Remote Sensing Observations of the Subsidence Zone Within the Eye of Typhoon Nuri in Hong Kong in 2008 C.P. Wong1, P.W. Chan1 1Hong Kong Observatory, 134A Nathan Road, Kowloon, Hong Kong, China, [email protected] ABSTR ACT to the south of Tsing Yi Island, and turned northwards There are a number of ground-based remote sensing to cross the northeastern part of Lantau Island, Tuen equipment operating at or near the Hong Kong Inter- Mun and Yuen Long that ev ening. Nuri then crossed national Airport (HKIA), including two wind prof ilers, a Deep Bay, the western part of Shenzhen and the Pearl minisodar (sonic radar) and a multi-wavelength micro- River Estuary that night and made a second landfall wav e radiometer. The wind profilers and minisodar near Nansha of the Guangdong province subse- are able to measure the three components of wind, quently . whilst the microwave radiometer provides the tem- Radar imagery in Figure 2 depicts that there was no perature profile of the troposphere continuously. In intense precipitation at 7 p.m. on 22 August 2008 at August 2008, during the period when Typhoon Nuri Siu Ho Wan and near the centre of Nuri. Only a f ew made landfall in Hong Kong, these equipment millimeters of rainfall were estimated in the imagery. prov ided continuous data f or close monitoring of the In f act, only a few millimeters of rainfall were recorded v ariation of dynamic and thermodynamic parameters by the automatic weather stations nearby Nuri’s eye. of the atmosphere. When the eye of Nuri approached HKIA, a subsidence zone with downward motion of up 2. REMOTE-SENSING EQUIPMENT to 6 m/s within the eye was captured by the wind pro- f iler and minisodar while rise of temperature of 6 deg The radar wind prof ilers located at Siu Ho Wan and C together with a temperature inversion at the lower Sha Lo Wan (locations in Figure 1) continuously moni- atmosphere were revealed by the microwave radiome- tored the v ariation of v ertical wind profiles when Nuri ter. This paper analyses the remote sensing data was ov er Hong Kong. Technical details of these pro- mentioned above. f ilers can be f ound in [1]. They belong to the boundary -layer type and operate at a f requency of 1299 MHz. Wind profiles, including the horizontal and 1. INTRODUCTION v erticalv elocities, are available every 10 minutes. There are a number of remote sensing equipment operating in Hong Kong, including the wind profiler, The minisodar (co-located with Siu Ho Wan wind minisodar and multi-wavelength microwave radiome- prof iler) uses acoustic waves of 4500 kHz to disturb ter. The wind prof iler and minisodar measure the the atmosphere. The turbulent eddies in the atmos- three components of wind, whilst the microwave radi- phere will cause the sound wave to become reflected. ometer provides the temperature profile of the tropo- From the backscattered signal, the three-dimensional sphere continuously. In August 2008, during the pe- wind components of the lower atmosphere could be riod when Ty phoon Nuri made landfall in Hong Kong, obtained. The detection height of the minisodar is 100 these equipment provided continuous data f or close m and the height resolution is 5 m. Wind profiles are monitoring of the v ariation of dynamic and thermody- av ailable every 5 minutes. namic profiles of the atmosphere. Nuri was the f ourth The microwave radiometer at the HKIA (location in tropical cyclone that necessitated the issuance of Figure 1) is similar to that described in [2]. It retrieves tropical cyclone warning signals in Hong Kong in 2008. the temperature and humidity profiles up to 10 km by It also necessitated the issuance of the Increasing measuring the radiation intensity at 14 channels in the Gale or Storm Signal No. 9. This was the f irst No. 9 microwave spectrum that are dominated by molecular signal since the passage of Typhoon Dujuan in Sep- oxy gen emissions (51-59 GHz) and atmospheric water tember 2003. v apor (22-30 GHz) respectively. The microwave On 22 August 2008, Nuri made landfall in Hong Kong radiometer is able to make both zenith scans (for tem- and its track within the territory is given in Figure 1. perature and humidity profiles up to 10 km) and eleva- Ty phoon Nuri was f irst located near the Hong Kong tion scans (for temperature measurements within the University of Science and Technology (H.K.U. of Sci & boundary layer of the atmosphere, up to 2 km). Tech) in Sai Kung area ov er the eastern part of Hong Kong at around 4:50 p.m. (08:50 UTC, local time = 3. WIND PROFILER OBSERVATIONS UTC + 8 hours) and