Reprint 322

From PPI to Dual Doppler Images

40 Years of Radar Observations at the Hong Kong Observatory

by

E W L Ginn, Hong Kong Observatory

Presented at the 32nd Session of the ESCAP/WMO Typhoon Committee

23-29 November 1999

Seoul, Republic of Korea

Introduction

Radar is a unique tool for continuous and close range observations of precipitation related weather systems such as rainstorms and tropical cyclones. The use of radar in weather offices dates back to the 50s. However, recent advancements in signal processing technology, computer hardware and software, have revolutionized capability of radars in observing the weather. This papers attempts to document the progress on the use of weather radar in a meteorological service from the 50s to the present. The focus will be from an operational point of view and the Hong Kong Observatory's efforts in radar observations of tropical cyclones will be used to illustrate the enormous development that has taken place in the last 40 years. A summary of the various radars used at the Hong Kong Observatory discussed in this paper is given in Table 1.

The Monochrome Analog era - the 50s to the 70s

The first weather radar for Hong Kong was a 3.2 cm X-band Decca 41 installed at Tate's Cairn, 580 m above MSL in 1959 (Fig 1). It has a double-curvature antenna and a magnetron transmitter capable of transmitting at a peak power of 20 kW.

The Decca 41 could perform horizontal PPI but did not had RHI capabilities. This radar could detect medium intensity precipitation (around 20 mm/h) out to about 200 km from Hong Kong but its useful range for detecting heavy precipitation (40 mm/h) such as those for a typhoon was reduced to less than 100 kilometres. The antenna was not covered by a radome (Fig 2) and had to be bolted down to avoid damage whenever winds exceeded around force 6. This further limited its usefulness during high winds.

Nevertheless, many valuable radar images of early typhoons were captured with this radar. Fig 3 is a Decca 41 PPI image of Typhoon Wanda in 1962, one of the most destructive typhoons to visit Hong Kong in recent history.

The problem of attenuation in heavy rain was well recognized and the Observatory started planning for the acquisition of an S-band weather radar in the early 60s. The project came to fruition with the installation of the Plessey 43S radar in 1966 (Fig 4).

The 43S was a S-band radar with a magnetron transmitter of over 650 kW peak power. It had an effective range of around 450 kilometres and was significantly superior to the Decca 41 in the monitoring of rainstorms and typhoons. In additional, this radar is capable of performing PPI at various elevation angles and also RHI.

It had a calibrated attenuator which, in a graded manner, enabled the estimation of radar reflectivity from the strength of the radar echoes. This is illustrated in Fig 5 for T Elsie in 1975. It also had an iso-echo function which was akin to a contour map of radar reflectivity, see Fig 6 for an example. Radar pictures of many typhoons were captured by the 43S. Fig 7 is a picture of Typhoon Rose in 1971 while Fig 8 is for Typhoon Hope in 1979. Both typhoons caused hurricane force winds in Hong Kong. Other interesting examples include the concentric eye of Typhoon Elise in 1975 (Fig 5) and the PPI and RHI images of STS Agnes in 1978 (Fig 9).

Although the two analog radars, the Decca 41 and Plessey 43S, greatly enhanced the Observatory's ability in tracking the movement and development of typhoons, their limitations were also obvious. The radar displays were monochrome CRTs and have to operate with a hood

(Fig 10) or in a dimly lit dark room (Fig 11). The data would be lost as next PPI sweep arrived and forecasters were basically working on one radar picture at a time. Although data could be recorded on open reel magnetic tapes for playback, the system was cumbersome and not easy to use. This limited the usefulness of the system and quantitative analysis of the data both operationally (on the radarscope) and during post analysis (on still photographs) were very difficult, if not impossible.

The Multi-colour Digital Era - the 80s

In the early 80s the Observatory worked towards acquiring a new breed of weather radar capable of digitizing the receiver signals and storing the data on hard disks for real time retrieval and on magnetic tapes for permanent archival. The Goodwood Digital Radar MR790S with a computer for data processing (Fig 12) became operational in 1983 and the 43S was relegated to standby status.

