Well, as a climate scientist, as you might imagine, I'm quite fond of precipitation or really maybe even the lack thereof. In the Arctic, we find very fascinating patterns of precipitation, areas which are extremely dry, areas which are actually quite wet. The key points I want to get across, one of them is just what I said, precipitation amounts vary widely across the Arctic. Some places are so arid, they're classified as desert, polar desert. Other areas of the Arctic on the Atlantic side of the Arctic are extremely moist, extreme variations in precipitation across the Arctic. As you might expect, since the Arctic is a cold place, a lot of that precipitation falls as snow. In other words, for most of the year, precipitation falls as snow. Of course, the Arctic does have summer, like everywhere else. But also, we need to remember that Arctic precipitation is especially difficult to measure. Well, let's first ask a question, starting with a few basics here. What are the requirements for precipitation? What do we need for precipitation to fall in the first place? Well, the answer here is all three of the choices that I've offered: Number one, we need water vapor in the atmosphere. Without water vapor, you just can't have any precipitation. The more of the water vapor in the atmosphere, the more the possibility of precipitation. In other words, if you want precipitation in terms of water vapor in the atmosphere, the more the merrier. You also need cloud condensation nuclei. These are tiny little particles in which water vapor can condense to form raindrops or ice crystals, which can then grow to snowflakes. Usually we have no shortage of cloud condensation nuclei in the atmosphere. But the amount of water vapor in the atmosphere can certainly be a limiting factor in terms of how much precipitation we get. You also need uplift and cooling of air. When a parcel of air, you can think of it as an amount of air, maybe a kilogram of it that we can put tags on each molecule. As a parcel is lifted up, it's going to cool because it's expanding and it's doing what we call work against the environment. Well, if it cools enough, it's going to get to what we call its saturation point. Which would be the case where the relative humidity is 100%, that is, that little parcel is holding as much water vapor as it can at that temperature. When we reach that 100% relative humidity, something also called the dew point temperature, we get condensation, and that condensation will lead to cloud cover and possibly precipitation. So we need all three: you need water vapor in the atmosphere, you need cloud condensation nuclei, and you need some kind of uplift mechanism. Well, what's the annual pattern of precipitation across the Arctic? What we find is that parts of the Arctic are desert, as I mentioned, parts of them are called polar desert. We see a lot of polar desert in Antarctica, we also see it in the Arctic. Both of our polar regions, we see polar desert environments. Now, why polar desert? There's just very little water vapor in the atmosphere. These areas are cold, and when they're cold that means that they just can't hold that much water vapor. How much water vapor the air can hold is a temperature dependent thing. The higher the temperature, the more water vapor the atmosphere can hold. In these cold environment, the atmosphere just can't hold much, so it's hard to get a lot of precipitation. But also, these areas are far from moisture sources, far from oceans which can provide a lot of moisture. Then you might say in the Arctic, well, don't we have an Arctic Ocean? Of course we do. However, that's covered in large part with sea ice and it basically acts as a lid which separates the ocean, which would be a moisture source from the atmosphere above, acts like a cap. Really, it's fairly dry over the Arctic Ocean in terms of the amount of water vapor in the atmosphere. So some areas are these polar desert. Some of these areas are very high in precipitation of the Arctic. Highest precipitation amounts on the Atlantic side of the Arctic. Why? There's a lot more water vapor in the atmosphere, and there's also very strong uplift mechanism. We have storms that move through there, what we call cyclones. These are the storms that we're familiar with in the middle latitudes that have their warm fronts and they're cold fronts. These warm fronts and cold fronts provide a mechanism to lift that air up so that it can cool and condense and for clouds to form and for precipitation to fall. Now, this figure here, that I'm showing, is an illustration of the annual average precipitation across the Arctic region. You'll first sees those areas in yellow, I'm pointing them out here with that arrow, very, very arid, these are your polar desert areas. These areas comprised the Canadian Arctic Archipelago, very dry. The Arctic Ocean itself is very dry in terms of precipitation, it's really a polar desert. But it was purple colors, I've just highlighted them there with another arrow, those are areas which are really very moist, very wet. It's really amazing the contrast we see across the Arctic from polar desert areas to areas that are really rather well watered. I mentioned that a key thing here is the amount of water vapor in the atmosphere. This here is an image from satellite data, actually it's from a combination of satellite data in an atmospheric weather model that is showing how much water vapor is in the air. Basically, it's showing it in millimeters of water vapor in the atmosphere. In other words, if we took all the water vapor in the atmosphere and squashed it down to liquid water, what kind of a water depth we would get? Well, the point is, is that in the Arctic regions, is showing that gray areas, it's quite low. There's just not a lot of water vapor in the atmosphere because it's cold and it's far from those moisture sources. You look down to the bottom of that image, that's showing the more southerly latitudes in those greens and blues, much, much more water vapor because you have a warmer atmospheric, it hold more water vapor and a lot of these areas are very close to moisture sources or riding right over them in terms of the ocean. What about these polar desert areas? This figure here that I'm showing you, it's showing the limits of polar deserts, it's in that dark shaded area, and you'll see that all of the Canadian Arctic Archipelago is polar desert. Parts of Greenland are polar desert, Novaya Zemlya, those has one of those big islands on the Russian side of the