Friday, September 16, 2016

Notes on Atmospheric River Research

Western Washington residents are familiar with the weather term "pineapple express". A pineapple express is a warm heavy rainstorm. The weather system that delivers these events is out of the tropics and hence temperatures will be mild with lots of rain.

Satellite images of water vapor over the oceans show atmospheric rivers hitting the U.S. West Coast in 2006 (top), 2009 (middle), and 2004 (bottom). (Images courtesy of NOAA.)

These events have been getting lots of attention by both meteorologists and climatologists. Because these storms can cause severe flooding and landslides, being able to predict them can improve the ability of communities and individuals to prepare and respond. Overall the prediction of these events has gotten pretty reliable. The harder part of the prediction has been some of the detail which can be very important. One prediction issue is just how much water is being carried out of the tropics and how much of it will be dumped as it makes landfall. Perhaps a bigger challenge is exactly where the landfall of the main section of atmospheric river will take place and Will the flow of the atmospheric river stay locked in one place or drift? The locked in one place scenario can mean a narrow band of the coast and inland will get huge rain fall with little to modest amounts to the north and south of the band. These locked in place events have proven to be major flood events and a cause of many landslides.

A few years ago I timed a filed project with the arrival of an atmospheric river in the northern Coast Range of Oregon. I am used to rain, but that experience was memorable. My crude rain gauge I left at the car (a bucket) measured nearly 7 inches of water in the 8 hours I was out in the field.  

The climate change question is Will these events increase in intensity and frequency? And Were there past natural variability in the pattern of these events? The more recent work seems to be pointing towards a modest increase in frequency for western Washington but heavier rainfall events when they take place.

From a hazard perspective considering landslides and flooding, this suggests more frequent floods and more landslides. But some consideration has to take into account the weather prior to the atmospheric river arrival. Cold wet weather with low land snow followed by a sudden warm rain with warm wind will greatly amplify the flooding and geohazards versus an isolated atmospheric river event.      

NCAR: atmospheric-rivers provides a press release on two recent papers by Shields and Kiehl Simulating the Pineapple Express and Atmospheric River Landfall. Both papers as well as others on the same subject are in the Journal Geophysical Research Letters - access to the full papers requires membership and subscription.

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