Wednesday, April 16, 2014

Atmospheric Rivers

Atmospheric rivers (ARs) are narrow bands of enhanced water vapor extending from the tropics toward the midlatitudes. ARs are responsible for most of the moisture transport between the equator and poles in the lower troposphere. A significant percentage of total precipitation on the west coast has been shown to come from ARs between the months November and April (Rutz & Steenburgh, 2012).

Neiman et al. (2008) identified ARs from SSM/I satellite measurements and defined them as regions with concentrated water vapor (integrated water vapor greater than 20 mm) that are at least 2,000 km long, but are less than 1,000 km wide. ARs can make landfall anywhere along the west coast from British Columbia to the Baja Peninsula, but their frequency varies throughout the year. For instance, ARs in the northwest tend to occur more frequently during November while ARs in the southwest occur more frequently during January (Rutz, Steenburgh, & Ralph, 2014). Moisture from these ARs can cause precipitation in Utah if that moisture propagates beyond the mountain barriers. Forecasters should be aware of ARs because they can cause heavy precipitation and flooding if an AR persists for a long period of time. For this reason, forecasting an AR’s location and duration should influence short-term precipitation and water management forecasts.

Since ARs correspond with transported tropical moisture, ARs are typically found in the low-level jet ahead of cold fronts in the warm sector of an extratropical cyclone (Ralph, Neiman, Kiladis, & Weickmann, 2011). ARs are also located between a cyclonic pressure trough and an anticyclone ridge when circulation in the trough extends into the moist tropics and transports water vapor northward. ARs tend to make landfall in the Pacific Northwest when the longwave trough is positions over the Pacific Ocean with a ridge over the western United States. When the trough axis is closer to the coast ARs will make landfall in southern California or cross the Baja Peninsula (Rutz, Steenburgh, & Ralph, 2014; Neiman, Ralph, Wick, Lundquist, & Dettinger, 2008).

The effects of atmospheric rivers are most apparent along the west coast where they first interact with the topography. As ARs advance inland the amount of AR precipitation decays because orographic lifting causes the moisture laden air to condense and precipitate. Consequently, ARs that interact with the High Sierras are less likely to penetrate inland (Rutz, Steenburgh, & Ralph, 2014). When an AR intersects the High Sierras, most of the moisture is rained out before that moisture can reach Utah. This rain shadow effect is evident in Rutz and Steenburgh’s (2014) analysis of AR frequency. Because of the rain shadow, the Great Basin is the most unlikely region in the Intermountain West to experience AR related precipitation.

ARs will more likely penetrate inland if they pass over smaller mountains or through mountain gaps. For example, Rutz and Steenburgh (2014) show that AR moisture can extend as far as Idaho and Montana when transported through the low elevations of the Columbia River Basin and Snake River Plain. AR moisture can extend into Utah when moisture is channeled around the High Sierras. Southern Utah is mostly influenced by ARs that cross the Baja Peninsula. Although ARs in the Southwest do not occur as often they tend to have a longer duration (Rutz, Steenburgh, & Ralph, 2014).

During winter months precipitation from ARs tend to increase the snow-water equivalent in the snowpack. This is because AR moisture originates from the tropics and is warmer than the winter storms coming from the northwest. These warmer storms often have higher freezing levels and bring valley rain (Neiman, Ralph, Wick, Lundquist, & Dettinger, 2008). Heavy precipitation from AR moisture is enough for flood conditions, but flooding can become more severe by the potential melting of the snowpack by warm rains (Neiman, Ralph, Wick, Lundquist, & Dettinger, 2008).

Future Research
ARs have been a focus of study in the Northwest where they occur regularly and cause extreme floods. However, more research is needed to show how ARs affect precipitation in the Intermountain West. Rutz and Steenburgh suggested their analysis is smoothed because of poor topography resolution. Using higher resolution topography to analyze ARs may result in a better understanding of how broken and decayed ARs affect precipitation patterns in Utah.


Reading List
Guan, B., Molotch, N. P., Waliser, D. E., Fetzer, E. J., & Neiman, P. J. (2010). Extreme snowfall
 events linked to atmospheric rivers and surface air temperature via satellite measurements. Geophysical Research Letters, 1-6.
Neiman, P. J., Ralph, F. M., Wick, G. A., Lundquist, J. D., & Dettinger, M. D. (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. Journal of Hydrometeorology, 22-26.
Ralph, F. M., Neiman, P. J., Kiladis, G. N., & Weickmann, K. (2011). A Multiscale Observational Case Study of a Pacific Atmospheric River Exhibiting Tropical–Extratropical Connections and a Mesoscale Frontal Wave. Monthly Weather Review, 1169-1189.
Rutz, J. J., & Steenburgh, W. J. (2012). Quantifying the role of atmospheric rivers in the interior western United States. Atmospheric Science Letters, 257-261.
Rutz, J. J., Steenburgh, W. J., & Ralph, F. M. (2014). Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States. Monthly Weather Review, 905-921.

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