to the 20's to 30's of the last century, Dr. Sakuhei Fujiwhara (1884 - 1950) already discovered that when two tropical cyclones approach each other, they tend to rotate anti-clockwise about a point between them. This phenomenon is usually known as the Fujiwhara effect. Research studies showed that tropical cyclones begin to interact more prominently when their centers come within around 1200 km of each other. The degree of interaction increases as the separation distance decreases[1], while the separation distance where interaction commences depends on the sizes of the tropical cyclones[2]. Other research studies also pointed out that the interaction between two tropical cyclones depends on the sizes and intensities of the tropical cyclones as well as the environmental steering flow; whereas tropical cyclones of unequal sizes likely to have greater interaction than two of similar sizes[3].

When two tropical cyclones come into proximity, it may bring about the following situations:

(1) The two tropical cyclones rotate in a stable orbit (Fujiwhara effect), followed by a release and escape (Figure 2). For example in 2009, tropical cyclone Parma near the Philippines interacted with another tropical cyclone Melor, and Parma underwent a looping motion during 5-7 October (Figure 3 and 4).

Figure 2

Figure 2      A conceptual model showing interaction of two tropical cyclones: approach and capture, followed by a stable mutual orbit (Fujiwhara effect), then release and escape. The diagram is adapted from Lander and Holland (1993).

Figure 3

Figure 3      The tracks of tropical cyclones Parma and Melor in September-October 2009.

Figure 4

Figure 4      Animation of infra-red satellite imagery showing tropical cyclones Parma and Melor from 3 to 12 October 2009. (HKT is local time, the satellite imagery was originally captured by MTSAT-2 of the Japan Meteorological Agency)

(2) One tropical cyclone is captured and "swallowed up" by another tropical cyclone, or the two tropical cyclones undergo a merger (Figure 5). This scenario is most likely to occur when one tropical cyclone is much larger and stronger than the other[4]. Examples are Zeb and Alex in 1998 (Figure 6); Namtheun in 2010 near the Taiwan Strait interacting with Lionrock over the northeastern part of the South China Sea, weakening and dissipating afterwards (Figure 7 and 8).

Figure 5

Figure 5      A conceptual model showing interaction of two tropical cyclones. When they orbit and come closer and closer together,
they eventually undergo a merger. The diagram is adapted from Lander and Holland (1993).

Figure 6

Figure 6      Animation of infra-red satellite imagery showing tropical cyclone Zeb and Alex from 10 to 13 October 1998. (UTC is Coordinated Universal Time, the satellite imagery was originally captured by GMS of the Japan Meteorological Agency.
The animation is adapted from the University Corporation for Atmospheric Research (UCAR))

Figure 7

Figure 7      The tracks of tropical cyclones Lionrock, Namtheun and Kompasu in August - September 2010.

Figure 8

Figure 8      Animation of infra-red satellite imagery showing tropical cyclones Lionrock, Nantheun and Kompasu
from 30 August to 1 September 2010. (HKT is local time, the satellite imagery was originally captured by
MTSAT-2 of the Japan Meteorological Agency)

(3) The two tropical cyclones only exhibit "semi-direct" interaction, and their tracks are largely determined by the steering flow associated with other synoptic weather systems[4][5] (Figure 9).

Figure 9

Figure 9      A conceptual model showing other synoptic weather system (e.g. subtropical ridge) provides the main steering flow to the tropical cyclones. The diagram is adapted from Carr and Elsberry (1997).

Whenever there are interactions between two or more tropical cyclones, they will drag each other, rotate, cause one of them weakening, merge together or escape from each other, etc. These are superimposed on the steering flow of the synoptic environment. The tracks of the tropical cyclones will then become rather complex, making forecasts more difficult. Nowadays, we have a grasp of the basic conceptual models, and computer numerical models can also generally capture the interaction processes between tropical cyclones. However, as many factors come into play (including changes in intensities, sizes and relative positions, etc. of the tropical cyclones), there will be discrepancies between different model forecasts, posing great challenges to forecasters. In any case, members of the public are advised to pay close attention to the latest tropical cyclone information and weather forecasts issued by the Observatory.

