Publication Date

Fall 2021

Degree Type


Degree Name

Master of Science (MS)


Meteorology and Climate Science


Sen Chiao

Subject Areas



Atmospheric Rivers (ARs) can provide much needed water to drought-ridden locations, however, can also lead to potential devastating flooding events. Thus, AR forecasts must be improved to keep the public safe. This case study used the Weather Research and Forecasting (WRF) model to compare the impact of including COSMIC-2 GPS Radio Occultation (RO) measurements using 4D-VAR data assimilation techniques. Each WRF simulation analyzed the vertical motion, pressure, mixing ratios with respect to ice, rain, and vapor, and precipitation fields. To provide observational precipitation data for model comparison, the Integrated Multisatellite Retrievals for Global Precipitation Measurement (IMERG) satellite and the Santa Clara Valley Water rain gauges were used. Vertical temperature and moisture profile were compared with the National Oceanic and Atmospheric Administration (NOAA) soundings. Results showed: (1) COSMIC-2 GPS RO 4D-VAR data slightly improved the position of the trough; (2) 4D-VAR significantly improved the vertical structure of the AR especially in the vertical motion field: (3) 4D-VAR redistributes mixing ratio through the different phases; and (4) that the 45-hour 4D-VAR showed the most accurate precipitation results between the control experiment (CNTL) and 24-hour simulations. These results provide confidence that ingesting COSMIC-2 GPS RO data using 4D-VAR data assimilation improves the upstream environment, which leads to more accurate vertical motion fields and thus improved precipitation results.