The Digital Radar could perform volume scans once every 6 minutes according to a pre-defined fixed scan strategy. The radar data could be displayed on a colour graphic monitor with different colours representing different reflectivities or rainfall rates. Apart from PPI, other radar products such as CAPPI (Fig 13), vertical cross section (Fig 14) and echo top display (Fig 15) were available. Animation sequence of radar images were also available and up to 4 recent images could be accessed with readout of cursor positions allowing tropical cyclone centres to be located with greater ease.

The resolution of the data however, was not high by today's standard due mainly to the relatively simple radar signal processor and the limited computer resources available at that time. The graphics displays were also of low resolution (512 x 512 pixels) compared to the present high resolution displays (1280 x 1024 pixels). The software was computer and operating system specific and could not be easily ported to another computer. Printouts of radar pictures were available but only in the form of alpha-numeric characters from a line printer.

Despite these limitations, this radar revolutionized weather forecasting operations at the Observatory. For the first time, forecasters could analyse the data quantitatively on an operational basis and radar data was archived and available to researchers for post analysis. The Observatory witnessed the genesis of nowcasting with the implementation of the Goodwood Digital Radar. With digital radar data available, efforts were directed towards systematic radar reflectivities and rainguage rainfall comparison and correlation. This paved the way for realizing an operational nowcast system of short term quantitative rainfall forecast.

The Exciting Doppler Era - the 90s

One of the greatest leaps in weather radar technology was the emergence of the Doppler technique. A Doppler weather radar can observe radial velocities of weather targets in addition to reflectivities. Doppler weather radar had been around since the 60s, however, many of these systems were targeted for research purposes and the first operational Doppler radar for Hong Kong, the EEC DWSR-93S was only installed in 1994 (Fig 16).

The EEC radar has a magnetron transmitter with peak output of 500 kW and an analog receiver. The main difference between this radar and its predecessors is that it is equipped with a fast digital signal processor and a versatile software which operates under a UNIX environment.

The radar signal processor, RVP6 is capable of generating high resolution reflectivity and Doppler velocity data. Amongst its many available functions, it can perform pulse pair processing for clutter correction and dual PRF processing for extending the unambiguous range for Doppler velocity data.

The software operates under a UNIX operating system thereby making it much easier to take advantage of advances in computer systems. As a result, the software and computer workstations have since been upgrading several times with a minimum of effort. Apart from PPI, CAPPI, echo top and vertical cross section, the radar software can generate a host of other useful products for forecasters in assessing the intensity and development of storms and the mean wind field near the radar. These include, Vertically Integrated Liquid (VIL), rainfall accumulation during the past 1 to 24 hours, maximum reflectivity projections and Velocity Volume Processing (VVP) winds, etc. (Fig 17).

Dual-Doppler and the 3D Era - Towards the Next Millennium

The Observatory installed the second and more advanced Doppler weather radar, the Mitsubishi RC-38, in early 1999 (Fig 18). This radar situates at the top of Tai Mo Shan, the highest peak in Hong Kong, almost 1 km above mean sea level and is unobstructed by local terrain in all directions.

The Tai Mo Shan weather radar is equipped with a 8.5-m dish high precision antenna, with a beam width of 0.9 degrees and a maximum side lobe of about - 30 dB (Fig 19). Instead of a magnetron, the radar utilizes a stable klystron transmitter with a peak power of 650 kW and has ground clutter rejection ratio of better than 55 dB. A digital receiver and powerful radar signal processor, RVP7, is employed, which provides much wider dynamic ranges (78 dB linearity for both Doppler and reflectivity modes) than traditional analog receivers and largely simplifies the radar hardware. The radar signal processor also features Fast Fourier Transform (FFT) and random phase processing which gives more accurate clutter correction and estimates of the Doppler velocity and spectrum width.

Apart from the advances in radar hardware, the most distinctive feature of the radar system is its ability to ingest and assimilate data from other radars. By networking with the EEC radar at Tate's Cairn, the system is capable of generating dual-Doppler winds within about 40 km of Hong Kong (to be elaborated in more details below).

