Much of the precipitation along the U.S. West Coast is delivered by phenomena known as “atmospheric rivers”—narrow bands of moist air that may extend for thousands of miles across regions outside of the tropics, and play a critical role in regional water supply and storm activity. Atmospheric-river events play a beneficial role in building up Western water supply and snowpack but are also the source of a large majority of floods in the region. Many uncertainties about key processes that affect storm development within atmospheric rivers limit the ability to predict atmospheric rivers and associated precipitation. An improved understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions. From January-March 2015, the joint NOAA, NASA, and DOE CalWater-2 campaign collected a comprehensive dataset in environments where atmospheric rivers develop and make landfall, including data on how atmospheric aerosols influence precipitation.

Aircraft instruments sampled aspects of atmospheric rivers and their associated environment, and researchers aboard a NOAA Research Vessel operated NOAA and DOE instrumentation, measuring energy flow between the ocean and the atmosphere and its influence on atmospheric rivers. The campaign also built on the new NOAA Hydrometeorology Testbed (HMT) ground sites, which contributed measurements of precipitation, winds, snowpack, soil moisture, snow level, and surface weather. Scripps Institution of Oceanography installed additional instrumentation at the Bodega Bay HMT site to study aerosol chemistry. This data will be used to improve short- and long-term precipitation predictions and develop decision-support tools for extreme-precipitation events, hazard response, and water-resources management.