Near Real-time Analysis of the Wind Structure of Tropical Cyclones

Nathaniel T. Servando

Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) Quezon City, Philippines

Ping-wah Li, Edwin Sau-tak Lai

Hong Kong Observatory, 134A Nathan Road, Kowloon, Hong Kong

ABSTRACT Real-time information on the wind structure of tropical cyclone is very useful for weather forecasters, in particular in the assessment of landfall impact. In this study, LAPS (Local Analysis and Prediction System, originally developed by FSL of NOAA) is used to objectively analyze near real-time wind observations on a high-resolution three-dimensional grid. The data used for the analysis come primarily from SYNOPS, automatic weather stations (AWS) over Hong Kong and the neighbouring Guangdong province, rawinsonde, wind profiler, Doppler and radar-based TREC winds, SATOBS and QuikSCAT. Wind fields from the Operational Regional Spectral Model (ORSM) of the Hong Kong Observatory (HKO) are utilized as first guess. The re-analyzed wind fields are then used to study the wind structure and intensity of tropical cyclones. Comparisons of wind analyses before and after landfall using three tropical cyclone cases are presented and the observed changes in wind structure and intensity described. Ingestion of more wind observations in such high-resolution re-analyses reveal some interesting features in the cyclone circulation, especially in the evolution of wind asymmetry within the vortex structure as a result of land-sea differences during landfall. 1. Introduction

Considering the amount and variety of weather data pouring into the forecasting offices these days, it is reasonable to assume that effective processing and utilization of such information would naturally lead to more accurate analyses and hence more reliable forecasts and warnings. For tropical cyclones suffering from the perennial problem of insufficient observational data, the challenge is to assimilate different data types from conventional as well as unconventional sources into one coherent framework for forecasters’ ready interpretation. The problem becomes more acute as tropical cyclones approach land and forecasters have to make timely operational decisions in relation to the location of wind maxima and the extent of gale force winds. In turn, disaster managers will rely on such decisions to implement mitigation measures and contingency plans to safeguard lives and property within the community. In this paper, near real-time wind data as received at the Hong Kong Observatory are ingested into an objective analysis system, LAPS (Local Analysis and Prediction System, originally developed by Forecast Systems Laboratory of NOAA), in an attempt to produce a frequently updated 3-D cyclone circulation based on the latest available information. The first guess field is provided by the Observatory’s Operational Regional Spectral Model (ORSM). As an exploratory exercise based on cases of landfalling cyclones near Hong Kong in recent years, the main objectives of this study are: (a) to assess the impact of additional wind data in terms of system robustness and operational viability; and (b) to evaluate possible changes in tropical cyclone wind structure and intensity before and after landfall. In Section 2, analysis procedures and description of the data used are presented. Brief history of the tropical cyclone cases included in this study can be found in Section 3. Discussion of results and summary of findings are given in Sections 4 and 5 respectively.

2. Methodology

The methodology adopted involves three essential components: (a) near real-time wind observations with sufficiently high resolution in horizontal and vertical coverage; (b) high-resolution NWP output as first guess; (c) a flexible enough objective analysis system to integrate a variety of data types from different sources.

2.1 Data description

The analysis domain for this study is bounded by 112.0°E, 116.2°E, 20.7°N and 24.1°N centred over Hong Kong. In the event of a tropical cyclone approaching the south China coast, a full array of observations from the local and regional networks, conventional information from the GTS network, as well as remote sensing data from a variety of sources, would be made available for processing and analysis. Specifically, for wind information, the data set includes: (a) surface and upper-air synoptic reports over the region; (b) surface reports received from ships and buoys; (c) surface observations from the networks of automatic weather stations over Hong Kong and the neighbouring Chinese province of Guangdong; (d) wind profiler data from Hong Kong; (e) TREC winds derived from radar echo movement at selected CAPPI levels (Lai et al., 1999); (f) Doppler radar velocity data; and (g) ocean surface winds derived from QuikSCAT .

