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The Meso-cyclone over Hong Kong on 8 May 2004:

Doppler Radar Observation and Numerical Simulation

L.O. Li, W.K. Wong & C.C. Lam

International Conference on Mesoscale Convective Systems and

Heavy Rainfall in East Asia (ICMCS-IV),

Beijing, China, 15-19 November 2004

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The Meso-cyclone over Hong Kong on 8 May 2004: Doppler radar observation and numerical simulation

Luen-on Li, Wai-kin Wong and Queenie C.C. Lam

Hong Kong Observatory, Hong Kong, China. E-mail: loli@hko.gov.hk

1. Introduction

The rapid spinup of a meso-β-scale cyclone produced a rainstorm over Hong Kong on 8 May 2004. This paper presents an analysis of the structure of the meso-cyclone as it passed over Hong Kong on 8 May 2004. Data from a mesoscale observation network that included four Doppler weather radars were used in the analysis. Numerical model simulations results were also used to understand the evolution of the meso-cyclone and its associated high winds and heavy rain.

2. Observations and Analyses

Three-dimensional analyses of the meso- cyclone were carried out. Data from four Doppler radars, three in Hong Kong (TMS, TCR and TDWR shown in Fig. 1), and one in Macao (MAC), as well as four wind profilers and several tens of automatic weather stations were employed in this study.

Fig. 1: Tracks and positions of the parent meso-cyclone and its embedded smaller scale vortex (solid line), as well as other weaker vortices in its vicinity (dashed line) at 2-km height. Numbers beside the crosses indicate times of passage (Hong Kong Time, HKT = UTC + 8 hours) on 8 May 2004.

The meso-cyclone first formed at the southern periphery of a deep westerly trough. It tracked east-northeast (Fig.1), moving to the right of the upper-level southwesterly environmental flow ahead of the trough. The meso-cyclone lasted around 3 hours. Smaller scale vortices formed within and in the vicinity of this well organized parent meso-cyclone.

The meso-cyclone was about 100 km in diameter and its cyclonic circulation reached a

height of 5 to 6 km. Two hook echoes were observed and the vortex centres coincided with the weak echo regions. Tornado vortex signature of strong azimuthal shear and a maximum Doppler velocity of about 25 m/s in low levels were observed in the eastern part of Hong Kong at around 07:00-07:30 HKT. Large gradient of reflectivity, which marked the region of inbound air (Doviak and Zrnic 1993), was found in the wake of the strongest azimuthal shear. The centre of the meso-cyclone tilted forward and to the left in the vertical (Fig. 2). Coupled with surface observations and damage evidence, a tornado of F0 class in the Fujita Scale which was embedded in the parent meso-cyclone touched down the ground shortly after 07 HKT. The tornado and the parent meso-cyclone weakened rapidly after the tornado reached its mature stage at 07:30 HKT.

Fig. 2: Vertical cross section of Doppler velocity along the line XY (lower left) and AB (lower right) of the top figure at 07:12 HKT.

3. Numerical Model Simulation

Numerical simulations of the meso-cyclone were carried out using the 20-km Operational Regional Spectral Model (ORSM) (Lam and Yeung 2003) and the 5-km Non-hydrostatic Model (NHM) (Fujita et al. 2002).

Simulation results of the model run initialized at 15 UTC (23:00 HKT) 7 May 2004 were examined. The genesis of the meso-cyclone was found to be successfully forecast by ORSM and NHM. The system-relative cyclonic centre at low-levels passed over Hong Kong and that matched with the radar observations (Fig. 3). The cyclonic centre on higher level tilted to the

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left, which coincided with the ridge axis of the vertical cross section of equivalent potential temperature (θe). The supply of warm and moist southwesterly air over the South China Sea, as shown by high θe tongue in model forecast, gave rise to intensification of the low-level vortex by creating a convectively unstable environment. Moisture and heat fluxes were transported upward to a lower θe region aloft, maintaining the cyclone for further development (Zhang and Bao 1996).

