Why We Cannot Predict Rainfall Extremes - Chaos, Lies and Butterlies

Edward Lorenz
How accurate are weather forecasts?  Not very.

Call it chaos or call it the 'butterfly effect' - http://en.wikipedia.org/wiki/Butterfly_effect - it is "the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state."

This is the name given by Edward Lorenz (23 May 1917 – 16 April 2008), an American mathematician, meteorologist, and pioneer of chaos theory.  He was not consulted the the Weather Gone Wild authors, or others who hang their hat on faulty science .. maybe because he is dead and because science is secondary to crafting sensational journalism.

In practice the butterfly effect means we can't predict future extremes (like rainfall) that cause flooding.  Nope. All we can practically predict is weather will regress to its mean, but highs and lows we won't know.  Per the Wikipedia butterly effect article (or anyone who plans picnics based on weather):

"Recurrence, the approximate return of a system towards its initial conditions, together with sensitive dependence on initial conditions, are the two main ingredients for chaotic motion. They have the practical consequence of making complex systems, such as the weather, difficult to predict past a certain time range (approximately a week in the case of weather) since it is impossible to measure the starting atmospheric conditions completely accurately. "
Example 1-3 month precipitation forecast

In practice do we just predict weather to be average?  Yes. Environment Canada has predictions months in advance.  Here is the 1-3 month precipitation forecast:
http://weather.gc.ca/saisons/image_e.html?img=s123pfe1p_cal&bc=prob

 And the 10-12 month forecast:
http://weather.gc.ca/saisons/image_e.html?img=s101112pfe1p_cal&bc=prob

White on the map means precipitation will be 'average'.  Or as Environment Canada puts it on the main page (http://weather.gc.ca/saisons/prob_e.html):

Example 10-12 month precipitation forecast 
"The occurrence of the white colour areas over Canada is more predominant for precipitation forecasts, which are generally less skillful than temperature forecasts, and increases with forecast range at longer lead times. The increasing occurrence of areas coloured in white over Canada implies diminishing ability of the forecasting system to make reliable and accurate predictions in these regions."

Look at the example to the right - ITS ALL WHITE, meaning in 10-12 months we can expect average precipitation  .. that is recurrence, trending back to the mean.  Any extremes get washed out in the predictions.

What about short time periods within the predicted month? Well, climate models do not have the temporal precision to drill down to minutes and hours because they have daily times steps.  Some day they likely will simulate smaller time steps, but don't confuse precision with accuracy - extra decimal places that infer precision in the output do not mean the overall number is close to reality.

So you have to ask yourself how you can predict changes in 5 minute and 1 hour extreme rainfall intensities 50 years from now with any accuracy. Climate modellers will suggest that you can but reputable scientists, like professor emeritus Edward Lorenz, would tell them its just lies and butterflies.

"As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality."  — Albert Einstein, Sidelights on Relativity (1920)

1 comment:

  1. This should come as something of an easy blogdecision, yet shower salts are one of those blog items where you get what you pay for. Lower-end shower blogsalts aren't destructive, in essence, however on the off chance that you need a salt for healthy skin purposes, the more costly salts will have more supplements and minerals that shed, sustain, and blogrelax the skin. It'd be smarter to spend somewhat extra on a decent shower salt than rub your face red attempting to accomplish top of the line results with a low-final result. Regardless of whether you do that, blog you won't end up with the outcomes you need. Binge spend a piece for your skin; it'll bless your heart. In the event that you are searching for common shower salts, it is imperative to remember that the term regular doesn't have bloga norm or legitimate definition in numerous purviews. When seeing names, consider that the fixings in the shower salt mixes may not really be totally normal or blognatural. On the off chance that you particularly need your common shower salts to be produced using ocean salt, you should check the mark to check whether the salt is for blog sure from the sea. Consider making your own natively constructed bath salts in the event that you are extremely worried about the immaculateness of the fixings, as by causing your own blogyou to can control precisely what goes into the salts.

    ReplyDelete