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Weather Forecasting

(continued)

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6. Numerical Weather Forecasting

• Rely on extensive calculations. That’s where “numerical” comes from.

• Use equations, including the gas law, conservation of mass, conservation of energy (1st law of thermodynamics), conservation of momentum (Newton’s 2nd law), and conservation of water vapor. Conservation means “all accounted for”.

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Numerical Weather Forecasts

• Couldn’t be accomplished without computers to do the many calculations

• Concept was established by L.F. Richardson in England in the early 1900s

• Start with current observations, called initial conditions

• Then many small forecasts over short time periods “step forward” in time the equations.

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Steps in Numerical Weather Forecasting

• 1. Weather Observations

• 2. Data Assimilation

• 3. Forecast model integration (of the equations with time)

• 4. Forecast tweaking and Broadcasting

• Models can be short-range or longer

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Lewis F. Richardson

• English scientist who did the first numerical weather forecast

• Forecast was for a part of Europe

• His initial conditions allowed gravity waves to grow

• His time step was numerically unstable—advection changed the winds with errors during his time step

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More Problems

• He made a numerical error early on in the extensive series of calculations

• The result showed a low pressure center that did not occur

• But the method he developed is used today with great success, because of computers, better mathematical methods, more and better initial data

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Initial conditions

• Today there are many sources of data, spanning the globe, both surface and upper-air, and remote sensing instruments like radar and satellites.

• Data assimilation merges all this world-wide data, checking for errors, and assigns it to appropriate positions within a grid of points.

Fig. 13-7, p. 386

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Fig. 13-7a, p. 386

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Fig. 13-7b, p. 386

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Fig. 13-7c, p. 386

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Fig. 13-7d, p. 386

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Fig. 13-7e, p. 386

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Fig. 13-7f, p. 386

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Fig. 13-7g, p. 386

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Fig. 13-7h, p. 386

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Fig. 13-7i, p. 386

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Fig. 13-7j, p. 386

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Fig. 13-7k, p. 386

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