Topic: Rainfall Rate

[Intheswamp]

As a newbie I've got a question.  Have the differences been seen both during heavy straight down rains and in blowing rains?  Could the wind be messing with the tipper...tipper fills up, tips and dumps it's load, and a heavy wind "flutters" the tipper?  This would account for possibly the "over reporting" issue.  It's pretty well sealed under there but the drain screens are just below the tipping cups.  Just thinking outside the...bucket.

Something that I've noticed is that when watching a heavy rain the data may jump .02 rather than just .01 at one time.  Is this just the console "catching up" or something?

Ed

[moehoward4]

Intheswamp....    it's the wind blowing the rain away from the bucket that causes under reporting, but if the ISS isn't well secured to something, the wind 'could' move the ISS and and cause 'bad' tips.

[Intheswamp]

Intheswamp....    it's the wind blowing the rain away from the bucket that causes under reporting, but if the ISS isn't well secured to something, the wind 'could' move the ISS and and cause 'bad' tips.

But if the wind is blowing the rain away from the bucket then it stands to reason that the wind is also blowing rain *towards* the bucket...isn't it?  Thus a balanced condition?

Ed

[Cutty Sark Sailor]

For Davis, the rain rate is calculated by measuring the time interval between each rainfall increment. When there is rainfall within the archive period, the highest value is reported. If no rain, the rain rate slowly decays based on the elapsed time since last measurement. Archive interval can be set from 1 min to 2 hrs.

Here's a simple description regarding wind and rain measurements:
http://www.newton.dep.anl.gov/askasci/wea00/wea00198.htm

Severe turbulence can vibrate the spoon in the Vue, but then it would over report. My ISS is on a 23' mast, with guys to stabilize, rather than support, for that very reason... Iss also needs to be level.

[dalecoy]

Intheswamp....    it's the wind blowing the rain away from the bucket that causes under reporting, but if the ISS isn't well secured to something, the wind 'could' move the ISS and and cause 'bad' tips.

But if the wind is blowing the rain away from the bucket then it stands to reason that the wind is also blowing rain *towards* the bucket...isn't it?  Thus a balanced condition?

Ed

In the limit, imagine that the wind is blowing the rain horizontally.  

[dendrite]

Intheswamp....    it's the wind blowing the rain away from the bucket that causes under reporting, but if the ISS isn't well secured to something, the wind 'could' move the ISS and and cause 'bad' tips.

But if the wind is blowing the rain away from the bucket then it stands to reason that the wind is also blowing rain *towards* the bucket...isn't it?  Thus a balanced condition?

Ed

It doesn't work that way. The problem is turbulence over the gauge. As already mentioned, the tippers can lose some liquid during +RA during tips too. Something like this could help with the wind induced errors...

http://www.novalynx.com/260-952.html

I'm sure someone here has made their own at some point.

[LFWX]

The Davis recorded more?

Looking back through my June records, the Davis recorded 1.908 in, the manual CoCoRaHS recorded 1.70 on June 10. They are on opposite sides of the same post. Usually they compare well w/in 0.01 inch or so.

It rained very hard that night. I edited my records to reflect the manual amount. This was the largest difference I've noticed in 2.5 years. Perhaps it was an anomaly, but both WL & WD both recorded the higher amt

Ed the reason I asked is because it has always been the other way around for me. The Davis always under reports in heavy rain. Other then heavy rains the two gauges are usually 1-2 100's off.

...it's also possible for the CoCoRaHS to under-report because rain is bouncing out of the short funnel used on that gauge. Removing the funnel and inner tube (which CoCoRaHS has told me is fine) has brought my VP2 measurements within 0.02" of my CoCoRaHS measurements during almost all types of rain. (knock on wood)

BTW - Maximums I've recorded:
Rainfall Rate 17.45"/hr June 2, 2010 @ 6:08 pm
15 Minute Rain 1.13" August 24, 2008
1 Hour Rain 2.81" June 26, 2009
24 Hour Rain 4.56" June 26, 2009

[Intheswamp]

Great posts, everybody.  Lots of variables involved.

That wind shield looks interesting.  One day I might try a variant of that...one day.

Does a larger catch area make a rain gauge more accurate?  I can see where the funnel of the CoCoRaHS might cause hard rain to bounce out of it....maybe a deeper funnel design is needed?

Ed

[Weather Spares]

I suppose the easiest way to understand the effect that wind has on rain collection is as follows.