Among these data, the radar-based winds provide the highest resolution coverage, both spatially (horizontal as well as vertical) and temporally. There are two Doppler weather radars in Hong Kong, both operating with an effective range of about 500 km. TREC (Tracking Radar Echoes by Correlation) analysis derived from two consecutive radar images is updated every six minutes. It is part of the routine analysis products from the Observatory’s nowcasting system, SWIRLS, primarily designed for rainstorm monitoring and forecasting (Li et al., 2000). As such, there are certain limitations that may restrict TREC’s interpretation for tropical cyclone winds. Its wind vectors averaged over a 6-minute period (i.e. duration between two radar scans) have to be evaluated against the more conventional 1-minute or 10-minute winds. TREC vectors may also be complicated or contaminated by echo growth and decay, and analysis below say 1 km becomes impractical due to interference from orography and other physical blockage. The translational component due to cyclone motion itself has also not been explicitly resolved. Alternatively, dual-Doppler analysis, on paper, should give a more accurate reflection of the cyclone circulation. However, with the two radars in Hong Kong spaced about 10 km apart, it is only applicable when the cyclone gets close enough. In practice, only Doppler radial velocities are available most of the time. Despite such limitations, both TREC and Doppler winds do provide valuable details that cannot be otherwise obtained from more conventional sources (see in Fig. 1 example for Typhoon Maggie taken from Lai, 1999), and it is a matter of how such information can be meaningfully integrated for objective interpretation.

2.2 NWP first guess The first guess background field comes from ORSM, a hydrostatic model originally developed by the Japan Meteorological Agency. In the configuration adopted by the Observatory (NPD/JMA, 1997), the model has a 60-km outer domain and a nested 20-km inner domain. Wind fields used in this study are taken from the 20-km ORSM which is run every three hours for a forecast range of 24 hours. For a typical hourly analysis cycle carried out on a near real-time basis, the background fields to be extracted from the 3-hourly operational schedule of 20-km ORSM runs are summarized in Table 1.

2.3 Analysis scheme

LAPS is the objective analysis scheme used to assimilate the wide array of

observations in a near real-time mode. The algorithms used by LAPS are discussed in details in Albers 1995. Parameters of LAPS are adjusted to create a domain suited for the specific purpose of this study. The spatial domain is centred at 114.3°E and 22.3°N near Hong Kong. The analyzed fields reside in a 3-D 125 x 105 x 21 grid, with 5-km resolution in the horizontal and 50-hPa resolution in the vertical.

Although LAPS has to rely on ORSM for the background fields, there is no redundancy in data utilization between the two systems in the sense that observational data used in LAPS being already included in the initialization process of ORSM. In practice, LAPS only makes use of ORSM forecasts as first guess (as shown in Table 1). For example, for a timely LAPS analysis at 00 UTC, the best operational option is to utilize the ORSM 3-hour forecast based on 21 UTC data as first guess, instead of a LAPS re-analysis of ORSM 00 UTC analysis which will not be available for another couple of hours.