Fig. 3: System relative wind overlaid with potential vorticity (in 0.1 PVU) at 950-hPa level in 20-km ORSM 9-hour forecast valid at 08 HKT 8 May 2004.

In the 5-km NHM forecast using the 20-km ORSM output as the initial and boundary conditions, the axis of the simulated meso-cyclone was found to slant forward and to the left side of the storm motion (Fig. 4). The low-level θe anomaly was more pronounced. It should have played a crucial role in enhancing the vertical transport of moisture and heat budgets which resulted in a deeper surface cyclogenesis and larger amount of forecast precipitation. The tilting of model cloud water content in T+7 hour forecast matched well with the observed radar reflectivity. Though the forecast position of the surface cyclone centre was due north of the observed or the ORSM forecast, the minimum pressure predicted by NHM was closer to the observed 1001 hPa recorded at Waglan Island.

4. Discussions and Concluding Remarks

The evolution and structure of a meso-β-scale cyclone were studied using mesoscale observation network including four Doppler weather radars, and real-time simulations of 20-km ORSM and 5-km NHM. The 3-dimensional structure of the meso-cyclone and development of smaller scale vortices including a tornado within the parent meso-cyclone were examined.

Dual Doppler wind field retrieval from multiple radars was found to be a powerful tool in monitoring and tracking meso-cyclone and the embedded vortices.

Fig. 4: 3-dimensional structure of the simulated meso-cyclone in 5-km NHM T+7 hour forecast valid at 06 HKT 8 May 2004. PV iso-surface of 1 PVU in orange, vertical cross section cloud water content in purple contours, system relative winds at 900-hPa level in green vectors and hourly rainfall in colored filled region.

Both ORSM and NHM successfully forecast the genesis and evolution of the meso-cyclone. It was not surprising to note that smaller scale vortices were not depicted by models due to relatively coarse model resolution and limitations in physical parameterizations. The leftward and forward tilting of the system-relative cyclonic centre matched with radar observations. As revealed from model simulation results, a high θe tongue associated with the warm and moist southwesterly air from the South China Sea and the relatively low θe air descending on the rear side of the system was responsible for the rapid intensification of the meso-cyclone over Hong Kong.

Compare to ORSM, the higher resolution NHM shows better performance in cyclone intensity forecast and quantitative precipitation forecast. However, the cyclone position forecast may not be necessarily better. Scale interactions among synoptic scale, mesoscale and convective scale are very complex and further studies are desirable. Assimilating Doppler velocities from multiple radars or incorporating tendencies of atmospheric fields measured by frequently update mesoscale observations and more sophisticated physical processes in mesoscale models would be explored in the future with a view to further improving quantitative forecasts of wind and rain.

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Acknowledgements

The authors would like to thank Messrs K.H. Yeung, H.G. Wai, K.C. Tsui and Miss Sharon S.Y. Lau for their comments. Thanks are also given to Mr. Paul K.L. Ho for radar data retrieval and Mr. C.K. Chow for preparation of model graphical outputs. The radar data in Macao was obtained from Macao Meteorological and Geophysical Bureau. NHM and ORSM were developed on the basis of Japan Meteorological Agency NHM and RSM respectively.

References

Doviak, R.J. and Zrnic D.S., 1993: Doppler Radar and Weather Observations. Academic Press.

Fujita T., K Saito, Y. Yamada, J. Ishida, M. Narita, S. Goto, C. Muroi, T. Kato and H Eito, 2002: Development of a nonhydrostatic model for very short-range forecasting at JMA. Proceedings, 15th AMS Conf. Numerical Weather Prediction, 355-356.

Lam, Queenie C.C. and L. H.Y. Yeung, 2003: Impact of radar data on model forecast of heavy rain associated with the landfalling tropical cyclone. Proceedings of International Workshop on NWP Models for Heavy Precipitation in Asia and Pacific Areas, 106-113.

Zhang D.L. and N. Bao, 1996: Oceanic Cyclogenesis as Induced by a Mesoscale Convective System Moving Offshore. Part II: Genesis and Thermodynamic Transformation. Mon. Wea. Rev. 124, 2206 – 2224.