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  • Uncertainty in genesis of tropical cyclone

  • Thursday, 18th August 2016

Since midweek last week, the Observatory has anticipated the formation of tropical cyclones over a stretch of area from the northern part of the South China Sea to the western North Pacific this week, yet the formation location was rather uncertain at that time. Why is there such an uncertainty? To start with, we need to talk about the favourable conditions for the genesis of tropical cyclone.

One of the prerequisites for tropical cyclone formation is the presence of cyclonic convergence (anti-clockwise flow of airstream in the Northern Hemisphere) at the lower levels of atmosphere. One of the systems that are favourable for tropical cyclone formation is the Intertropical Convergence Zone, where the southwesterly airstream flowing from the Southern Hemisphere to the Northern Hemisphere meets the northeasterly airstream at the southern periphery of the subtropical high in the Northern Hemisphere. The convergence zone favours the development of cyclonic airflows and convective activities. The satellite imagery (Figure 1) this Tuesday (16 August 2016) noon depicts a rather long band of convective clouds extending from the northern part of the South China Sea to the western North Pacific, indicating the position of the Intertropical Convergence Zone.

Figure 1

Figure 1      Satellite imagery at 12 noon on 16 August 2016 with yellow arrows indicating the direction of movement of airstreams. (Source of satellite imagery: Japan Meteorological Agency)

Although different computer prediction models had already forecast earlier the formation of a tropical cyclone in the Intertropical Convergence Zone this week, there were differences between various models on when and where the cyclones would form along the convergence zone, which spanned a few thousands of kilometres. This added uncertainty to the forecast. In fact, with reference to the radar imagery captured at 6 am on Tuesday (Figure 2), a broad area of low pressure appeared over the south China coast and the northern part of the South China Sea, where there were more than one centres of cyclonic flow or low pressure. Some of them existed for only a while and dissipated. As to which one could finally develop into a tropical cyclone would depend on various factors such as the sea surface temperature, transport of moisture, vertical variations of winds, and divergence of airflow at the upper atmosphere. The Observatory kept monitoring and assessing the development of this broad area of low pressure. Eventually a centre of cyclonic flow developed at about 200 kilometres southwest of Hong Kong over the coastal waters of western Guangdong. It subsequently intensified into a tropical depression on Wednesday morning (17 August), warranting the issuance of tropical cyclone warning signal by the Observatory.

Figure 2

Figure 2      Radar imagery at 6 am on 16 August 2016 with the symbol "L" indicating the positions of the centres of low pressure.

Indeed, whenever the Intertropical Convergence Zone or a broad trough of low pressure is close to Hong Kong, no matter whether a tropical cyclone will ultimately formed, there will be chances of squally showers, thunderstorms and swells in Hong Kong. The public is therefore advised to keep track of the latest weather information from the Observatory. Those engaging in water sports or staying near the shore should take extra care against the danger brought by the rough seas.

C.Y.Y. Leung and L.S. Lee

Last revision date: <09 Sep 2016>