Table 2 summarizes the types of wind data that are ingested into LAPS and ORSM. Whereas LAPS horizontal resolution is chosen to be 5 km from the selectable options of 2, 5 or 6 km, the ORSM resolution is relatively coarse at 20 km. The ORSM is run 3-hourly and therefore does not include observations at asynoptic hours. On the other hand, LAPS analyses are done on an hourly basis and target in-between observations for a frequently updated situation. In tropical cyclone situations, a bogus vortex is included in the ORSM analyses which passes onto LAPS analyses through the first guess background fields. The wind analysis procedure for u and v (wind components in x and y directions, respectively) makes use of all available data sources in a two-pass Barnes objective analysis. Fig. 2 illustrates how the data are processed in LAPS. In the first pass, SYNOPS, observations from local and regional AWS networks, TREC, SATOBS, wind profiler, QuikSCAT and aircraft reports (AMDAR) are used to generate a preliminary analysis. The second pass is similar to the first except that Doppler radial velocities are added. Doppler wind vectors are mapped onto the LAPS grid and compared against the first-pass analysis at each grid point. After obtaining the corrections in the radial and tangential components, the Barnes analysis is run again with the complete ensemble of data, now enhanced with radar information. Products from LAPS include gridded fields of surface and upper-level winds, pressure, temperature, dewpoint, cloud coverage, cloud base, cloud top, precipitable water and radar reflectivity. In this study, we shall focus on the wind aspects in an attempt to evaluate the intensity and structural changes of tropical cyclones before and after landfall. The wind fields are represented by vectors and contour plots. Isotach analyses are performed to depict the gale extent. Vertical cross-sections of wind speeds at selected locations, either at constant latitude or longitude, are drawn to reveal cyclone circulation in depth. 3. Tropical Cyclone Case Reviews A brief review of the history of tropical cyclone cases used in this study is presented. Selection of cases is based primarily on their landfall locations relative to Hong Kong to ensure reasonable radar coverage. Owing to data availability, particularly with respect to Doppler data and ORSM products, eligible cases coming close enough for the purpose of this study are: Typhoon Sam in 1999 and Typhoons Utor and Nari in 2001. Fig. 3 shows the HKO best tracks of the three typhoons. 3.1 Typhoon Sam (9910) Sam hit Hong Kong directly on 22 August 1999. It intensified on approach and reached typhoon strength for a short time less than 6 hours prior to landfall and then weakened rapidly into a severe tropical storm after moving inland. Before and after the passage of Sam, northerly and southwesterly maximum hourly winds both exceeded 90 km/h respectively, with the former slightly stronger having a peak value of 104 km/h. Peak gusts on high ground and in exposed places generally exceeded 140 km/h and were as high as 160 km/h. As Sam weakened after landfall, the southwesterlies trailing in its wake brought record-breaking rainfall of more than 600 mm to Hong Kong. 3.2 Typhoon Utor (0104)

Typhoon Utor was characterized by its large circulation, with its radius of circulation reaching a distance of 1,000 km. It caused considerable damage to life and property in Taiwan and the Philippines before entering the South China Sea. Record-breaking 24-hour rainfall of over 1,000 mm was registered in the central mountains of Luzon. Landing over the south China coast about 100 km east of Hong Kong on 6 July, Utor proved quite tenacious and retained its circulation for more than 40 hours while moving further inland. Already 200 km away over inland Guangdong on the night of 6 July, violent squalls associated with the intense outer rainbands of Utor affected Hong Kong. Southerly maximum hourly winds exceeded 80 km/h, with winds gusting to 120 – 130 km/h on high ground and in exposed places. 3.3 Typhoon Nari (0116) The track of Typhoon Nari was highly unusual. After some erratic movement due to the Fujiwhara effect caused by Typhoon Danas and several cycles of intensification and weakening over the Pacific near the Ryukyus, it took on a rarely seen southwestward course across Taiwan and towards the south China coast. It weakened into a severe tropical storm as it made landfall about 200 km east of Hong Kong around noon time on 20 September. Tracking westwards and weakening further over land, Nari was closest to Hong Kong that evening when it was about 100 km to the north. Wind damage was generally less severe compared to Sam and Utor. Northerly maximum hourly winds were only about 30 km/h offshore and around 50 km/h on high grounds, with gusts exceeding 60 km/h on approach. 4. LAPS Near Real-Time Wind Analyses 4.1 Data impact The case of Typhoon Nari at 12 UTC on 20 September 2001 is used to illustrate the impact on the analysis after additional and unconventional data are ingested through LAPS. At the time, Nari has already made landfall, passing to the north of Hong Kong on a westerly track. At an altitude of say about 3 km (i.e. 700-hPa), LAPS has the benefit of a relatively good coverage of Nari’s circulation through Doppler radial velocities as well as TREC winds (Fig. 4a). Doppler coverage at the 850-hPa level is more limited (Fig. 4b) but wind structure around the core region can still be discerned with wind speed exceeding 50 km/h northeast of the eye. TREC winds at such low levels, though available, are not used due to increasing risk of data contamination as a result of potential interference from orography and other physical obstacles. At the surface, major injection of additional information comes from the dense AWS networks within Hong Kong as well as over the neighbouring Guangdong province (Fig. 4c).

