What is an Atmospheric River?
An atmospheric river (AR) is a narrow corridor of moisture and moisture transport in the atmosphere. Zhu and Newell (1998) estimate that the moisture flux in a typical AR is comparable to the flux in the Amazon River, roughly 160 million kg/sec. ARs play a crucial role in heavy precipitation and flooding, including the so-called mega-floods (Dettinger and Ingram, 2013; Ralph and Dettinger, 2012; Moore et al., 2012).
ARs are classified based on the source of the moisture. ARs with the moisture source in the East Pacific are often called “Pineapple Express” storms (Dettinger, 2004), and occur more in northern hemisphere winters. Moisture transport from the West Pacific produces the Bai-u / Mei-yu (as it’s known in Japanese / Chinese) rainy season in northern hemisphere summers (Knippertz and Wernli, 2010). Moisture from the Gulf of Mexico and the Caribbean Sea reaches the inland continental United States via the Great Plains Low Level Jet (Barandiaran et al., 2013). This moisture transport occurs most frequently in northern hemisphere summers. And the North Atlantic is a moisture source for Europe (Lavers et al., 2011; Stohl et al., 2008), with maximum transport in northern winters.
Particularly interesting are cases where ARs meet high land topography because the upslope flow increases orographic contributions to precipitation rates.
Identifying Atmospheric Rivers in Satellite Microwave Data
Passive microwave satellite remote sensing provides highly accurate measurements of total column integrated water vapor (Wentz et al., 2007). Neiman et al. (2008) and Dettinger et al. (2011) have used SSM/I water vapor to identify ARs based on the criteria that an AR has > 20 mm of water vapor and is > 2000 km long and < 1000 km wide. Water vapor is available from many microwave sensors including AMSR-E, AMSR2, GMI, SSM/I, SSMIS, TMI, and WindSat. ARs can also be identified based on moisture transport, but estimating moisture transport requires vertical profiles of vapor and wind, which cannot be accurately measured by satellites (Hilburn, 2010). In addition to moisture, the production of heavy precipitation also requires atmospheric instability and a source of lift. The heavy rainfall in California in December 2014 highlighted the importance of strong Aleutian low dynamics in producing heavy precipitation (Hilburn and Wentz, 2014).
Description of Atmospheric River Images
The images on this page provide the daily average water vapor and the daily maximum wind speed from all available RSS satellite data. The areas of high vapor (in blue shades) show ARs and the areas of high wind speed (in red shades) show strong storm energy. The wind speed estimates come from the same satellites as the water vapor estimates. Wind speed cannot be estimated in rain locations, and water vapor cannot be estimated in heavy rain. Winds from SSMIS are the “medium frequency”, while winds from AMSR2 and WindSat are the “low frequency” winds. There are only slight differences between medium and low frequency winds.
Images are updated every hour on this web page, and we provide the last seven days to help one locate the moisture source of any developing AR. Note that these images are not forecasts, but are looking backwards in time. Since the images combine 24 hours of data for each image, you may observe “double fronts” where meteorological features moved during the day and were sampled at different locations by different sensors or by the same sensor at two different times. Light gray areas exists were there are no measurements available or if over land and coastal areas.
We provide these images year-round. Since ARs affect the US West Coast primarily in the winter months, the images will show other strong and moist events such as tropical cyclones in the summer and fall seasons.
Daily Watch of Atmospheric Conditions
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Movie of Last 7 Days
Barandiaran, D., S.-Y. Wang, and K. Hilburn, 2013: Observed trends in the Great Plains low-level jet and associated precipitation changes in relation to recent droughts, Geophys. Res. Letts., 40 (23), 6247-6251, doi: 10.1002/2013GL058296.
Dettinger, M., 2004: Fifty-two years of “pineapple-express” storms across the west coast of North America, California Energy Commission Public Interest Energy Research Program Project Report CEC-500-2005-004.
Dettinger, M. D., F. M. Ralph, T. Das, P. J. Neiman, D. R. Cayan, 2011: Atmospheric rivers, floods, and the water resources of California. Water, 3, 445-478; doi: 10.3390/w3020445.
Dettinger, M. D., and B. L. Ingram, 2013: The coming megafloods. Scientific American, January 2013, 64-71.
Hilburn, K. A., 2010: Intercomparison of water vapor transport datasets. Abstract H31H-1098 presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec.
Hilburn, K., and F. Wentz, 2014: An inter-calibrated passive microwave brightness temperature data record and ocean products. Abstract A51I-3147 presented at the 2014 Fall Meeting, AGU, San Francisco, CA, 15-19 Dec.
Knippertz, P., and H. Wernli, 2010: A Lagrangian climatology of tropical moisture exports to the Northern Hemispheric extratropics. J. Climate, 23, 987-1003.
Lavers, D. A., R. P. Allan, E. F. Wood, G. Villarini, D. J. Brayshaw, A. J. Wade, 2011: Winter floods in Britian are connected to atmospheric rivers. Geophys. Res. Letts., 38, L23803, doi:10.1029/2011GL049783.
Moore, B. J., P. J. Neiman, F. M. Ralph, F. E. Barthold, 2012: Physical processes associated with heavy flooding and rainfall in Nashville, Tennessee, and vicinity during 1-2 May 2010: The role of an atmospheric river and mesoscale convective systems. Mon. Wea. Rev., 140, 358-378.
Neiman, P. J., F. M. Ralph, G. A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSM/I satellite observations. J. Hydrometeor., 9, 22-47.
Ralph, F. M., and M. D. Dettinger, 2012: Historical and national perspectives on extreme West Coast precipitation associated with atmospheric rivers during December 2010. Bull. Amer. Meteor. Soc., 93, 783-790, doi: 10.1175/BAMS-D-11-00188.1.
Stohl, A., C. Forster, H. Sodermann, 2008: Remote sources of water vapor forming precipitation on the Norwegian west coast at 60°N – a tale of hurricanes and an atmospheric river. J. Geophys. Res., 113, D05102, doi:10.1029/2007JD009006.
Wentz, F. J., L. Ricciardulli, K. A. Hilburn and C. A. Mears, 2007: How Much More Rain Will Global Warming Bring?, Science, 317, 233-235.
Zhu, Y., and R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725-735.