Arctic polar desert. This image here really doesn't show what's happening over the Arctic Ocean, but I can assure you that the Arctic Ocean is really quite dry and I would say it qualifies very much as a polar desert. What things can grow in polar desert? As you'd imagine, not a whole lot. This image on the left is showing saxifrage, it's one of the plants that can grow there. A beautiful plant with those nice purple flowers. But the image on the right, it's showing me, this was taken way back in 1982 and as you can see, there's not much growing there. There's likens, things like that, you'll find some of those areas of saxifrage, beautiful flowers. There's other little things that can grow, but it's a desolate environment but its got a beauty to itself as well. But this is what you might find in a polar desert environment. The other issue I want to get across is that precipitation is really hard to measure. There's not many gauging sites, in other words, these are sites where we would actually measure precipitation with what we call a rain gauge or a precipitation gauge. This other issues is that this big undercatch of blowing snow, most of the precipitation in the Arctic is snow and it's blowing snow and that's actually hard to capture in your precipitation gauges, so you tend to have an undercatch. Precipitation is also often in trace amounts in the Arctic, just a little bit of precipitation falls and that can be hard to measure, you really can't measure it much at all. We can use data from satellite retrievals, weather models to help us get at precipitation amounts in the Arctic. But the point is, precipitation has always been hard to measure in the Arctic and is likely to remain so for quite some time. This figure I'm showing here is it's showing the number of gauge sites across most of the northern hemisphere. Every one of these little dots is a gauge site, well you can see on the North American side, on the left of this figure, the dots are so dense it looks black. But if you look over to the high latitudes towards the middle of that figure there's not a lot of dot. That just means that there's not a lot of sites where we actually directly measure precipitation. This is a problem. We get this blowing snow issue which causes gauge undercatch. This is just an image of myself, I think my advisor in grad school, Ray Bradley took this. I was trying to measure the wind speed out. Blowing snow environment, a rather nasty day as I recall but it was a long time ago, but yeah, classical case of blowing snow. This is just looking at a couple of different types of precipitation gauges that are used across the Arctic. The one on the left is the one used in North America by the United States. The one on the right is called a Tretyakov gauge, that's what the Russians have been using, very different gauges. It turns out that different gauges are, well some of them are better in some situations than others. So we have a variety of gauge types used across the Arctic and so that just adds to the problem of trying to measure precipitation in any kind of accurate way. That's how figure I'm showing now is called a Wyoming snow gauge. The idea is that we build the structure around a precipitation gauge which is located in the middle of that structure. Oh, that structure is maybe 15 feet across, something like that, and the idea is that the structure breaks up the wind so we can get a more accurate measurement of precipitation. But I can tell you from first-hand experience that they don't work very well. This particular one is in the [inaudible] creek drainage on the North slope of Alaska. Trace precipitation, I also mentioned this. This is really a hard thing to measure. It's a hard thing to measure traces of precipitation. You can see some beautiful little snowflakes on this image. But this thing we get a lot of in the Arctic, that the precipitation falls, but it falls in just these tiny, tiny little amount, which really an individual one can be hard or even difficult to measure. But there is a lot of them so they do end up mattering. This image here I'm showing is trying to illustrate this. This is showing precipitation over the Arctic Ocean, average precipitation over the Arctic Ocean by month, reading from left to right is January, February, March, April, May, June, July, August, and to the end of the year, and on the y axis is the precipitation amount. Now, what you see here is, just by the total length of those bars over the Arctic Ocean, precipitation actually tends to maximize in the summer or late summer month. Makes sense because that's the warmest situation and so the atmosphere is going to hold the most water vapor at that time. But what's done here is the area is the darkest areas, the darkest part of that bar is what was actually measured. The other parts with the lighter shading and the white at the top, these were adjustments that were made to try and to account for this gauge under catch of blowing snow and these trace amounts. You'll see with these adjustments that the precipitation patterns really change. You get a lot more precipitation. Are these adjustments any good? Some people say they're pretty good, some people say that they're not, but it simply illustrates the point that Arctic precipitation is not something that is easy to measure. A question, why might Arctic precipitation increase through the 21st century? It's actually predicted that it will increase through the 21st century, why might that be? Well, the answer is that number 1, there's a warmer atmosphere and so the air can carry more water vapor. So we have the potential of more water vapor in the atmosphere simply because it's warmer. Also, where might that water vapor come from? We're going to have less sea ice so there's more local moisture sources. I noted earlier that the sea ice essentially acts as a lid and it separates the Arctic Ocean, which is an ample moisture source, from the atmosphere. It basically separates them from the two. As we move through the 21st century we're going to have less sea ice and so we can put more water vapor in the atmosphere and the atmosphere is going to be warmer. So the answer here was C, that both A and B, these two things, a warmer atmosphere carrying more water vapor, less sea ice so more moisture sources. That's it. It's not more cloud condensation nuclei. We've never had a shortage of those. We'll never really have a shortage of them. Thank you, and I hope you've learned a little bit now about patterns of Arctic precipitation.