After including these as well as other available additional information, significant differences in the resultant LAPS analysis can be seen when compared with the initial first guess field from ORSM (i.e. 3-hr forecast from ORSM run based on 09 UTC analysis on 20 September 2001). Winds and isotach analysis at selected levels at 500-hPa, 850-hPa and 950-hPa are shown in Fig. 5. In the first guess fields, the centre of Nari was shifted to the south away from the land mass. Reinforced by additional information, LAPS is able to come up with a circulation that is better defined and correctly places the centre of Nari further north over the coastal region of Guangdong.

4.2. Wind analyses verification

The reliability of LAPS objective wind analyses is evaluated quantitatively through an experiment in which wind profiler data from Hong Kong are not included for LAPS but are used instead as independent observations for verification purpose. Selected vertical wind profiles at the location of the Hong Kong wind profiler are extracted from the LAPS wind fields for comparison with the wind profiler data. In Fig. 6, time series of the two sets of vertical wind profiles between 00 UTC and 13 UTC on 6 July 2001 are plotted. This corresponds to the period of closest approach in the case of Typhoon Utor. Although there are some quantitative differences in wind speeds, the trends displayed are largely similar between the

two sets of vertical wind profiles. Both indicate a strengthening in the local winds at 11 UTC, the time of southwesterly gale onset in the wake of Utor.

The vertical profiles of u, v and wind speed based on the wind profiler, LAPS and ORSM first guess at 12 UTC 06 July, 2001 are plotted in Fig. 7. If profiler data are taken to be the ground truth, then there is a consistent improvement in the zonal component all the way up to the mid-troposphere as a result of LAPS analysis. LAPS advantage, however, is less obvious for the meridional winds, with a tendency of over-correcting at certain levels. The overall impression is that LAPS analyses do make an earnest effort in dragging the first guess profiles towards the ground truth, with improvement particularly significant around the 700-hPa level where the impact of radar data is most keenly felt. Such a notion is objectively confirmed by the lower average bias and root-mean-square residuals of LAPS wind as compared with the first guess ORSM winds. 4.3 Tropical cyclone wind structure analyses Apart from data impact and analysis reliability, we would also like to examine whether the injection of additional data helps us capture the landfall characteristics of the three typhoons under study. Particular attention is given to the changes in wind structure before and after landfall: (a) near-surface (900-hPa level, about 1 km above ground) wind distribution of strong winds (> 23 knots), gale force winds (>33 knots), storm force winds (> 48 knots) and the hurricane force winds (> 63 knots); and (b) vertical cross-sections of maximum wind speeds. 4.3.1 Typhoon Sam Distributions of near-surface winds six hours apart before and after the landfall of Sam are compared in Fig. 8. The isotach analysis shows an envelope of strong to gale force winds wrapping fully around the eye of Sam before landfall. Hurricane force winds are located over the southeastern quadrant with the radius of maximum winds (i.e. the axis of isotach maximum) about 100 km away from the eye. This is followed by a general decrease in wind speeds after landfall, particularly over the northwestern quadrant where winds are less than strong. The area of hurricane force winds noticeably shrinks with the radius of maximum winds dilating outwards to a distance of 200 km, a tell-tale sign that the cyclone is weakening. The LAPS analysis is consistent with the operational assessment of Sam’s intensity changes reaching marginal typhoon strength before landfall and degenerating rapidly into a severe tropical storm as it passes to north of Hong Kong (see discussion in Section 3.1). The north-south cross-section of winds across Sam’s circulation is shown in Fig. 9. Before landfall, a shallow core of high winds at an elevation of 1 km is clearly depicted on the southern (seaward) side of the eye. Winds on the northern (landward) side are not as intense but extend much deeper from the low levels all the way up to about 500-hPa in pressure level. The eye region is well defined, and also well aligned in the vertical. Six hours later when Sam is over land, winds generally decrease both north and south of the eye. The eye region itself becomes dilated and its base appears to be somewhat lifted away from the surface. 4.3.2 Typhoon Utor The evolution of Utor’s near-surface winds before and after landfall according to LAPS is in Fig. 10. In contrast to Sam, the wind structure of Utor is highly asymmetric throughout its passage, with high winds confined mostly to a band of southwesterly jet running more or less parallel to the coastline. Even though winds generally weaken after landfall, gales remain quite extensive within Utor’s large circulation. Again, LAPS analysis is in general agreement with observations from Hong Kong and the neighbouring areas. Most of the damage is associated with the southwesterly gales sustained over a prolonged period in the wake of Utor (see discussion in Section 3.2).

