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.