February 6th Case Study of an Atmospheric River Event Using GPS Radio Occultation

Atmospheric rivers (ARs) are of high concern in California, bringing significant rain to a region over extended time periods of up to 5 days, causing floods and triggering extreme values of streamflow. Since ARs are a major water source for CA, a year with few events may result in a devastating drought. The CalWater project focuses on predicting the variability of West Coast water supply, including improving AR forecasting. Unfortunately, data collection over the ocean remains a challenge. One novel technique to address that issue includes GPS radio occultation (RO), or using broadcast GPS signals from space to measure the signal ray path bending angle and refractivity to retrieve vertical water vapor profiles. The Global Navigation Satellite System Instrument System for Multistatic and Occultation Sensing (GISMOS) system was developed for this purpose for recording and processing high-sample rate (10MHz) signals in the lower troposphere. Previous studies (Murphy et al, 2014; Zhou & Chen, 2014) have shown promising results in acquiring and assimilating GPS RO data, comparing it to dropsondes and numerical weather models. CalWater launched a field campaign in the beginning of 2015 for testing GPS RO for AR data acquisition, flying into the February 6th AR event that brought up to 35 cm of rain to parts of central California. This case study will demonstrate the coverage of the GPS RO observations and use GFS reanalysis models and dropsondes to show the vertical structure of the frontal system from that same day to guide the analysis of GPS RO data.