ETL Explores Tropical Origins of Atmospheric Rivers Using the NOAA P-3 Research Aircraft

Contact: Paul Neiman

The pre-cold-frontal low-level jet (LLJ) in land-falling extratropical cyclones that approach the West Coast of the United States each winter plays a critical role in transporting water vapor into the coastal mountains, resulting in orographic enhancement of precipitation that can generate devastating flooding and debris flows. The LLJ, which resides at approximately 1 km MSL, represents the lower-tropospheric component of a deeper corridor of concentrated water vapor transport in the pre-cold-frontal environment. Because these corridors tend to be quite narrow (2000 km), and yet are responsible for almost all of the meridional water vapor transport at midlatitudes, they are referred to as atmospheric rivers. Most (~75%) of the water vapor transport within these rivers occur within the lowest 2.5 km of the atmosphere. In addition to causing flooding rains in the coastal mountains and playing a critical role in the global water cycle, atmospheric rivers are integrally tied to water resource issues in the semi-arid West, where a majority of snowfall in the higher elevations ultimately provides fresh water to the population.

Satellite imagery reveal that a subset of atmospheric rivers in the midlatitudes protrude northward from the moisture-rich tropics. However, it is not well understood why the tropical moisture becomes entrained into some atmospheric rivers but not others. Hence, ETL teamed up with NOAA's Aircraft Operations Center in March and April 2005, to fly the P-3 research aircraft from Honolulu, Hawaii, into several atmospheric rivers at the tropical-extratropical interface.

Two consecutive flights were carried out on March 24-25 and 25-26, 2005, through a developing atmospheric river that eventually extended from the tropics to the Pacific Northwest of the United States. The P-3 successfully released 44 dropsondes in two parallel curtains (~60 km horizontal resolution between drops) across the developing river north of Hawaii on the first flight. A follow-on flight the next day obtained 1-second resolution lower tropospheric flight-level data through the strengthening atmospheric river, after which the aircraft released another high-resolution curtain of 23 dropsondes across the river. The day after the second flight, the mature atmospheric river slammed into the Pacific Northwest, resulting in heavy rains that offered temporary relief (but also generated flooding) in the drought-stricken region. Initial perusal of the data from these flights reveals that our team did a remarkable job at capturing the mesoscale thermodynalic and kinematic structure across this river, where peak values of integrated water vapor within the core region exceeded 4 cm. The P-3 data gathered during the two flights will help further our understanding of cool-season tropical-extratropical interactions and their role in generating significant hydrologic impacts along the U.S. West Coast.

A third P-3 flight on April 8-9, west of Hawaii gathered mesoscale data from 66 dropsondes a region where an extratropical frontal zone penetrated the Hadley subsidence belt and directly tapped moisture from the tropics. Ultimately, however, a well-defined atmospheric river never developed. This case will provide additional invaluable insights into the dynamics that are required to generate atmospheric rivers originating from the tropics and extending deep into the midlatitudes. The analysis of the P-3 observations should provide a scientific and practical basis for improving West Coast nowcasting in ways that can aid the issuance of watches and warnings by the National Weather Service and help in decision making by forecast users, especially in terms of flooding.

More Information

• NOAA Hydrometeorology Testbed Program
• NOAA Regional Weather and Climate Applications Division