The eye of Utor shows up much better in the north-south cross-section of LAPS winds in Fig. 11. While the southwesterly gales are much in evidence from the 900-hPa fields, Fig. 11 reveals two cores of high winds on the landward side as well, one narrow strip very much near the surface beneath the 900-hPa level and the other broader and higher up near the 700-hPa level. While Hong Kong appears to have been spared the full impact of the narrow strip of high winds just north of the eyewall during Utor’s closest approach, it does feel the force of the extensive southwesterly gales twelve hours later. At the same time, the LAPS analysis actually shows that high winds are relatively better sustained on the landward side over the inland region of Guangdong and the central core region of Utor is essentially maintained, consistent with observations at the time concerning the tenacity of Utor as it ploughs further inland. Such analyses demonstrate the need to interpret cyclone intensity and structural changes from a full 3-D perspective, and not to over-rely on wind fields from selected horizontal slabs which can be misleading at times. 4.3.3 Typhoon Nari Compared to the other two typhoons in this study, Nari is very much weakened by the time it passes to the north of Hong Kong. Because it lands further east over the coast of Guangdong, it does not have the benefit of additional data coverage (especially from the radar perspective) for the LAPS analysis. Evaluation can only be made soon after Nari’s landfall at 06 UTC on 20 September 2001 and six hours later when Nari makes its closest approach to Hong Kong. The near-surface winds from LAPS are shown in Fig. 12. In the case of Nari, wind distribution is highly asymmetric, with the intense winds mostly confined to the northern half of the circulation. Hong Kong stays just outside of Nari’s strong wind areas during the landfall stage; and by the time of closest approach, even the strong winds have moderated, leaving only a narrow strip over the northwestern quadrant of the circulation. This is consistent with the generally subdued wind conditions experienced in Hong Kong during the passage of Nari (see discussion in Section 3.3). Fig. 13 shows the north-south cross-section of Nari’s winds from LAPS analysis. The very weak nature of Nari’s circulation with poorly defined core region is in sharp contrast to the structure depicted in the cases of Sam and Utor. Soon after landfall, a shallow isotach core on the seaward side and a deeper isotach core north of the cyclone centre can still be seen. Six hours later, both high wind regions have largely fizzled out and Nari ceases to be a system of much significance. 5. Summary of Findings Through the use of LAPS and the mesoscale observational data obtained from Hong Kong and the surrounding region, this study has provided a proof of concept in the development of a near real-time analysis system for tropical cyclones winds. The case reviews of three tropical cyclones coming near Hong Kong in recent years have shown that: (a) the idea is operationally viable; (b) the system is sufficiently robust and reliable; (c) there is positive data impact in terms of both the location of cyclone centre and wind distribution; (d) the analysis produces reasonable results that are generally consistent with the weather scenarios observed during the passage of the three cyclones under study. With encouraging results from this exploratory work, plans to implement the system on an operational trial basis are being considered at the Hong Kong Observatory. The ultimate goal is to establish a system that will enable forecasters to make timely operational decisions with respect to the location of wind maxima and the extent of violent winds before and after the cyclone makes landfall. In turn, it is reasonable to assume that disaster managers will be better informed and better prepared in implementing mitigation measures and contingency plans for the safeguard of lives and property. For example, a frequently updated isotach analysis such as that provided by LAPS incorporating the latest wind information as received will greatly facilitate operational