Other advanced features of the Tai Mo Shan radar include :

• Composite reflectivity images which overcome the problem of physical blockage experienced by individual radars;

• 3-dimensional visualization of storm structures (Fig 20); • Forecast radar images based on previous images and user specified model (Fig 21); • Automatic warning and tracking of intense radar echoes based on user defined thresholds

(Fig 22) and alerting forecasters by voice; • Automatic switching of operation mode, such as the activation of the standby radar by the

operational radar when it detects severe weather in the vicinity of Hong Kong; • Remote control and monitoring of radar equipment and station facilities at Tai Mo Shan by

staff at the Observatory Headquarters (Fig 23).

The 1999 typhoon season was one of the most active on record. During the year, four tropical cyclones passed through Hong Kong (Fig 24), a first in the past half a centuary. Vital radar data for these storms was captured by the Observatory's Doppler radars at Tai Mo Shan and Tate's Cairn.

The availability of the Doppler winds provided valuable quantitative information on the intensity of typhoons. The case of Typhoon Dan, illustrates that good objective estimation of maximum winds for typhoons more than 400 kilometres from the radar (Fig 25), albeit at a significant height above the sea surface, can be obtained by an experienced forecaster.

The Tai Mo Shan and Tate's Cairn radar sites are separated by a distance of 11 kilometres and the dual-Doppler observations provide a reasonable coverage of Hong Kong area. At close range, these observations provided detailed 3D wind structure of tropical cyclones when they are over Hong Kong.

On 7 June 1999, STS Maggie crossed Hong Kong and passed between the 2 radar sites. As a result, the detailed 3D wind structure of Maggie was captured. Fig 26 is the dual-Doppler winds of Maggie at 0400 H on 6 June 1999. This is probably the first such observations in the world. The circulation of Maggie was highly asymmetric. Winds were significantly stronger in the northern semi-circle. In general, winds were strongest at the 2-km level and started to decrease above this height. From surface observations and the dual-Doppler winds, it can be seen the local terrain apparently had significant effect on the track of Maggie near the surface. Above 3 km, the track of the centre was largely unaffected by the local terrain.

The dual-Doppler observation shed new lights on the effect of terrain on the vertical structure of tropical cyclones as they hit the coast. It provides 'ground truth' 3D wind fields to verify numerical simulations of tropical cyclones. Operationally, it enabled forecasters to accurately judge the location and intensity of storms and issued appropriate warnings accordingly.

More Accurate Measurements and Virtual Reality - the Next Millennium

Looking ahead, we can expect advances in more accurate precipitation measurements and identification of rain, ice and hail using dual-polarization technique. Better removal and recovery of second trip echoes and delineation of noise and interference from real weather echoes can be expected with phase tagging of the transmitted radar pulses and corresponding decoding of the received pulses.

More powerful and faster computers will usher visualization of radar images into a new era. Forecasters will be able to 'fly' inside a typhoon, observe its rainbands, maximum winds and locate its centre with greater ease and accuracy.

Coupled with the next generation non-hydrostatic Numerical Weather Prediction (NWP) models, not only will forecasting of typhoons be more accurate, we can probably create virtual reality of a typhoon before its arrival and forewarn the public of the hazardous weather associated with some of the most destructive storms in the world.

References

Apps R F and Lo Y Y, 1963, : Comparison of the Theoretical Performance of Various Radars. Hong Kong Observatory Technical Note (Local) No.2.

Battan L J, 1973 : Radar Observation of the Atmosphere. Univ of Chicago Press, Chicago, Illinois.

Brown, R A, D W Burgess, J K Carter, L R Lemon and D Sirmans, 1975 : NSSL dual-Doppler radar measurements in tornadic storms : A preview. Bull Amer Meteor Soc, 56, 524-526.

Brown W B and Hansford R F, 1957 : Weather Radar for Hong Kong - Decca Radar Siting Report.

Davies-Jones, R. J., 1979: Dual-Doppler radar coverage area as a function of measurement accuracy and spatial resolution. J Appl Meteor 9, 1229-1233.

Doviak J R and Zrnic D S, 1993 : Doppler Radar and Weather Observations. 2nd Ed Academic Press.

