Dusty Atmospheric River Strikes California

It is the time of the year when deep plumes of tropical moisture known as atmospheric rivers tend to bring heavy precipitation to the west coast.  A particularly strong atmospheric river impacted the west coast this weekend (6-8 February) bringing over 10 inches of rainfall to parts of northern California and nearly 20 inches of snowfall across portions of the Sierra Nevada.  The CIRA total layered precipitable water product clearly shows the large amount of moisture streaming into California from the tropics.

Image 1.  SPoRT-CIRA total layered precipitable water valid on 7 February 2015 at 12 UTC.

Image 1. CIRA total layered precipitable water valid on 7 February 2015 at 12 UTC.

However, there is another component to this atmospheric river event that does not get much attention but they can significantly impact precipitation from these storms.  They are dust aerosols originating from the deserts in Asia, the Middle East, and Africa, and pollution aerosols from the megacities of eastern Asia.  These aerosols are transported across the Pacific most frequently during the late winter and early spring when the wind pattern is ideal.  In fact, a currently ongoing NOAA-led field investigation (i.e. CalWater 2) is focused on investigating the interaction between aerosols and atmospheric rivers to better understand how they modify precipitation.  SPoRT is supporting this effort by disseminating a near-real time product that combines aerosol optical depth (AOD) retrievals from geostationary and polar-orbiting satellites to monitor and track the long-range transport of aerosols.  Extensive cloud cover over the Pacific can often make it difficult to track the aerosols via satellites, therefore, the SPoRT product takes advantage of MODIS, VIIRS, MTSAT, and GOES AOD retrievals in order to provide a more comprehensive look at aerosol activity across the Pacific.  SPoRT is disseminating 6-hourly and daily AOD composites throughout the extent of CalWater 2 field campaign set to end mid-March with the overall goal of supporting their aerosol forecasting and flight planning activities.  Shortly before the atmospheric river made landfall on 7 February, the SPoRT daily AOD composite shows unusually high AOD of 0.3 to 0.5 (Image 2) in the vicinity of the atmospheric river in Image 1.  Higher values of AOD indicate an increasing amount or loading of aerosols in the atmosphere.  AOD is typically minimal (< 0.2) over the eastern Pacific when long-range transported aerosols are not present.

Image 1. NASA SPoRT daily AOD composite valid on 7 February 2015 at 00 UTC.

Image 2. NASA SPoRT daily AOD composite valid on 7 February 2015 at 00 UTC.

The CalWater 2 field investigators flew an aircraft equipped with advanced instruments directly through this atmospheric river on 7 February, which sampled an abundance of long-range transported dust according to CalWater investigator Dr. Jessie Creamean.  Further investigation is warranted to determine the source region of the dust.  With weeks to go still in the CalWater 2 campaign, field investigators are hoping for more opportunities to fly through aerosol laden atmospheric rivers.  If the trend from the past week continues, the CalWater 2 team should be able to gather a wealth of unique measurements that will ultimately lead to improved forecasting of these atmospheric river events.

Aerosol activity over the Pacific was relatively calm during January, but February kicked off with a bang.  The SPoRT daily AOD composite on 3 February shows a very thick, extensive aerosol plume propagating from eastern Asia to the western Pacific (Image 3).  Closer analysis suggested this plume was a mixture of dust and pollution aerosols.

Image 3.  NASA SPoRT daily AOD composite valid on 3 February 2015 at 00 UTC.

Image 3. NASA SPoRT daily AOD composite valid on 3 February 2015 at 00 UTC.

SPoRT AOD composites can be found at http://weather.msfc.nasa.gov/sport/aod/aodCalendarView.html

2 thoughts on “Dusty Atmospheric River Strikes California

  1. Nice post! Does having more aerosols near the atmospheric river feature result in a higher likelihood of precipitation (or higher precipitation) occurring when the feature makes landfall? In other words does the presence of aerosols allow the clouds able to hold more water? Or is this an area of more study that you refer to in the post?

    • Thanks Brad! It is definitely a complex issue that warrants further investigation as it depends on the aerosol types involved and meteorological conditions. However, results from several recent studies have suggested that the dust aerosols increase the riming rates of supercooled water droplets onto ice particles within mixed-phase orographic clouds which ultimately increases precipitation, in particular snowfall. However, if pollution aerosols are dominant, then precipitation may actually decrease due to increasing the number concentration of CCN and reducing the cloud droplet sizes. Pollution aerosols are much different than dust in that they do not have a significant impact on ice microphysical processes. Hope that answers your questions!

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