assessment on the timing of the onset and duration of gales affecting Hong Kong. Often in the case of landfalling cyclones passing to the north of Hong Kong, southwesterly squalls in the wake of the cyclone’s closest approach can be even more dangerous; and an accurate analytical depiction of such a possibility will certainly enhance forecasters’ awareness and anticipation. As wind damage is also very much linked with surface gustiness, which is partly related to the downward transfer of momentum within the boundary layer, a 3-D wind structure with full information on the wind profiles in the vertical can also allow forecasters to make educated guesses on the likely wind scenarios during the cyclone’s passage. The proposed system, at this early stage of development, is not without its drawbacks and limitations. While near-surface winds are no doubt of great interest to the forecasters, radar-based winds below 1 km are not reliable enough for ingestion into the analysis systems. How to reliably resolve the wind structure in the boundary layer with better observational data remains a problem that needs to be addressed. On a more general point, error characteristics and reliability of some of the new data types ingested are still poorly understood or studied; and this would have consequences on proper data quality-control and utilization in operational implementation. The way the observational data are distributed may also lead to a biased interpretation of the cyclone’s wind structure. For example, a lot of the land-sea asymmetry seen in the wind structures of the three typhoons under study may actually be the result of denser observations over land. More data sensitivity studies need to be carried out and in the longer term, the balance may hopefully be redressed through the use of more remote sensing information over the data-sparse oceanic regions. If more observational data, particularly from remote sensing platforms, do become routinely available in the coming years, the prospect of a frequently updated analysis approach will be even more promising and rewarding. It means that rather than relying on dense in-situ observations which tend to be concentrated in the developed countries, data coverage will become extensive enough to offer possibilities of applications over a much wider region. While meteorological services with sufficiently powerful computing facilities can go for the more elaborate and expensive 4-D variational analysis approach (and even then, new analysis would probably only be done every three or six hours with results available after a finite amount of intensive computing time), a frequently updated alternative (say done every hour with results to be made available in terms of minutes) based on a regional analysis system such as LAPS can allow centres with less resources to tackle specific operational forecasting problems of interest. If neighbouring centres can pool their resources together for the common good, then such a system may even offer the best solution in terms of utilization of information from local sources which, even if they are made available on the international circuit, may easily get submerged in the sea of data that goes around. Acknowledgement This research project is made possible through the ESCAP/WMO Typhoon Committee Research Fellowship Scheme and the attachment programme greatly facilitated by the financial and logistic support kindly offered by the Hong Kong Observatory. The lead author in particular would like to thank the staff members of the Forecast Development Division of the Hong Kong Observatory for the assistance and invaluable suggestions generously provided throughout the study. References Albers, S.C., 1995: The LAPS wind analysis. Mon. Wea. Rev., 10, 342-352. Lai, E.S.T., 1999: TREC application in tropical cyclone observation. ESCAP/WMO Typhoon

Committee Annual Review 1998. Li, P.W., W.K. Wong, K.Y. Chan & E.S.T. Lai, 2000: SWIRLS - An Evolving Nowcasting

System. Technical Note 100, Hong Kong Observatory.

NPD/JMA, 1997: Outline of the Operational Numerical Weather Prediction at Japan

Meteorological Agency, JMA.

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Figure 1 – (a) TREC-analyzed wind field (128-km range) and (b) dual-Doppler wind field (42-km range) of Maggie at 03:36 HKT on 7 June 1999.

Figure 2 – Schematic diagram showing how the data are processed in LAPS.

Figure 3 – HKO best tracks of Sam, Utor and Nari.

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Figure 4 – Some of the additional information ingested by LAPS for the case of Nari at 12 UTC on 20 September, 2001: (a) Doppler radial velocities and TREC winds at 700-hPa level; (b) Doppler radial velocities at 850-hPa level; (c) surface winds from Hong Kong and Guangdong AWS. The analysis domain is bounded by 112.0°E, 116.2°E, 20.7°N and 24.1°N centred over Hong Kong. Shaded areas in (a) and (b) are radar reflectivity.

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ORSM first guess

LAPS analysis

500-hPa level

500-hPa level

850-hPa level

850-hPa level

950-hPa level

950-hPa level

Figure 5 – Comparison of ORSM first guess fields (i.e. 3-hr forecast based on 09 UTC analysis, left panels) with LAPS analysis (right panels) valid at 12 UTC on 20 September 2001 at 500-hPa, 850-hPa and 950-hPa levels. Wind barbs plotted on grid and contours are isotachs (at intervals of 2 knots for 500-hPa and 950-hPa winds and 5 knots for 850-hPa winds).

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