Author Topic: Hard rain causes marked differences in rain totals?  (Read 1620 times)

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Offline DaleReid

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Hard rain causes marked differences in rain totals?
« on: September 22, 2016, 09:53:19 PM »
Over the last 24 hours or so the area I live in near Minneapolis, MN had some intense rain.  While the official total at the Mpls airport was 1" some suburbs to the north got over 10".  I live about 90 miles east and had the opportunity to watch the storms with the Blitzortung lightning data and be at home to have cell after cell pass by, with what I would consider 'hurricane strength' rain.  This isn't something we normally get.  A thunderstorm and hard rain for a few minutes, or an all afternoon slow soaker is much more common.

But, as each cell passed, with some very light hail sometimes, there would be much cloud to ground lightning, almost no wind and very very heavy rain.

My total was about 5.51" all measured with tipping bucket devices.

Here's the odd thing, as reported by a friend who also likes weather and has two tipping buckets and one 3" tube type collector.

After a whole afternoon of storms, he decided he'd better empty his tube gauge.  It was very close to 4.0".  The two tipping buckets were at about 3.5" which is understandable since I believe they tend to under report in heavy rain.  This fellow is not one for hyperbole or stretching the facts, a lifelong trained engineer who's daily work required precise measurements.  The gauges are as perfectly placed as one might be able to get them, no trees, overhead diverters, etc. and again wind was not a factor.

After dumping the 4.0 from the tube gauge, he went to bed and this morning went to see the additional total, which was now 4.9'.  So his total according to the tube was 8.9", while the total on the tipping buckets was between 5.5 and 6".  This total was in comparision to my 5.51", although we are about a mile or so apart.

He has been baffled for an explanation.  I have no problem accepting the likely truth to the 8.9", local variation was certainly evident with each cell passage, and it was absolutely torrential when it came down with some of passages.

He is pondering how there could be such a difference in his measurements.  I would assume that the gold standard would be the tube gauge.  Can very heavy rains get a tipping bucket that far behind real accumulation? 

Being in the country we even speculated as to some creature urinating in the tube gauge, but the bear would have to be exceptionally nimble and bigger than most to get the perfect shot into the gauge.

I'm hoping someone with an interest in rain and measurement would have some insight.  About the only place I could think would have these impressive amounts of rain would be the hurricane areas, and perhaps monsoon areas in the SW but usually I don't see accumulations anywhere close to what we had in about 18 hours.

Comments?  And no, neither one of us brews our own moonshine or are prone to hallucinations!

Dale
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Offline CW2274

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Re: Hard rain causes marked differences in rain totals?
« Reply #1 on: September 23, 2016, 12:58:40 AM »
For sure tippers will under report in heavy rain, but certainly wouldn't expect that kind of discrepancy. Must have really come down! Did either of you get a max per hour rate?

Offline ocala

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Re: Hard rain causes marked differences in rain totals?
« Reply #2 on: September 23, 2016, 05:51:45 AM »
That is a bit excessive. It sounds like his tippers need to be re calibrated.
I have seen almost an inch difference from very a heavy slow moving thunderstorm at my station so it is conceivable that when you get the training effect the difference could be large.
After mine was re calibrated there was still a large spread for very heavy downpours but not as much as before.   

Offline ValentineWeather

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Re: Hard rain causes marked differences in rain totals?
« Reply #3 on: September 23, 2016, 05:54:59 AM »
I know people sometimes think about doing something but don't. (I've done it myself) Its very possible he only thought he dumped the 4" out of gauge so when he came back the next day it was 4.9". However the gauge never was dumped. This could easily be confirmed if his data from tipping buckets was being recorded somewhere.
Randy

Offline DaleReid

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Re: Hard rain causes marked differences in rain totals?
« Reply #4 on: September 23, 2016, 06:47:32 AM »
All good comments, thanks.  Yes, we thought of him NOT emptying the tube gauge as a thought, but then to see only 0.9" more rain when we got a couple inches more would seem odd.  We do things out of routine, or think we do, and our brains won't accept any ambiguity, so sometimes we are sure we did something when we really didn't.  He is pretty sure he did empty, but very sure he didn't partially empty the tube which would be an explanation.

Unfortunately his station isn't linked to any computer logging or rate determining programs, at least as of yet.  My rain rates when I had the chance to check them were in excess of 6" per hour for brief periods.

I'm using a tipping bucket on all my stations.

He spent a couple weeks last year doing calibrations (retired engineer with a slow drip method (old IV bag and tubing) with a few runs minimum and up to 5 or so on one tipper which he no longer has.

As I mentioned one can speculate forever and he raised the question since it was not easily explained. 

Probably will never know, but it makes me think that next season, or the rest of this one, that I will get my old copper Taylor tube type out again and run it for comparison.

With weather, whether rain, humidity, FARS, you name it, there is always something to hold your attention.

Thanks for the comments.  I'll pass them along. 
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Offline WeatherHost

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Re: Hard rain causes marked differences in rain totals?
« Reply #5 on: September 23, 2016, 04:14:26 PM »
I can see it, but I'm not sure I can explain it.

Single tipper or dual/seesaw might matter too.

Basically  ...  so much water so fast that some is bypassing the tipper(s) during the tipping process.  Think of holding tablespoons under a gutter downspout and trying to catch it all and pass it into a bucket and keep count.  Ain't gonna happen.

Then feed that same downspout directly into a barrel with graduated markings.

Will that account for such a wide difference?  Dunno.  Would take somebody with more ambition that me to test it.

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Offline Old Tele man

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Re: Hard rain causes marked differences in rain totals?
« Reply #6 on: September 23, 2016, 04:28:25 PM »
Back in 1967, on Guam, I was fueling one wing-tip fuel tank on our Lockheed EC-121K "Warning Star" plane as a storm cloud passed overhead and dumped its rain. I, who was on the port wing, got totally drowned, but the guy fueling the starboard tip tank, 123-feet away, didn't even get wet -- rain fall can obviously be locally sporadic.
« Last Edit: September 23, 2016, 06:56:54 PM by Old Tele man »
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