If you look directly at the rain gauge from above you have a perfect circle catching precipitation. As the wind increases it's coming in at an angle, so looking at a rain gauge from a 45deg angle you find there is a reduction in surface area, and the circle is reduced to an ellipse.

[SlowModem]

I suppose the easiest way to understand the effect that wind has on rain collection is as follows.

If you look directly at the rain gauge from above you have a perfect circle catching precipitation. As the wind increases it's coming in at an angle, so looking at a rain gauge from a 45deg angle you find there is a reduction in surface area, and the circle is reduced to an ellipse.

I never was good at geometry.   If I'd know I'd be playing with circles, ellipses, and rain gauges some day, I would have studied harder!   

[LFWX]

I suppose the easiest way to understand the effect that wind has on rain collection is as follows.

If you look directly at the rain gauge from above you have a perfect circle catching precipitation. As the wind increases it's coming in at an angle, so looking at a rain gauge from a 45deg angle you find there is a reduction in surface area, and the circle is reduced to an ellipse.

Well...not exactly true.

The rain is falling as drops, not a block. A horizontal cross section of a round shaft of rain falling into wind and being blown at an "angle" will also be an ellipse. We are measuring the accumulation of rain in a given area by depth, which the rain gauge still does when the wind is blowing. We are measuring the accumulation on the ground, we are not measuring the quantity at the base of the cloud. I made an illustration of this in the past when this same subject came up...I'll see if I can find it.

BTW - If all the rain from a storm fell from a cloud directly over your property, but was all blown onto the property next door...the correct observation at your location would be 0.00" measured. If it would have measured 1.00" on your property, but was spread over the next two properties to a depth of 0.50"...the correct measurement would be 0.50" at those locations, not 1.00". That 1.00" on your one acre (27,154 gallons of water) would be 0.50" on the other two acres (still a total of 27,154 gallons of water). This is why zooming in on the radar to see which street it's raining on is not a good thing for TV mets to be doing.

The real problem with a rain gauge in high winds results from the fact that it is above the ground and it has vertical sides which create turbulence over the gauge. The "perfect" rain gauge would be in the ground with the top even with the soil...but only if you could somehow keep the surrounding rain from bouncing into the gauge and/or flowing into the it from the surrounding area!

[Intheswamp]

I'm like Greg...if I'd known I was going to actually be interested in some of these things later in life I might've paid better attention in school...but, I did have a good time...the 70's were pretty good years.  

Here is a highly artistic drawing of a rain gauge bucket and rain entering it straight down and at an angle.  This convinced me that the angle of the rain could significantly. The lines represent rain drops falling and are spaced evenly parallel across the bucket opening.  There are eleven perpendicular "rain drops" but only seven angled "rain drops".  Maybe not too scientific, but looking at it it makes sense to me. 

Ed

[hankster]

One answer to that same question:

Dear Michael-

You are correct in assuming that strong winds can bias rainfall measurements. The rain gages are calibrated using a known area of collection, which is related to the diameter of the collection tube. Strong winds effectively reduce the area of the collection tube by increasing the "slant" (angle of rainfall from the vertical) of the rain. It is doubtful that the difference would be as much as you suggested in your example, though.


Wendell D. Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

[dalecoy]

This discussion is somewhat related to the following:

The other day, my grandson got wet walking home from the bus stop while it was raining.  I asked him why he didn't run.  He told me that "everybody knows" that it doesn't matter how fast you walk or run, you'll collect the same amount of water (i.e., get equally wet).

So, I asked him what would happen if he just stood still.

[BCJKiwi]

The diagram looks good but I do wonder if the spacing is the same when the rain is at an angle?

Assume (always dangerous) that the rain starts falling vertically and then lower down it gets blown sideways. Wouldn't the 'lines' of rain be closer together now - ie, the same amount of rain is falling. If this is the case, the angled lines in the diagram would be closer together than the vertical lines.

Time for someone to do a test with a garden mist spray and a fan?

[Cutty Sark Sailor]

[dalecoy]

There are a large number of scientific observations regarding performance of rain gauges of various types, under various conditions.  Many of them documented in the open literature.

Most of them do not actually prove why the effects exist, although they have reasonable theories.

As I recall, early ones were in the 1880 time frame and concerned height above ground.

