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• NWP is a method for predicting future atmospheric conditions such as wind, temperature and humidity based on a set of physical equations by a super-computer. The computer program is called NWP model.
• The globe is covered by a regular 3-d grid and the initial values are set at each grid point. Then the equations are integrated with time starting from the initial condition. The observed data is merged to the short-term (say, 6-hour) prediction and make the initial condition 6-hour later. This cycle is called forecast-analysis cycle.
Observation
Analysis
forecast
Observation
Analysis
forecast
time
NWP model
Chaos and preditabilityA small initial error grows but does not diverge
Lorenz model
• dx/dt=-10x+10y
• dy/dt=28x-y-xz
• dz/dt=-8/3z+xy
• This is a non-linear equation, which can be solved numerically.
Chaos and predictability
• In NWP, small errors in the initial condition are unavoidable. Two initial states with small differences evolve to completely different states after certain time, due to a chaotic nature of the atmosphere. The atmosphere has a finite predictable period. In the mid-latitude atmosphere, the predictability limit is about 2 weeks.
• For this reason, weather forecast is based on ensemble forecast. The ensemble forecast is many forecast ( about 50 in JMA) starting from slightly different initial conditions. By ensemble forecast, probability of weather can be obtained. Also, the reliability of forecast can be known.
Ensemble forecast
Japan Meteorological Agency HP
Weather Agencies in the world
• Japan Meteorological Agency (JMA)
• Korea Meteorological Administration (KMA)
• European Centre for Medium-range Weather Forecasts (ECMWF)
• National Center for Environmental Prediction (NCEP), National Weather Service