T Kung and C C Lam


[1] Brand, S., 1970: Interaction of binary tropical cyclones of the western North Pacific. J. Appl. Meteor.,9, 433-441.

[2] Introduction to Tropical Meteorology, 2nd Edition, Chapter 8: Tropical Cyclones, MetEd (2010) (, COMET Program, UCAR.

[3] Prieto, R., B. D. McNoldy, S. R. Fulton, and W. H. Schubert, 2003: A classification of binary tropical cyclone-like vortex interactions. Mon. Wea. Rev., 131, 2656-2666.

[4] Lander, M. A., and G. J. Holland, 1993: On the interaction of tropical-cyclone-scale vortices. I: Observation. Quart. J. Roy. Meteor. Soc., 119, 1347-1361.

[5] Carr, L.E., III, and R. L. Elsberry, 1997: Objective diagnosis of binary tropical cyclone interactions for the western North Pacific basin. Mon. Wea. Rev., 126, 1734-1740.

  • Where is the "Hiatus" in Global Warming?

  • Friday, 26th June 2015

In the past several years, climate deniers have been bleating about the slowdown of global warming since 1998, or the so-called "hiatus". Against the background of increasing atmospheric carbon dioxide concentration in recent decades, some deniers even claim that carbon dioxide does not cause global warming and hence there is no need to curb emissions. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change has explained that short-term trends, such as the trend over 1998-2012,  would be very sensitive to the start and end dates selected and would not in general reflect the long-term climate trend (e.g. the trend over 1951-2012).  Short-term temperature fluctuations of the globe can be strongly affected by natural variations of the climate system, such as the El Niño that we are experiencing now,  as well as by volcanic and solar activities.  In our earlier blog "Has global warming stopped?", we showed how climate deniers used cherry-picking tactics to magnify selected short-term trends in support of their claims while ignoring the big picture.

Estimating the Earth's temperature has always been a challenge to climate scientists because of differences in evolving observing practices, changes in instrumentation, methods and locations of measurement, as well as the uneven and insufficient coverage of temperature measurement, especially over the oceans. Some studies have suggested that natural variations, such as the increase in heat uptake by the oceans, has an important role to play in determining the global temperature in the past couple of decades. Recent developments however have enabled scientists to come up with a better estimate of global average temperature: over land, a much larger surface temperature dataset (viz. the International Surface Temperature Initiative databank [1]) has been released and the number of stations available for analysis has doubled; while over the oceans, surface temperature observations on buoys have increased and at the same time a new method has been developed to improve the correction of the systematic bias of ship data.

Figure 1 shows the old and new data analyses of the US National Oceanic and Atmospheric Administration (NOAA) [2] that depict a global warming trend since the 1950s and, based on the analyses, Figure 2 compares the old and new warming rates over different periods of time. While the long-term overall warming rates in NOAA's old and new analyses are rather similar (0.117 and 0.129 oC/decade respectively) during the period of 1951-2012, the warming rate for the period of 1998-2012 has more than doubled from 0.039 oC/decade in the old analysis to 0.086 oC/decade in the new analysis. Even more intriguing is the fact that if the latest data of 2013 and 2014 are included in the new analysis, the warming rate over 1998-2014 would become 0.106 oC/decade, bringing it even closer to the long-term overall warming rate. As such, the fragility of the so-called "hiatus" claim of significant warming slowdown is evident for all to see as it has obviously failed to withstand the test of time!

Figure 1

Figure 1      NOAA's old and new analyses of global temperature anomaly (data source: NOAA)

Figure 2

Figure 2      NOAA's analyses of global warming rate over different periods of time (data source: NOAA)

S M Lee


[1] Release of the International Surface Temperature Initiative's Global Land Surface Databank

[2] Science publishes new NOAA analysis: Data show no recent slowdown in global warming