05 September 2017

Analysis of Coordinated Observations Reveals the Structure and Evolution of a Long-lived Atmospheric River

The first study of its kind provided researchers with new insights into atmospheric rivers over the open ocean.

Science

Atmospheric rivers (ARs) transport a large amount of water vapor as narrow bands in the atmosphere and, upon reaching land, dump heavy rain on regions such as the U.S. West Coast. To gain a more complete observational understanding of ARs over the data-sparse open ocean, scientists from the U.S. Department of Energy’s (DOE) Pacific Northwest National Laboratory and the National Oceanic and Atmospheric Administration (NOAA) analyzed diverse AR data collected during the CalWater-2015 field campaign. During this campaign, data gathered from multiple observing platforms provided new insights into the variability of thermodynamic, kinematic, and precipitation characteristics of the AR over time and space.

Impact

Researchers analyzed a unique suite of observations gathered aboard the NOAA Ronald H. Brown research ship, integrated with airborne measurements collected from the NOAA G-IV research aircraft, for a long-lived AR over the northeastern Pacific Ocean. The Ronald H. Brown hosted the DOE Atmospheric Radiation Measurement (ARM) Mobile Facility, among other instruments. This study offered new perspectives on the structure and evolution of atmospheric rivers that may be integrated into a modeling framework to ultimately improve forecasting of ARs.

Summary

Scientists investigated the structure and evolution of a long-lived AR modulated by six frontal waves that developed between warm and cold air masses over the northeastern Pacific Ocean from January 20-25, 2015. They used mobile observing platforms deployed on NOAA’s Ronald H. Brown ship and G-IV aircraft during the CalWater-2015 field campaign. Satellite observations and reanalysis diagnostics provided large-scale context, illustrating the warm, moist southwesterly airstream within the quasi-stationary AR located between an upper-level trough and ridge. Researchers acquired a comprehensive thermodynamic and kinematic depiction of the AR from rawinsondes. The unique data set revealed an upward intrusion of strong water-vapor transport in the low-level moist southwesterly flow during the passage of frontal waves 2 through 6. Scientists analyzed data from a co-located 1290 MHz wind profiler and noted an abrupt frontal transition from southwesterly to northerly flow below 1 kilometer mean sea level coinciding with the tail end of AR conditions. Additional data gathered from a shipborne disdrometer and radar provided key microphysical characteristics of shallow warm rain, convection, and deep mixed-phase precipitation. Rare, continuous observations of sea-surface fluxes in a midlatitude AR documented persistent ocean-surface evaporation and sensible heat transfer into the ocean. The G-IV flew directly over the ship, with dropsonde and radar spatial analyses complementing the time depictions of the AR from the Ronald H. Brown. AR characteristics varied, depending on the location of the cross section relative to the frontal waves. This first-of-its-kind AR study supplied new perspectives on the structure and evolution of atmospheric rivers over the open ocean, which will be useful for improving precipitation forecasts.

Contact
L. Ruby Leung
Pacific Northwest National Laboratory
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