Since we are now actively deploying sites in the field, I wanted to create a post explaining a little bit more about how we are doing our snowfall measurements. I know most of you don't have science degrees so I promise this post won't be super "sciencey" and full of buzz words that no one will understand. With that said, let's start with the basics. There are two ways to measure snowfall. The method most of you are familiar with is to measure the depth. Go out, stick a ruler in five or more different locations in a given area, average those measurements and you have your snow depth! (Or you can cheat like the television newscasters and find the deepest snow drift and take your measurement there). While depth measurements are certainly useful (that is one of the automatic measurements we are taking at our sites), what really matters in the world of science and transportation applications is the amount of water in the snow. Most of you will recall I covered this in an earlier blog post. What I didn't cover in that earlier was how we plan to address the issue of the strong winds they get down here, which can blow the snow more horizontally than vertically.
Most people that do snowfall measurements have recognized that you need to put some sort of shielding around the precipitation gauge to slow the wind so the snow falls more vertically (and thus, into your gauge instead of past your gauge). While you can put a precipitation gauge out without any shielding, the gauge will oftentimes act as a wind block and can create eddies in the air patterns that cause the snow to blow over and around the gauge instead of into the gauge. There are all sorts of shields out there that have been designed to counter this effect, but if you put five precipitation gauges in a field and surrounded them with five different shields, you would get five different measurements. So which one is correct? The accepted standard is referred to as the Double Fence Intercomparison Reference (DFIR) shield. As its name implies, it consists of two wooden fences, shaped in an octagon, with the precipitation gauge in the center. There is also a third, Alter shield (discussed a little later) that surrounds the precipitation gauge near the center (so it's technically a triple fence, but since the innermost shield isn't really a fence, we let the title slide ....). Ok, if this shield is the accepted standard, why not use it everywhere? The one thing I didn't mention is that the outer fence is 40-feet in diameter.
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| DFIR shield |
Not many people doing snowfall measurements have a 40-foot area to work with, especially in the mountains where terrain can be an issue, so it then becomes a challenge to determine what size of shield you can use and how well the measurements from the gauge inside that shield compare to the measurements from a gauge inside a DFIR shield. More on this in a minute .....
I'll quickly introduce you to a couple other shields that are being used. One of the most common shields is the Alter shield, invented in the 1930's by J.C. Alter (hence the shield name). This shield is a mere four feet in diameter and has metal fins, about 16 inches in length, that swing in the wind. This is also the shield used as the third innermost shield in the DFIR that I mentioned earlier.
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| Alter shield installed around a precipitation gauge. These shields and gauges are typically installed about six feet off the ground, contrary to what is shown in the picture. |
Another recent shield is the Belfort Double Alter (BDA). This shield is similar to the Alter but it has two concentric circular shields instead of one, hence the Double Alter name. The outer shield is eight feet in diameter and the inner shield is the standard four feet in diameter. The fin lengths between the two shields is also different (18 inches for the inner shield and 24 inches for the outer shield). Belfort was the company that produced this and they did some Computational Fluid Dynamic modeling (fancy term for using a computer to model flow past an object) to develop this shield.
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| Belfort Double Alter shield |
Alright, so back to the question of "how well do these shields do?" Below is a plot of measurements from four gauges during storm in April of 2014. One gauge is in a DFIR (black line), one is in a Belfort Double Alter (blue line), one is in a single Alter (green line) and one has no shield (red line). I've also plotted wind speed in gray so you can see how the winds were changing throughout the event. You can see that the blue line (the Belfort Double Alter) matches closest to the DFIR, but it's not the same as the DFIR. This is primarily due to the wind and since we have measurements from gauges in both shields, we can now calculate a wind speed correction factor to get the Belfort Double Alter to more closely match the DFIR (though I won't put you to sleep with the details on how we do that).
Now that you have a little better understanding of what we are doing, I can tell you that we will be installing Belfort Double Alter shields at all of our sites down here. We will also be installing a DFIR shield at Willie to derive our own wind correction.
Now that we've gotten past those details, on to some other more interesting pictures! We had a fairly significant storm hit last week and nearly broke the record for the most amount of snowfall to occur in November in a 24-hour period (which is roughly 8 inches ... I know everyone in Colorado is laughing at that, but considering the temperature was in the teens the whole time, that's fairly impressive to get that kind of accumulation). We went from this typical view out my lab window:
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| View from my lab winter with Discovery Peak (the summit is covered in clouds) in the distance and Black Island off to the left. |
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To this view out the window with winds of 25 knots gusting to 30 knots:
What does one do in these conditions? Walk out to Hut Point! Four of us decided to venture out to Discovery Hut and see what conditions were like out there. Needless to say, it was cold and very windy. Most of the pictures are on my camera and my laptop has decided it doesn't like reading my memory cards anymore so I'll have to post them later. (My laptop is not having a good time in Antarctica ....). Below is one I took with my phone on the way back from Hut Point.
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| The walk back from Hut Point. The dorms can be seen in the background. |
As the storm was clearing out, I got a nice shot of the Chalet with Observation Hill in the background. The Chalet is where everyone goes as soon as they land here for a quick briefing and to get their dorm keys and training schedules.
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| The Chalet and Observation Hill. |
Before I sign off, I wanted to send a quick THANK YOU to my IT staff back at NCAR, who shipped me a new laptop that actually has a usable screen! I can now use my laptop in locations other than the lab and I might be able to update my blog more frequently! Also, it looks like we might be delayed in returning as the weather hasn't been cooperating for us to get out to all of our sites. It now looks like we'll be returning next Friday instead of next Monday. Hopefully we can finish what we need in those few extra days.
More stuff to come as I will post about my first Twin Otter airplane trip to our third site as well as pictures from our other two sites that are finally coming online.
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