Nowcasting for the Beijing Olympics
By Angela TAM

The capital is buzzing with anticipation: the highlight of the track and field events, the 110 m hurdles showdown between China's poster boy Liu Xiang and rivals Dayron Robles, Terrence Trammell and David Payne is scheduled to take place in a few hours.

But then comes news from the weather centre: a heavy downpour is expected to occur over the venue at precisely the hour of the 110 m hurdles final. Fortunately, with the advance warning, the organiser is able to reschedule the event and avoid chaos. There is time to notify spectators of the change and the athletes are able to time their preparations accordingly.

China has spared no effort in ensuring smooth running of the Beijing Olympics, and part of this effort involves engaging the world's weather forecasters to work together in providing the most accurate predictions of weather changes that may impact on the Games. And among these weather experts is a team from the Hong Kong Observatory (HKO), whose 'nowcasting' system SWIRLS has impressed the Games organiser.

Short-range forecasting
SWIRLS stands for "Short-range Warning of Intense Rainstorms in Localised Systems". When we think of weather forecasts, we tend to think of what the experts identify as medium-range events: forecast of general weather conditions over one week to ten days (long-range forecasts refer to any forecasts that go beyond two weeks, and include climate change predictions). Short-range forecasts are related to weather phenomena that occur in the next 48 hours. Nowcast or very short-range forecasts however focus on predictions of short-lived weather systems up to 6-12 hours ahead.

SWIRLS was developed to target a very short range in terms of both time and space: what is called mesoscale weather events occuring with a spatial variation at 10-100 km range over six hours. The whole nowcast process is primarily based on rain gauge and weather radar data. Weather radars scan the atmosphere every six minutes to obtain distribution of radar echoes, or precipitation patterns, over a certain range of distance. Motion vectors are then estimated from consecutive scans and used to advect precipitation patterns up to six hours ahead. Based on the forecast pattern and calibrated relationship connecting radar observations (technically known as echo reflectivity) to surface rain gauge records, SWIRLS can then estimate the amount of rain in different areas within the next few hours.

Time really is of the essence when it comes to nowcasting. It becomes less useful, in terms of timeliness of rainstorm warnings, if it takes a long time to complete the whole nowcast computation. A superfast computer is therefore essential for producing rapidly updated predictions for the forecasters' reference.

According to the HKO scientific officer Wong Wai-kin, the research and development for SWIRLS began after the mid-1990s. Launched in 1999, it was initially used to predict precipitation and rainstorms up to three hours ahead. Later, the system was enhanced to provide more 'guidance products' to forecasters, for warnings of landslips over Hong Kong and flooding in the northern New Territories. Running on a Sun Enterprise 450 Ultra-Sparc server, the system could calculate rainstorm movements and related data within a nominal time of less than six minutes.

The second phase of SWIRLS' development took place in 2003. Three-dimensional atmospheric analysis was adopted to provide hourly updates of mesoscale analysis of data from radar, satellite and automatic weather stations and the algorithm for forecasting the location of radar echoes was improved. In 2005, HKO upgraded the computer hardware for SWIRLS to a high-powered IBM P690 Regatta Power 4 server with 32 CPUs. About one-quarter of its capacity was allocated to SWILRS while the other three quarters were given to the operation of a high-resolution numerical weather prediction (NWP) model with grid points 5 km apart. The use of this model has enhanced SWIRLS' ability to forecast the growth and decay of precipitation systems.

The development and implementation of algorithms to nowcast severe gusts, hail and lightning commenced in 2006. With these new products under development, more computing power was needed to conduct intensive experiments and trials. HKO received several proposals in response to an invitation to bid for the supply of a more powerful computer, including one from a relatively new firm set up by the Hong Kong electrical products maker Shell Electric Manufacturing (Holdings) Co Ltd (SMC).
Galactic Computing, the Shenzhen-based subsidiary of SMC, makes 'supercomputing blades' - essentially duplicate sets of computing nodes featuring high-performance chipsets (dual Intel Xeon), all linked together by powerful Infiniband switches and fitted into one cabinet to make a high-performance computer that runs on the open-source operating system Linux.

While other well-known computer vendors also had similar 'blade' concepts in their server computer products, Galactic's machine had the merit of having a relatively mature design for handling high-speed computation together with storage access through its Infiniband connections. Being made in Shenzhen and serviced by support staff who are personally familiar with the way the machine works meant the local company's bid was unbeatable on price, performance and service.

Forecast Demonstration Project
The Beijing 2008 Forecast Demonstration Project (FDP) is organised by the World Meteorological Organisation's World Weather Research Programme. The first FDP was implemented at the Sydney Olympics in 2000, to demonstrate and quantify the benefits of real-time nowcasting. In Sydney the participating systems came mainly from the US, UK, Australia and Canada. The FDP for the Beijing Games however will feature a total of nine systems from Hong Kong and China, as well as those that took part in the previous FDP. Each system has its strengths and is designed to focus on a specific aspect of nowcasting. One US system, for example, uses its track record of radar analysis of convergence lines to predict the initiation of thunderstorms.

SWIRLS' focus will be rainstorms and severe weather elements that will occur within the next six hours. The system is uniquely suitable for the Games because it was developed to predict rainstorms within high spatial and temporal resolutions (5 km between grid points compared with the more typical 20-60 km), making it ideal for predicting the precipitation and severe weather events over specific Olympic venues scattered across the Chinese capital.

Involvement in the Beijing Olympics is viewed by HKO as a valuable opportunity for testing the system and facilitating improvement of existing and new forecast modules. The FDP also facilitates the exchange of ideas and technology on more advanced nowcasting algorithms and understanding of nowcast techniques adopted by other systems.

SWIRLS was successfully adapted to work in Beijing during two trials in the summers of 2006 and 2007 and a number of modifications were made to address the significant difference in transmission and the format of the observation data provided by the Beijing Meteorological Bureau (BMB), as compared to those in Hong Kong. The data provided in real time to all nowcast systems are obtained from radar, automatic weather stations, radiosonde, wind profiler, lightning location network and weather satellites.

The computing facilities, including a superblade server with 15 computation nodes and some workstations for radar data processing and display of SWIRLS products, have been set up at BMB. The graphical workstations, which feature an interactive graphical user interface and applications written in Java, were set up along with the displays and workstations of the other nowcasting systems so that users and forecasters can view them more conveniently. To make it even more user-friendly, some nowcast products are integrated with geographical information using GoogleEarth. Remote control is implemented so that the HKO team can monitor system performance and carry out diagnostics in Hong Kong.

Data from all the systems are to be digested by a group of experienced forecasters who will have the responsibility for disseminating the summarised information or warnings to the Games organiser for action when necessary. A training programme was organised to facilitate effective use of all nowcast products. The first training seminar was conducted in April 2007 and another one will be held for the systems go live in 2008. The FDP is set to be commissioned in July 2008 and, following a month of installation and optimisation, will be launched on 6 August 2008, two days before the Games?opening. It will continue until September 20 so that it will be able to cover the Paralympics on September 6-17 as well.

Forecast scales. Images courtesy Hong Kong Observatory

SWIRLS' forecast domain in the Chinese capital

Galactic's superblade

The SWIRLS setup for the Beijing Olympics

The FDP office set up at BMB

Nowcast information superimposed on GoogleEarth

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