rivers in the sky

Well, in a small step toward more focused posts, I wanted to write a few thoughts about a short paper that is in Geophysical Research Letters this month. The reference is:
Ralph, F. M., P. J. Neiman, G. A. Wick, S. I. Gutman, M. D. Dettinger, D. R. Cayan, and A. B. White (2006), Flooding on California's Russian River: Role of atmospheric rivers, Geophys. Res. Lett., 33, L13801, doi:10.1029/2006GL026689. [LINK]
The authors are using satellite observations of atmospheric moisture, and comparing coherent (but transient) structures to precipitation events (storms), with particular emphasis on flood events. These coherent structures are called atmospheric rivers by the authors, who have been championing the term for a couple of years. Residents of California will recognize the particular events discussed in the paper as the "pineapple connection" or the "pineapple express." This is basically a narrow band of very moist air that stretches from the tropical central to eastern Pacific northeastward toward California. When these atmospheric rivers form, they tend to bring humid, more tropical conditions to California. I've always thought of these events happening in spring, and delivering warm, humid conditions, usually associated with mid-level to high-level clouds. The authors are looking at winter storms though, and they find that atmospheric rivers are associated with the warm-sector of storms, and also they are associated with strong low-level winds (a low-level jet).

When the low-level jet gets wrapped up with a developing storm, forming the connection between the tropics and the extratropics manifest as an atmospheric river, it can deliver a lot of precipitation to coastal areas. This study is an attempt to show that atmospheric rivers are associated with floods in the Russian River area of northern California. The way it works is that the storm approaches the coast, with the atmospheric river sort of preceding it (or riding on the warm sector). The atmospheric river is confined to the lower levels in that low-level jet, which then intersects the land. The trouble with land is that it isn't flat, and when the low-level jet meets the coastal mountains, it has to go somewhere. It turns out that relatively low mountains don't deflect the flow very much, but instead the jet goes up and over the mountains (that's the dynamics). As the moist air rises, it cools, and cooler air has a lower saturation humidity so the water begins to condense and rain out (that's the physics). This is orographic precipitation. So as long as that low-level jet is carrying such warm, moist air and is intersecting the mountains, there's going to be serious rain. The paper shows a case study from February 2004, and it is clear that as long as there is upslope winds (the low-level jet going over the mountains), it is raining like crazy.

The authors suggest, not very strongly (because this is kind of a new way of thinking about this), that a large fraction of coastal flooding in California is connected with these filaments of moist air originating in the tropics and attached to winter storms.

What does this have to do with climate and/or climate change? Well, a lot of people like to talk about changes in the distribution of extreme events in a warming world... like more/stronger hurricanes, more frequent floods and droughts, etc. This is a possible mechanism for flooding along coastal areas (where most people live), so if we want to understand how flood events will change in the future, we need to understand this connection between atmospheric rivers and orographic precipitation. That includes better understanding of both the dynamics (how and why the atmospheric rivers form) and the physics (how mountains force precipitation). Even more interesting for some of us... well, me... is that a lot of the water in the atmosphere comes from the tropics, and there is some evidence that a large fraction of that transport is in the form of these narrow filaments. If the formation or characteristics of atmospheric rivers are sensitive to, say the distribution of moist convection in the deep tropics, or the structure of the north-south atmospheric circulation (the Hadley circulation), then there might be important consequences for extratropical atmospheric moisture in a changing environment. That's just a poorly-formed idea rattling around in my head though, so don't take it too seriously, but these atmospheric rivers are probably playing a more important role in the distribution of water in the climate system than has been appreciated before, and that is interesting.

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