Try this paper:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=http%3A%2F%2Fcarbon.ucdenver.edu%2F~jguo%2FPaperWeb%2F%28W1%29RainUnderCatch.pdf&ei=cantUdDBEYS-rQGes4BQ&usg=AFQjCNHJ5E9YukZreTcHW7mXrnt-v-LCPQ&bvm=bv.49478099,d.aWM

(sorry for the long link)

[floodcaster]

The real problem with a rain gauge in high winds results from the fact that it is above the ground and it has vertical sides which create turbulence over the gauge. The "perfect" rain gauge would be in the ground with the top even with the soil...but only if you could somehow keep the surrounding rain from bouncing into the gauge and/or flowing into the it from the surrounding area!

Yes, increasing height and resultant wind exposure is a major issue with accurate rainfall measurements. In fact, for open areas CoCoRaHS recommends installing the gauge lower. I also thought that this was standard in the UK and Australia for manual gauges but I may be mistaken.

[dalecoy]

As I recall, early ones were in the 1880 time frame and concerned height above ground.

Correction: 1866.

[dendrite]

Wind hitting a rain gauge creates updrafts over the gauge that cause some of the precip to overshoot the opening. The stronger the wind, the stronger the updrafts/turbulence. Theoretically, I don't think the angle is directly the problem. Wet snow falling at a 45 degree angle over a field will accumulate the same amount as the same snowfall falling with calm winds (assuming no ratio loss from drifting snow). The difference is there's no turbulent updrafts over ground level.

If you're sitting at a red light in a convertible while in a rain shower you're going to get soaked. If you're traveling 70mph, some of that precip is going to accelerate over your windshield and overshoot you the same way precip would over a rain gauge. In that example, the windshield provides the updraft. The wind shields are made to "break up" the wind coming in contact with the side of the gauge. Some actually deflect he air downwards. I'll see if I can find a picture.

[Cutty Sark Sailor]

Wet snow falling at a 45 degree angle over a field will accumulate the same amount as the same snowfall falling with calm winds (assuming no ratio loss from drifting snow). The difference is there's no turbulent updrafts over ground level.

Hmmm, this isn't the reason that CoCoRaHSians will take several snow measurements at various locations, and report the average as snow depth. It Varies.
The Solution to This IS:
Everybody run out and buy 1000 gauges. Place 'em as close together as you can, in layers from ground level to 9 feet. When it rains, go out an measure 'em all, then take the average.,.

Me, I'm gonna continue to record and report what falls in my one lonesome gauge. That's what fell in. In that location. During that time frame. Period.

[dendrite]

Wet snow falling at a 45 degree angle over a field will accumulate the same amount as the same snowfall falling with calm winds (assuming no ratio loss from drifting snow). The difference is there's no turbulent updrafts over ground level.

Hmmm, this isn't the reason that CoCoRaHSians will take several snow measurements at various locations, and report the average as snow depth. It Varies.
The Solution to This IS:
Everybody run out and buy 1000 gauges. Place 'em as close together as you can, in layers from ground level to 9 feet. When it rains, go out an measure 'em all, then take the average.,.

Me, I'm gonna continue to record and report what falls in my one lonesome gauge. That's what fell in. In that location. During that time frame. Period.

I was assuming a flat field with no snow transport which is obviously a hypothetical. You're missing my point though.

[dendrite]

This is the Vaisala QMR102 which is designed to reduce the updrafts over the gauge and undercatch.



http://www.vaisala.com/en/meteorology/products/weatherinstruments/rainandprecipitationsensors/Pages/QMR102.aspx

[LFWX]

I'm like Greg...if I'd known I was going to actually be interested in some of these things later in life I might've paid better attention in school...but, I did have a good time...the 70's were pretty good years.  

Here is a highly artistic drawing of a rain gauge bucket and rain entering it straight down and at an angle.  This convinced me that the angle of the rain could significantly. The lines represent rain drops falling and are spaced evenly parallel across the bucket opening.  There are eleven perpendicular "rain drops" but only seven angled "rain drops".  Maybe not too scientific, but looking at it it makes sense to me. 

Ed

The "seven angled rain drops" are the ones you are measuring!!! You are measuring the accumulated rainfall over a given area, not the greatest amount of rain you can collect in a given area.

BCJKiwi is correct. In the diagram, the spacing between the lines would be the same measured horizontally, not at an angle as you have drawn it. All the drops are being pushed by the wind, but in your diagram they are being turned by the wind as if the entire area of rain is solid block of ice.

[Intheswamp]

I can see too many holes (or is that rain gauge openings) in a lot of ideas (mine including).  I think I will do the scientific, highly technical and precise thing and follow CSS's lead.  

Just to approach NASA specs I may add a CoCoRaHS gauge!

Ed