The Impact of Assimilating Field Campaign and Radio Occultation Observations on Forecasts of an Intense Atmospheric River during CALWATER2015

The overarching motivation of this study is to assess the impact of assimilating intensive offshore observations from a field campaign into model forecasts of landfalling atmospheric rivers. A useful case study to evaluate the impact of these observations occurred during the CALWATER2015 campaign, which took place off the west coast of California and observed a particularly strong atmospheric river before it made landfall on 6 February 2015. This event was associated with copious precipitation in northern and central California, both along the coast and in the Sierra-Nevada Mountains. In addition to numerous dropsondes, research flights also collected airborne Global Navigation Satellite System (GNSS) radio occultation (RO) observations using the GNSS Instrument System for Multistatic and Occultation Sensing (GISMOS). Airborne radio occultation (ARO) uses broadcast GNSS signals from setting satellites recorded from side-looking antennas to measure the signal ray path bending angle and refractivity, and further retrieve vertical water vapor and temperature profiles below the aircraft altitude. ARO observations captured the larger scale environment around the flight path, including the frontal structure, while the dropsondes sampled directly beneath the aircraft. Aside from the data collected by aircraft during the field campaign, spaceborne GPSRO observations of the offshore atmospheric river are available from COSMIC and other low earth orbiting satellites.

This study assesses the impact of assimilating the available field campaign and GPSRO observations on high spatial resolution mesoscale forecasts of the atmospheric river event. The impact of assimilating these observations is first assessed through the resulting mesoscale analyses, the quality of which are evaluated using the available microwave satellite data and other offshore observations. Next the impact of the data assimilation on key features of the offshore atmospheric river, including the integrated vapor transport, are identified. Finally the impact of the assimilation of the various observations on the forecasts of the low-level winds, moisture, and precipitation over California is quantified.