Ginn E W L, S C Tai and C Y Lam, 2000 : Dual-Doppler observations of Severe Tropical Storm Maggie 1999 (to be published).

Lam C Y, 1984 : Digital radar data as an aid in nowcasting in Hong Kong. Proc Nowcasting-II Symposium, Norrkoping, Sweden, 3-7.

Lhermitte, R. M., and L. J. Miller, 1970 : Doppler radar methodology for the observation of convective storms. Preprints 14th Conf Radar Meteorology, Tucson, Amer Meteor Soc, 133-138.

Rinehart R E, 1997 : Radar for meteorologist. 3rd Ed Rinehart Publication.

Sachidananda M and Zrnic D S, 1999 : Systematic phase codes for resolving range overlaid signals in a doppler weather radar. J of Atmospheric and Oceanic Technology, 16, 1351-1363.

SIGMET, 1999: IRIS/OpenTM User's Manual Version 7.05. SIGMET Inc, Westford, Massachusetts.

Table 1 Summary of Weather Radars of the Hong Kong Observatory

Decca 41 II Plessey 43S Goodwood MR790S

EEC DWSR-93S

Mitsubishi RC-38

General

Model 41 43S MR790S DWSR-93S RC-38

Year 1959 - 1979 1966 - 1992 1983 - 1999 1994 - 1999 -

Period of service (year)

20 25 16 over 5 -

Radar site Tate's Cairn Tate's Cairn Tate's Cairn Tate's Cairn Tai Mo Shan

Display (resolution)

CRT CRT RGB Monitor (512 x 512)

High-resolution

colour monitor (1280

x 1024)

High-resolution

colour monitor (1280

x 1024)

Max range (km) 200 * 450 512 512 512

Antenna

Type double-curvature,

solid surface

parabolic dish,

solid surface

parabolic dish,

solid surface

parabolic dish,

solid surface

parabolic dish,

mesh surface

Diameter / width (m)

4.27 3.66 3.66 4.3 8.5

Gain (dB) 36 38 38 39 44

Beamwidth (deg) 0.75 (Horizontal) 2.8 (Vertical)

2 2 1.8 0.9

AZ rotation speed (rpm)

5 10 / 20 6 0 - 5 0.5 - 6

Table 1 Summary of Weather Radars of the Hong Kong Observatory (cont)

Decca 41 II Plessey 43S Goodwood MR790S

EEC DWSR-93S

Mitsubishi RC-38

Transmitter

Frequency range & (operating value) MHz

9320-9500 (9375)

2700-3050 (2800)

2700-2900 (2800)

2727-2925 (2920)

Preset at factory (2820)

Peak Power (kW) 20 665 650 500 650

Transmitter device magnetron magnetron magnetron magnetron klystron

Modulation pulse pulse pulse pulse pulse

PRF (Hz) 250 275 250 250 /623/934 250 - 1000

Pulse width (us) 0.35 / 2.0 2 2 0.8 / 2.0 1.0 / 1.8

Polarization N.A. Vertical Vertical Horizontal Horizontal

Receiver

Receiver MDS (dBm)

-92.5 -101 -110 -108 (Doppler)

-109 (Reflectivity)

-114 (Doppler)

-117 (Reflectivity)

Dynamic range (dB)

N.A. N.A. 80 26-80 (Doppler)

90 (Reflectivity)

88 (Doppler)

91 (Reflectivity)

IF (MHz) 29.5 30 30 30 30

Bandwidth (MHz) 4 6 1 2.0 (Doppler)

0.75 (Reflectivity)

1.18 (Doppler)

0.59 (Reflectivity)

**

Local oscillator reflex klystron reflex klystron Voltage Control

Oscillator with Phase Locking

Loop

Synthesizer Stable Local Oscillator (STALO)

Table 1 Summary of Weather Radars of the Hong Kong Observatory (cont)

Decca 41 II Plessey 43S Goodwood MR790S

EEC DWSR-93S

Mitsubishi RC-38

Radar signal processor

N.A. (analog) N.A. (analog) Front-End Video

Processor (FEVP)

