Flooding in the Russian River valley of California.


Satellite image of atmospheric river during the event.


Integrated water vapor measured at Cazadero, CA.


P3-flight across a developing atmospheric river north of Hawaii on March 24, 2005.


Flight across the strenthening atmospheric river on March 25, 2005.

Finding the Source of Atmospheric Rivers

Scientists and forecasters have long understood that the continental United States looks to the Pacific Ocean for its rainfall. Part of a complex global cycle, water evaporated from the Pacific Ocean travels in clouds to produce rain and snow over land. To better explain and predict droughts and floods, scientists are examining the processes which govern these flows.

Seeking to improve severe storm forecasts on the Pacific Coast, researchers during the California Jets (CALJET) and Pacific Jets (PACJET) experiments worked to understand structures in the atmosphere called rivers which transport large amounts of water vapor from the equitorial Pacific to the U.S. West Coast.

Atmospheric rivers are formed as part of the "warm conveyor belt", a hemispheric cycle by which the cold, dry air of the Arctic flows southward, and warm, humid air from the tropics is forced northward. This "conveyor belt" plays a key role in the formation of Pacific storms. Modeling studies concluded that more than 90% of the total water vapor transported toward the pole was concentrated into these rivers.

Flying into the River

While satellite observations reveal the global extent of these systems, understanding their dynamics requires direct, high-resolution observations. Much like probing the structure of a hurricane, the NOAA P-3 aircraft flew a number of flights through the rivers to measure the humidity, winds and pressure present to gain a more comprehensive understanding of this phenomena. These direct observations will be used to improve the modeling of these flows to more accurately predict their formation, trajectory and timing.

Hydrometeorological Testbed (HMT) 2005

During HMT 2005, 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.

Dropsondes are instruments which measure temperature, humidity, air pressure and altitude as they are dropped from the aircraft and transmit the data back to a data center via satellite. These high resolution measurements are needed to understand the structure and dynamics of the atmospheric river.

A follow-on flight the next day obtained one-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 thermodynamic 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.

Short-term Impacts and Long-term Perspective

Atmospheric rivers form a critical link between weather and climate scales. They strongly influence both short-term weather and flood prediction, as well as seasonal climate anomalies and the global water cycle, through their cummulative effects. In the short-term observation of atmospheric rivers can improve the forecasting of severe storms and potential flooding. A longer term view may allow us to better couple climate forecasts with seasonal weather forecasts to improve water usage during drought and flood seasons.

References

• Ralph, F.M., P.J. Neiman, and G.A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern North-Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, 1721-1745.
• ETL Explores Tropical Origins of Atmospheric Rivers Using NOAA P-3 Research Aircraft