ATOC 1050-Weather Discussion: Chapter 4

Ensemble Forecasting Example

Ensemble Forecasting Example

Chapter 4 of the course textbook describes ensemble forecasting. By following this link you will see an animation of an ensemble forecast of 500 mb heights. The animation shows forecasts that extend out for 360 h (or 15 days) from the initial model time (which for this example happened to be 00UTC Monday April 15, 2002). The forecast state at 500 mb is shown every 24 h in this animation.

This type of plot is called a spaghetti plot because the 500 mb height contour lines look like spaghetti by the end of the forecast period.

The lines shown in this spaghetti plot include:

red – 579 decameter height contour (5790 m height contour)
blue – 552 decameter height contour (5520 m height contour)
white – 579 and 552 decameter height contours from a specific numerical model called the MRF

yellow – 579 and 552 decameter height contours from a specific numerical model called the AVN
white – climatological 579 and 552 decameter height contours

Things to notice on this animation:

At the initial time notice that all of red and all of the blue lines are fairly close together. The different locations of the red and blue lines indicates the amount of uncertainty in the observed state of the atmosphere (and more specifically the uncertainty in the location of the 579 and 552 decameter height contours at the 500 mb level).

As the animation progresses notice that the red and blue lines begin to spread out, and eventually there appears to be little correlation between the different blue or red lines. This spreading out of the lines is caused by differences in the numerical model predicted state of the atmosphere that arise due to the initial uncertainty. As we can see, even small differences in the initial state can lead to very large differences in forecasts that extend out just a few days.

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