Rainfall adjustment

[WessexWeather]

After complaints about low wind speeds, last summer I mounted my Vantage Vue station on the roof, which solved that.

Unfortunately during our windier winter months my rainfall readings have understandably been inaccurate, as rainfall should be recorded close to the ground.

I how have a roof-based anemometer, whilst all other parameters are recorded at 2m, so all good from now.

However I now have several months of inaccurate rainfall data, and have calculated revised monthly totals, taking wind speed into account. Some software allows for adjustments of rainfall data, but Meteotemplate by nature of its using its own database, doesn’t easily allow this. I am not complaining as this is why I started to use MT rather than home-based software or third party databases like (unreliable) Weather Underground.

Has anyone any suggestions as to how I could easily revise my previous rainfall data within the alldata database, or would this just be too complicated and time consuming?

[Jachym]

Hi,
try the "bulk editing" feature in the control panel

[Dehatter]

Unfortunately during our windier winter months my rainfall readings have understandably been inaccurate, as rainfall should be recorded close to the ground.

Why would rainfall be different at 2m than 15m? Just wondering, All the research I read says there is no difference with minor elevation changes in the same location.

Tom

[Dehatter]

Well I found a paper that make the assumption the the horizontal velocity of rain is reduced to zero at ground level. Not sure that I agree with that assumption unless there is an object blocking the wind, without blocking the approach of the drop. Wind speed still exists at 2m, at least here. I wish I had 2 stations.

Tom

[Jachym]

Placing the rain gauge near the ground mostly stems from the fact it needs to be accessible. You might need to clean it from dirt, snow etc. Also in the past manual rain gauges had to be emptied and the content measured. So it cannot be like the wind sensor. Placing it on a flat roof, 10m above the ground and you will most likely measure the exact same result. It is very unlikely that those 10m would change the state of the drops or their direction.
The only other reason is that at a higher wind speed you are introducing more bias as the drops fall at an angle. So higher above means on average higher wind speeds. This will improve the gauge catch.

In open areas you can place it even lower, eg 2ft, in developed around 5ft. Rain gauge does not have to be at 2m height, the professional one we have here at our institute is about 1m above the ground, easy to access and look into when necessary. 2m is strictly given only for temperature and humidity.

[WessexWeather]

A quick Google throws up quite a number of references to loss of recorded rainfall with height/wind speed actually. The subject is well documented, and I’m quite surprised to see you refute that Jachym...

http://tulliemet-perthshire.org.uk/main ... ecords.pdf

https://www.wmo.int/pages/prog/www/IMOP ... rement.pdf

http://nora.nerc.ac.uk/id/eprint/514384/1/N514384PO.pdf

And indeed Davis have recently completely redesigned their VP2 gauge to minimise this: -

https://www.davisnet.com/new-rain-cone- ... -stations/

In my own case the losses during November, December and January were 13%, 29% and 35% respectively. The mean wind speeds for those months were 3.5, 4.7 and 5.7mph.

One of the most interesting papers I found on the subject was this one: -

http://web.tari.gov.tw/csam/CEB/member/ ... 1)/008.pdf

Hope these links (and there are many more) help.

[Jachym]

It is ineed better to place it at 2m, but whether you will see a significant difference depends on where you live and the surroundings.

I also said that if possible, it is even better to place it lower than 2m, 1m is fine.

[WessexWeather]

You also said...

Placing it on a flat roof, 10m above the ground and you will most likely measure the exact same result. It is very unlikely that those 10m would change the state of the drops or their direction.

I and others have found major differences between our gauges at 2m and 10m. I’m recording gust speeds some 20mph higher since raising my anemometer from 2m to 10m, which has clearly had an effect on rainfall measurement.

[Jachym]

Yes, so you are one of those where it makes a difference, I would then recommend placing it even lower if possible.

In my case for example, because I live between high urban landscape, the wind speed at 10m is almost equivalent to 2m....

[WessexWeather]

The problem with integrated sensor suites as in the Davis Vantage Vue, is that you can never get perfect standardised readings - ie wind at 10m, temperature at 1.25m and rainfall at 0.5-1m.

That is why I now have two Vues broadcasting on different channels, one on the roof measuring wind speed and the other at 2m measuring temperature and rainfall. The Vue console can then be set to receive wind data from the roof and the other parameters at 2m.

If I were to lower the ISS further then temperature would become inaccurate, especially on cold winter nights. This I think is the best compromise.

[Jachym]

I 100% agree, Ive had several arguments with people over on the wxforum.net about this when I said I would rather buy a 100USD FineOffset station than a much more expensive all-in-one Vue

[WessexWeather]

On the other hand the Davis Vsntage Vue is extremely durable and accuarate if sited carefully. My oldest (roof mounted) Vue has been recording continuously since 2011.

[Jachym]

Yes, but thats exactly the issue:

if sited carefully

My view is that an all-in-one station can NEVER be sited ideally no matter what you do (ok, unless you rip it apart )