Wednesday, March 25, 2015

Reliving storms past

I can't bring rain to drought-plagued California, but I can bring virtual showers by making movies showing historic Atmospheric Rivers (ARs) that hit California.

ARs can be defined in terms of satellite measurements of "20 mm of water vapor and is > 2000 km long and < 1000 km wide."  They average about 400 km wide.

First, I made a global animation of Dec 26, 2004 to January 11, 2005 so you can see atmospheric rivers peeling off from the moisture belt of the tropics and subtropics.  I used the NOAA/CIRES Twentieth Century Global Reanalysis Version 2c (20thC R2c) because I want to highlight a newly available dataset by showing cool things you can do with it.

Global animation of NOAA/CIRES Twentieth Century Global Reanalysis Version 2c for December 26, 2004 to January 11, 2005.
Precipitable water is a measure of the total, column-integrated, water vapor in the atmosphere at that point.  If all the water vapor condenses, the equivalent depth of the water can be measured as a length (often mm), or as mass per unit area (kg per square meter).  1 kg m-2 of water is equivalent to 1 mm of water.  In the global animation, ARs are light cyan to red in color.

Precipitable water can be estimated (pretty well!) using microwave remote sensing from satellites.  Modern Global scale models usually assimilate all available data, including satellite and ground-based measurements, to generate estimates of physical parameters, including precipitable water. But, because 20thC R2c goes back to 1851, this analysis was performed using just the three types of data available in 1851: surface pressure (from land-based ground stations), sea surface temperatures (from ship observations) and sea ice extent.

Then I zoomed in to the regional-scale and changed the color range of the precipitable water scale from [0.1, 70] to [0.1, 40] to better show the structure in mid-latitudes like California.  In this animation, ARs are yellow to red.
Regional-scale animation of NOAA/CIRES Twentieth Century Global Reanalysis Version 2c for December 26, 2004 to January 11, 2005.

Notice a first pulse of moisture that came in through the Gulf of California, responsible for the record-setting heavy rains in Death Valley and the inland deserts of California that filled up Lake Manly.  Yes, that is a kayaker in Death Valley.

Then an atmospheric river hit the coast of California with multiple pulses of heavy moisture. According to NOAA's California Nevada River Forecast Center's Heavy Precipitation Report for this event, these two storms dropped an impressive amount of rain in southern California, including a whopping 51.77 inches at Opids Camp.

 Can you feel the rain?

I made the visualizations with the help of NASA's free Panoply data viewer and GIFMaker.  You can easily make your own, too.

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