Wandering Water

Guest Post by Willis Eschenbach

I see in a most recent post here at WUWT that UC Irvine researchers have used “27 state-of-the-art climate models” to predict what will happen by the year 2100 to the “tropical rain belt”. They describe this as “a narrow band of heavy precipitation near the equator”.

By coincidence, I’d just downloaded the ECMWF reanalysis results for what is called the “TPW”, the total precipitable water vapor in the atmosphere. This is how many kilos of water there are in the column of air above each square metre of surface. The results cover 1979 to mid 2019.

So to start with, here’s the long-term average of the precipitable water in the atmosphere.

Looks simple, right? You can see the tropical rain belt above the equator. As you might imagine, it’s where the total precipitable water is at its highest, because you can’t get precipitation without precipitable water. So we just need to measure how far that rain belt moves …

But how stable is the tropical rain belt? Well … not very. Here’s a movie of the month-by-month averages in the rain patterns.

As you can see, the rain belt is quite mobile, and that’s just with monthly averages. It still seems like it might be kinda predictable … but those are still averages.

The individual months, however, are all over the map. Here are a couple of years worth of individual monthly changes.

So despite the fact that they’ve used 27 climate models, and despite the fact that each and every one of the models was “state-of-the-art”, despite the fact that there were no “average” or “good” models, despite the fact that not even one of the models used yesterday’s technology much less that of the day before yesterday, and despite the fact that all 27 of the climate models talked it all over and they all came to one conclusion … I fear I’m gonna put zero confidence in their results. I’m sorry, but that is a seriously chaotic system, and the idea that we can project it out until the year 2100 is a joke.

Now, I actually got the TPW data for a different purpose, so I’m gonna leave this there. I just couldn’t let the ludicrous claims of the UCI researchers to stand unchallenged. I do not think that such chaotic systems as the global rainfall distribution can be modeled out eighty years into the future. We can enjoy the glorious might of the thunderstorms, we can wonder in awe at the complexity of this monumental heat engine that we call the “climate” … but predict the details of it out to the year 2100?

The word that comes to my mind for the idea we can foretell the future of the rain belt in the year 2100 is not “science” … it is “hubris”.

I’ll be back at some later date to discuss my further meanderings in this realm.

And in the meantime, I could do with some rain, it’s another dry year here in Northern California. So my best of the sunshine to all, stay well, stay healthy.

w.

PS—Some may say “That’s total precipitable water, not rainfall”. And that’s true. However, you don’t get precipitation without precipitable water, so their distributions are tightly correlated … and I don’t have a global rainfall dataset. I do, however, have a movie I made of rainfall, so you can compare the rain and the precipitable water for yourself. It’s from the Tropical Rainfall Measurement Mission, so it only covers the tropics plus a bit. Here you go.

What a planet! …