weakened into a sev ere tropical Figure 3 depicts that, at around 7 p.m. on 22 August storm. Under the effect of terrain, the circulation of 2008 when the eye of Nuri moved closer to the Siu Ho Nuri re-organized itself. The original centre moved Wan profiler, changes in the v ertical wind prof ile northwestwards and dissipated rapidly. A new centre occurred at altitudes of 1800 m to 3000 m abov e Siu f ormed near Tseung Kwan O and turned to move Ho Wan. Within this y ear, the wind speed reduced westwards, passing over the eastern part of Victoria signif icantly, changing f rom f resh northwesterly to light Harbour and the southern part of Kowloon Peninsula westerlies, and finally picking up as strong southwest- within 1 km south of the Hong Kong Observatory erlies. Similar variation of the wind profile occurred at (HKO) headquarters. The centre of Nuri then passed altitudes of 3000 m to 4200 m later at about 8 p.m., 6. COMPARISON WITH OTHER STUDIES and similar changes also occurred near the surface Because of the high temporal resolution, wind profilers (between the ground and 600 m) at about the same become more important in daily meteorological time. The appearance of the tilting of the circulation operations. Reference [3] made use of wind profiler centre with height within the eye of Nuri suggests that data f rom Macao and Hong Kong to analyze the tropi- the intensity of Nuri had weakened. cal storm Kompasu. Their result showed that under If we consider the vertical motion measured by the Siu the influence of a tropical storm, the change in wind Ho Wan profiler, subsidence occurred from a height of direction could be v ery f ast and there was an obvious 4200 m to near the surface at about 6:30 p.m. (Figure change in the wind direction before and after a tropical 4). The blue colour in the figure represents downward storm made landfall. In the present study, besides the motion. Approaching 8 p.m., the subsidence was change in wind direction with the passage of Nuri, the ev en stronger than before with vertical v elocity subsidence associated with the cyclone is also (downward) reaching 5.8 m/s. Precipitation could also examined from v erticalv elocity data of wind profiler. appear as downward motion in the data of boundary - There are v ery f ew studies to observe the warm core lay er wind profiler. However, the radar imagery in structure of tropical cyclones using ground-based Figure 2 showed that there was no intense precipita- microwave radiometers so f ar. There have been tion near Siu Ho Wan around the time. Theref ore the studies to observe the warm core using satellite-based downward motion in Figure 4 is believed to mainly microwave radiometer, e.g. [4]. At the warm core of a arise f rom the subsidence within the eye of Nuri. tropical cyclone, the temperature difference at the higher lev els of the troposphere (at around 250 hPa) 4. MINISODAR OBSERVATIONS could reach 16 degrees Celsius. This paper gives a Figure 5 shows that when Nuri passed over Siu Ho preliminary study on the warm core structure of a Wan, there was an obvious change in the wind tropical cyclone in the boundary layer using a ground- direction in the lower atmosphere as measured by the based microwave radiometer. minisodar. The lower lev el wind changed f rom north- westerly to northeasterly at around 6 p.m. initially. 7. CONCLUSIONS After Nuri departed Siu Ho Wan, southwesterlies pre- The remote sensing equipment operating in Hong v ailed at the lower levels after 8 p.m. Kong, including the wind profiler, minisodar and multi- Figure 6 depicts the distribution of the vertical wav elength microwave radiometer, provide continuous v elocities at the lower levels of Siu Ho Wan at around data f or the close monitoring of the v ariation of 7 p.m. It is observed that subsidence occurred at the dy namic and thermodynamic profiles of the atmos- lower lev els of Siu Ho Wan with the maximum down- phere. Their data are used in the present paper to ward v elocity of about 3.2 m/s. In between 7 p.m. and document the eye structure of Typhoon Nuri when it 8 p.m., the subsidence persisted (not shown) and the made landf all over Hong Kong. The wind prof iler and maximum downward v elocity reached about 4.3 m/s. the minisodar capture the subsidence motion within This v alue was close to that measured by the wind the eye. The microwave radiometer measures the prof iler in Section 3. warming of the boundary layer associated with the subsidence. The remote-sensing equipment would be used to study the structures of other tropical cyclones 5.