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Before the weather computer would do anything, it needed to know the initial values for temperatures and winds at all levels at all grid point locations. How could Melissa possibly have that information? Fortunately, her father works for the National Weather Service, and he kindly gave her access to the National Weather Service's analyses of temperature and wind. Then it was just a matter of interpolating the analysis values to the particular grid points that Melissa had selected. At last all was set, and Melissa started the weather computer.
By now the weather computer was computing winds and temperatures at lots of levels and lots of grid points, and it was taking much longer to make its forecast. But finally it finished. Melissa examined the results. It proved to be a dull forecast. Because nothing was changing along the edges of the domain, 100 miles from the sixteenth green, not much was happening elsewhere, either. The temperature rose and fell just a little bit, but the winds stayed fairly constant.
Melissa knew that if she wanted to forecast the winds, she would have to do better. So she asked her father for access to the National Weather Service gridded forecasts. Her idea was that she could use those forecasts to update the data along her 100-mile perimeter, rather than keeping those values constant. That way, as her perimeter changed, the pressure, wind, and temperature variations would make their way inside the domain and affect the forecasts of wind and temperature at the sixteenth green.
It was a good idea, but her father wouldn't release the forecast data. (The National Weather Service was more proprietary with its data in those days.) So Melissa came up with another idea: she would extend her forecast domain so that it was 1000 miles in all directions. She knew that typical weather systems would take a day or so to cover that distance, and that as long as she started with accurate weather information, the winds and temperatures ought to evolve fairly accurately within her model domain, at least for the first twenty-four hours. Also, to forecast the evolution of weather systems, she extended her domain upward to a height of 10 miles, to include the jet streams and steering currents.
So she got the analysis over the larger area, interpolated it to her grid point levels, and started the forecast. The weather computer immediately spitted and told her "Error 47". Shocked, Melissa got the manual and looked up Error 47. She read:"Error 47: a ridiculous number of grid points. Reduce the number of points, or the computer will never be able to do all the calculations." Melissa checked some books, and found that the horizontal spacing of her grid, 20 miles, was small but reasonable, but that the vertical spacing was a bit excessive. So she cut back to a vertical spacing of 1000 feet for her levels, which was much more reasonable. Even so, that left her with about 50 vertical levels (actually 55, since she kept a higher resolution near the ground where it counted) and 100 grid points in the north-south and east-west direction, for a total number of grid points of 100 x 100 x 50 = 500,000. A long, long, time ago, Melissa was doing a forecast for a single point, and now she was doing a forecast for 500,000 points! No wonder it took longer! But at least with the new grid spacing, the weather computer was working again. And eventually it gave her her forecast.
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