RVP6 RVP7

Digitization resolution

N.A. (analog) N.A. (analog) 6 bits 8 bits 12 bits

Main processing computer

N.A. (analog) N.A. (analog)

Manufacturer N.A. (analog) N.A. (analog) Data General IBM HP

Model N.A. (analog) N.A. (analog) Eclipse S140 RISC 6000 Model 360

VISUALIZE C200

CPU speed N.A. (analog) N.A. (analog) <1 MFLOPS 13 MFLOPS 172 MFLOPS

Hard disk capacity N.A. (analog) N.A. (analog) 20 MB 1.3 GB 18 GB

Memory N.A. (analog) N.A. (analog) 512 KB 128 MB 1 GB

Operating system N.A. (analog) N.A. (analog) R-DOS UNIX / X-Windows

UNIX / X-Windows

* Maximum range for clear air detection of remote thunderstorms (20 mm/hr)

** Configurable in the radar signal processor

Fig 1 Tate's Cairn Meteorological Station. The antenna of Decca 41 was installed

on top of the circular building.

Fig 2 The 4.3-m double-curvature antenna of the Decca 41.

Fig 3 PPI image of Typhoon Wanda in 1962 captured by the Decca 41

Radar.

Fig 4 Radome of the Plessey 43S Radar.

Fig 5 Radar Observations of T Elise in 1975. Different attenuation levels were

applied to obtain estimates of the intensity of the radar echoes. Note also the concentric echoes near the eye.

Fig 6 'Iso-echo' display of rain bands. Different echo intensities were

displayed with different grey scales.

Fig 7 PPI image of Typhoon Rose in 1971 captured by the Plessey

43S.

Fig 8 PPI image of Typhoon Hope in 1979 captured by the Plessey

43S.

Fig 9 PPI image of Severe Tropical Storm Agnes in 1978 (left) and the RHI image

at the bearing of the storm centre (right) captured by the 43S.

Fig 10 Radarscope of the Decca 41 radar with a hood installed.

Fig 11 A forecaster monitoring the radarscope of the Plessey 43S in a

dimly lit room.

Fig 12 Data processing and display system of the Goodwood Digital

Radar.

Fig 13 3-km CAPPI image of Typhoon Ellen in 1983 captured by the Goodwood

Digital Radar.

Fig 14 Vertical cross section of an intense storm captured by the Goodwood Digital

Radar.

Fig 15 Echo top display of the Goodwood Digital Radar.

Fig 16 Radome of the EEC Radar, the first Doppler radar of the Hong Kong

Observatory.

Fig 17 VVP winds displayed in the form of a time series.

Fig 18 The Mitsubishi RC-38, installed at Tai Mo Shan, the highest peak in Hong

Kong, in 1999.

Fig 19 The azimuthal antenna radiation pattern of the Mitsubishi radar. The maximum side lobe was -31 dB down from the main beam.

Fig 20 3-dimensional composite radar image of Typhoon

York when it passed to the south end of Hong Kong at about 00UTC on 16 September 1999.

Fig 21 The Tai Mo Shan radar features automatic warning and tracking of

intense echoes based on user-defined thresholds. Echoes meeting the warning criteria are highlighted with hatched ellipses, and a voice warning is issued to forecasters. Forecast tracks are calculated based on past movement of the centroids of the echoes and are displayed as red arrows.

Fig 22 The TMS Radar system is capable of generating forecast CAPPI based on forecast wind fields (e.g. from NWP models). Top : current CAPPI image; Bottom : 2-hour forecast CAPPI image based on NWP winds.

Fig 23 A graphical menu showing the status of various parts of the radar system

as well as facilities of the radar station. The operator can remotely control some of the radar equipment and station facilities at the Observatory's Headquarters using this function.

Fig 24 Tracks of 4 tropical cyclones crossing Hong Kong in 1999

Fig 25 Doppler velocity field of T Dan. Note the extensive folding of velocity.

Fig 26 The dual-Doppler winds of Severe Tropical Storm Maggie at 3 km

height captured by the Observatory's Doppler weather radars at about 20 UTC, 6 June 1999.