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To gain a more complete observational understanding of atmospheric rivers (ARs) over the data-sparse open ocean, a diverse suite of mobile observing platforms deployed on NOAA’s R/V Ronald H. Brown ( RHB ) and G-IV research aircraft during the CalWater-2015 field campaign was used to describe the structure and evolution of a long-lived AR modulated by six frontal waves over the northeastern Pacific during 20–25 January 2015. Satellite observations and reanalysis diagnostics provided synoptic-scale context, illustrating the warm, moist southwesterly airstream within the quasi-stationary AR situated between an upper-level trough and ridge. The AR remained offshore of the U.S. West Coast but made landfall across British Columbia where heavy precipitation fell. A total of 47 rawinsondes launched from the RHB provided a comprehensive thermodynamic and kinematic depiction of the AR, including uniquely documenting an upward intrusion of strong water vapor transport in the low-level moist southwesterly flow during the passage of frontal waves 2–6. A collocated 1290-MHz wind profiler showed an abrupt frontal transition from southwesterly to northerly flow below 1 km MSL coinciding with the tail end of AR conditions. Shipborne radar and disdrometer observations in the AR uniquely captured key microphysical characteristics of shallow warm rain, convection, and deep mixed-phase precipitation. Novel observations of sea surface fluxes in a midlatitude AR documented persistent ocean surface evaporation and sensible heat transfer into the ocean. The G-IV aircraft flew directly over the ship, with dropsonde and radar spatial analyses complementing the temporal depictions of the AR from the RHB . The AR characteristics varied, depending on the location of the cross section relative to the frontal waves.

Atmospheric rivers (ARs) are long and narrow corridors of enhanced vertically integrated water vapor (IWV) and IWV transport (IVT) within the warm sector of extra tropical cyclones that can produce heavy precipitation and flooding in regions of complex terrain, especially along the U.S. West Coast. Several field campaigns have investigated ARs under the CalWater program of field studies. The first field phase of CalWater during 2009–11 increased the number of observations of precipitation and aerosols, among other parameters, across California and sampled ARs in the coastal and near-coastal environment, whereas the second field phase of CalWater during 2014–15 observed the structure and intensity of ARs and aerosols in the coastal and offshore environment over the northeast Pacific. This manuscript highlights the forecasts that were prepared for the CalWater field campaign in 2015, and the development and use of an “AR portal” that was used to inform these forecasts. The AR portal contains archived and real-time deterministic and probabilistic gridded forecast tools related to ARs that emphasize water vapor concentrations and water vapor flux distributions over the eastern North Pacific, among other parameters, in a variety of formats derived from the National Centers for Environmental Prediction (NCEP) Global Forecast System and Global Ensemble Forecast System. The tools created for the CalWater 2015 field campaign provided valuable guidance for flight planning and field activity purposes, and they may prove useful in forecasting ARs and better anticipating hydrometeorological extremes along the U.S. West Coast.

The role of synoptic scale storms in the current and future climate continues to be an area of interest for the climate community. An increase in global population coupled with the devastation that can result from extreme weather events makes it important to understand how synoptic-scale storm tracks may be altered due to climate change. To accomplish this, a software package generates storm tracks by determining sea-level pressure minimum. The storm track program is then applied to data from the Coupled Model Intercomparison Project Phase 5 (CMIP5). A climatological comparison of storm track frequency, intensity, and precipitation for the historical run and the RCP4.5 and RCP8.5 scenarios is conducted to detect changes in storm frequency, intensity, and precipitation due to global warming. There is a large amount of variability between model runs and the model experiments in the Northern Hemisphere active storm track regions. The five-member model ensemble depicts a decrease in Pacific storm frequency, a decrease in the intensity of storms in the Pacific storm track region, and a decrease of precipitation in the North Pacific (south of Japan). In the North Atlantic active storm track region a decrease in storm frequency and intensity is projected with an increase in precipitation precipitation along the North American east coast. For the Aleutian region, storm intensity and precipitation are depicted to increase as baroclinicity changes and latent heat increases. However, in the Icelandic region, baroclinic changes are causing a decrease in storm intensity and precipitation as surface temperatures in the North Atlantic are cooler in the RCP experiments. The conveyor belt systems were also explored using the GFDL model, with the model capturing the typical conveyor belt model structure with some differences in the RCP scenarios.