Vue Accuracy

[ptb]

I'm in the process of moving from a Fine Offset 3083 to vantage vue, running them in parallel on separate pc.
The FO is running on Cumulus 1 and the Vue on MX.
I'm pleased to report that the differences in temperature readings are less than +/- 0.2C at anytime over the past week, the rain gauge has yet to be troubled as it has been dry since installation, but the big difference I am seeing at the moment is wind speed recording. I realise the compromises of the height of the Vue will have an effect (it is on a two metre pole and the FO anemometer is at 3 metres) but I am surprised at the huge difference over such a small height difference.
Whilst the gust speeds generally tie up (max Vue is 10mph today and max FO 12mph) the "current" figures are considerably different eg as I write the FO states 9mph and the Vue is 3mph (recorded less than a second apart). With the present wind direction any obstructions are the same for both units (sited about 3 metres apart).
The other difference is the wind run - on the Cumulus 1 / Fine Offset the run today is currently 32miles. On the MX / Vue it is 4 miles - which is a huge disparity - I realise this is a calculated hypothetical figure; but does anyone know if Cumulus 1 use a different algorithm to MX. Before installing I thought if there was any major difference the Vue would give a higher figure due to the higher reporting frequency.

I realise that wind recording is highly subjective to the height of the unit but if I'm getting such a huge difference over 1 metre extrapolation of the figures would suggest at roof height we have gale force conditions! Which as the trees are only gently swaying is definitely not the case

Any thoughts would be welcome.

Paul B

[freddie]

Average wind speed doesn't increase linearly with height. The biggest increase occurs in the lowest few metres. See the diagram below:

B-L-shear.jpg

Having said that, it does seem a bit of a steep rise - although the mean speed from the FO does seem a bit high when compared to the gust speed. The good thing is that you see similar gusts at the two heights, which is typical of the boundary layer flow near the ground - you would see similar gusts too at rooftop level.

As you have pointed out, height of measuring equipment and upwind obstructions dramatically influence recorded wind speed. Atmospheric stability does too. Two measuring sites at the same height and exposure are likely to display differences in measurement at an instance in time, owing to the chaotic nature of atmospheric flow near the ground surface. Averaged over time, though, you will see those differences become minimal.

FWIW I think the Vue is a reasonable bit of kit, but you will find yourself having to mount it at a height where it is not optimal for two of the three main elements being measured (wind velocity, temperature, rainfall) - as the optimal (standard) height for measuring wind velocity is 10 metres, for temperature it is 1.25 metres, and for rainfall it is ground level. That's the drawback of buying kit which can't be (easily) separated.

[ptb]

Thanks for prompt reply Freddie
Interesting and useful stuff.
looking at the two instruments at time of writing it is clear that despite the FO anemometer being only a metre higher than the Vue something is way off.
The FO records a wind run of 57miles and the Vue just 9miles. I can't believe a one metre difference in heights would make so much difference. I think I will investigate further tomorrow