Weather computer zooms in

Weather computer zooms in

NWS' Louis W. Uccellini says accurate long-range forecasts are limited to about 14 days.

NWS' new IBM RS/6000 forecasts on a scale of a few hundred square miles

By Patricia Daukantas

GCN Staff

A week after a surprise Jan. 25 snowstorm blanketed the East Coast, National Weather Service scientists were still conducting post-mortem simulations on their just-installed IBM RS/6000 Scalable Parallel supercomputer.

The National Centers for Environmental Prediction in Camp Springs, Md., studied carefully how the supercomputer's models had belatedly predicted the path of the low-pressure storm but missed on the precipitation forecast, NCEP director Louis W. Uccellini said.

The 768-processor SP, which last year was ranked as the world's 26th-fastest supercomputer [GCN, Nov. 22, 1999, Page 1], will run numerical weather modeling programs on which meteorologists across the country base their daily forecasts for television, radio and newspapers.

Later this year, the weather service will more than double the number of processors, taking the SP higher in the elite ranks of ultrafast computers.

Weather models running on previous supercomputers have predicted weather features roughly the size of New Jersey, which is 7,836 square miles. The new machine can run more complex codes that anticipate weather patterns on the scale of individual counties, or a few hundred square miles.

Smaller grids

Until last month, NCEP's production weather forecasting simulations used a horizontal grid size of 32 kilometers for regional models and 95 kilometers for global models.

Starting Jan. 24, NCEP scientists began running a global-model code with 70-kilometer horizontal resolution on the new SP, Uccellini said. Coincidentally, a powerful winter storm was brewing over the Southeast, and the new code's first prediction was that the storm's northward track would fall farther west than in earlier estimates.

By late spring or early summer, NCEP meteorologists will start using a regional model with 22-kilometer horizontal resolution, Uccellini said. By the end of next year, the models will handle horizontal grid sizes of 10 kilometers on the regional scale and 50 kilometers on the global scale.

Also by next year, the models will have finer vertical resolution, which will give aviators better turbulence forecasts, Uccellini said.

The regional models now divide the atmosphere into 45 levels up to an altitude of 22 kilometers. By next year, the code should handle 70 vertical levels.

NWS this year will start issuing seven-day weather forecasts, and within five years the seven-day forecasts will be as accurate as today's five-day forecasts, Uccellini said. The SP is not designed for six-month to one-year climate-change predictions.

'The numerical forecast gives us the data,' Uccellini said. 'We can then make specific forecasts for individual cities and get them out not only to the local forecast offices but also the private sector and the media.'

Weather forecasts are theoretically limited to about 14 days in advance, Uccellini said, because during that time tiny errors within the model's initial state grow large enough to render predictions useless.

Last fall IBM Corp. installed the machine at the Commerce Department's Census Bureau Computer Center in Bowie, Md. NWS meteorologists watched to see whether the SP hardware and software ran stably before they declared the system operational on Jan. 18, Uccellini said.

During the shakedown, NWS made the system's data available to a wide community of meteorologists for independent review. The declaration that the SP is operational means that the forecast-generating software cannot be changed without following protocols for informing the community.

NCEP is leasing the SP for $35 million. When the lease runs out in 2002, the weather service hopes to replace it with something even more powerful, NWS director John Kelly Jr. said.

'Our challenge has been to keep the computational assets up with the data that's out there,' Kelly said, referring to the growing amount of information from remote-sensing satellites.

The SP, which has a theoretical peak speed of 690 billion floating-point operations per second, replaced a Cray C90 that was damaged by fire Sept. 27, just before deployment of the IBM hardware.

A year ago, NWS negotiated agreements with the Air Force and the United Kingdom Meteorological Office to assist it with weather forecasts in the event of computer outages, Kelly said.

'The fire taught us the importance of backup procedures,' he said.

NCEP's expansion plans call for upgrading the SP to 2,048 processors in September. That will boost theoretical peak speed to 2.5 TFLOPS'in the same ballpark as the Energy Department's largest classified supercomputers.

Kelly cautioned, however, that even the biggest computers won't always give entirely accurate forecasts. 'None of us has a 100 percent understanding of the weather,' he said.

Meteorologists have long been in the vanguard of supercomputing, as only powerful computers can solve the complex mathematical equations that simulate the Earth's atmosphere and produce forecasts before they become outdated.

Currently the Naval Oceanographic Office, the U.K. Meteorological Office, the Deutscher Wetterdienst in Germany and NASA's Goddard Space Flight Center have weather research supercomputers listed ahead of the NCEP machine among the world's top 500.

Uccellini said the new system is big and fast enough to run additional meteorological simulations along with the operational forecasts delivered to NWS customers.

'We had completely filled up the old computer with operational runs,' he said.

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