An Analysis of Coordinated Observations from NOAA’s Ronald Brown Ship and G-IV Aircraft in a Landfalling Atmospheric River over the North Pacific during CalWater-2015

From January to March 2015, government and university forecasters, research scientists, and engineers participated in the CalWater 2015 field campaign, which deployed state-of-the art monitoring equipment on a research ship, airplanes, and at ground sites to study two phenomena that play key roles in water supply availability and the incidence of extreme precipitation events along the U.S. West Coast: atmospheric rivers (ARs) and aerosol impacts on precipitation. In a new study led by ESRL’s Physical Sciences Laboratory, to be published in Monthly Weather Review, researchers examine observations from the NOAA ship Ronald H. Brown and a NOAA G-IV aircraft to document the structure and evolution of a long-lived AR that occurred over the northeastern Pacific Ocean in January of that year. The AR remained offshore of the U.S. West Coast but made landfall across British Columbia where heavy precipitation and high atmospheric melting levels resulted in flooding.

Forty-seven weather balloons launched from the ship provided a comprehensive depiction of the AR, including an influx of strong water-vapor transport in the low-level moist southwesterly winds during the passage of five storm systems along the front. A wind profiling radar located on the ship showed an abrupt frontal transition from southwesterly to northerly winds below 1 km coinciding with the tail-end of AR conditions. Observations from shipboard instruments captured key characteristics of shallow warm rain, convection, and deep precipitation that originated as snow in the atmosphere aloft. Continuous observations of the atmosphere above the sea-surface showed persistent ocean-surface evaporation into the AR, and exchange of sensible-heat from the AR into the ocean. The G-IV aircraft coordinated with the ship and flew directly overhead. Dropsonde and radar analyses from the aircraft complemented the depictions of the AR from the ship.

When ARs make landfall along the mountainous West Coast, they can generate beneficial precipitation for water resource management as well as damaging flooding. Advancing scientific knowledge about the structure and development of ARs prior to landfall is important for improving forecasts, and better understanding their potential benefits and adverse impacts.

Posted: June 26, 2017

Neiman, P. J., N. Gaggini, C.W. Fairall, J. Aikins, J. R. Spackman, L. R. Leung, J. Fan, J. Hardin, N. R. Nalli, and A. B. White (2017): An Analysis of Coordinated Observations from NOAA’s Ronald Brown Ship and G-IV Aircraft in a Landfalling Atmospheric River over the North Pacific during CalWater-2015. Mon. Wea. Rev., 10.1175/MWR-D-17-0055.