By now you’ve probably read about Wind speed trends over the contiguous USA by Pryor et al. (2009, in press, JGR)
There is also an Associated Press story which cites this as a “first-of-its-kind study, and suggests that average and peak wind speeds have been noticeably slowing since 1973, especially in the Midwest and the East.”
Steve McIntyre of Climate Audit takes the study to task, not only for its data and conclusions, but for the Shenanigans of Michael Mann and Gavin Schmidt when they try to backtrack post facto after giving independent interview to AP’s Seth Borenstein and suddenly finding themselves in disagreement over whether “climate change” is involved or not. Go team.
I’ll point out that measuring wind accurately over a long period is not easy, particularly because surface anemometers tend to be problematic by the nature of their mechanical design. Aerovanes are better, but still have some of the same issues.
There’s also issues with land use change around the sensor long term, which I’ll get to in a moment. But first, the instrumentation.
For the traditional anemometer, here are some issues I’ve identified:
- Cup anemometer accuracy is typically +/- 5% when new some are even less. It depends on cup size and how linear the detector is. Some are very non-linear at low speeds.
- Anemometers, being mechanical, age, just like automobile engines, they are only good for so many revs of the engine before stuff wears down, affecting accuracy.
- The most common aging problem of anemometers is dust/dirt accumulation in the bearings, which tends to slow them down. I used to have a collection of dead anemometers from the former California State Climatologist…the vast majority had sticky bearings. I’d use them as a prop then give them away to classrooms when I went out to give talks to kids.
- Aging tends to affect low wind speeds more, by virtue of friction and by the fact that the lions share of wind measurements are at low speeds below 10mph. 40+ mph winds are not an everyday occurrence in many parts of the world. Easy to see in in plot of wind speed distribution. Of course there are exceptions.
So, depending on how good the instrumentation is to start with, and how well it is maintained, we might see the slowdown in wind caused partly by long term sensor issues.
If maintenance of NWS/NOAA wind equipment used in this study is anything like what we’ve seen for the USHCN network, I think the answer will be self evident.
But there’s another issue- urbanization and the associated land use change.
On the Climate Audit thread, one of the commenters pointed out the wind record from the Great Blue Hill Observatory near Boston.
For those who don’t know, according to commenter MarkB “the Great Blue Hill observatory web site. GBH, is just south of Boston, USA, and supposedly has the longest continuous meteorology record in the country (1885-present). The observatory is at the top of the hill – 635 ft. – with a large woodland park all around. Wind speed is certainly down since 1980.”
Our own Evan Jones recently visited the Blue Hill Observatory for a TV interview on the surfacestations.org project by Mish Michaels at WBZ-TV in Boston. He pronounced it a CRN 3 by the way.
Here is a look at the observatory USHCN station. Click for large images direct from the Blue Hill website
The wind instruments are mounted on the old observatory tower:
See the graph of wind speed from the observatory below:
The graph brings some issues about urbanization to mind. The Boston area has been highly urbanized.
Urbanization could be roughing up the boundary layer.
Clearings, buildings, shopping malls, subdivisions, etc. create more friction and turbulence in lower level winds than say, open fields or vast swaths of forest top. More turbulence tends to inhibit smooth flow of wind, this decreasing the wind speed.
Granted Blue Hill is a bit higher than surrounding surface, but I think the the effect of increased turbulence and drag in the highly built up northeast will make an impact even at higher levels.
see this report by Zhang et al on urbanization impacts in China:
Urbanization leads to “heat island”, “dry island” and wind speed loss over urban area …
See the wind speed plots comparing urban/non urban on page 11
The wind speed slowdown due to urbanization has been known for quite some time. Here’s an older book on the subject:
Yet, the surface roughness of the city serves to reduce wind speeds…
Due to the friction and drag of buildings, there are wind speed losses over urban area in all cases (Fig.4). In summer, the reduce of wind speed can be 1.0 …
It seems then, since Blue Hill Observatory is in the middle of a large area of growth for the past 50 years, that some of this wind speed reduction related to land use change and urbanization would apply?
Have a look at this Google Earth 3D image showing the patchwork quilt of the terrain around the observatory. Note the may holes in the forest canopy around the observatory and the signs of ubanization. How much of that affects wind speed measured on the hill? How much of it was there 30, 40, 50 years ago?
I’d also like to find out when they last replaced/recalibrated their wind sensors. It is one of the sensors where detecting a problem is not always obvious, especially when they clog up with dust in the bearings.
When studies cite surface data from weather stations, it is always a good idea to look at land use/land change around the stations as well as the station instrumentation. Chances are you’ll find issues that may not have been considered.