Demonstration of Assimilation of CYGNSS Delay-Doppler Maps (DDMs) in a 2-Dimensional Variational Ocean Vector Wind Analysis

The Delay-Doppler Mapping Instrument (DDMI) on the Cyclone Global Navigation Satellite System (CYGNSS) constellation measures reflections of Global Positioning Satellite (GPS) signals from the ocean surface. The primary mission of the CYGNSS constellation is the retrieval of global ocean surface wind speed in all weather conditions from the measured Delay-Doppler maps (DDMs) of these reflections. We have developed methods for assimilating DDMs directly in a a 2-dimensional ocean vector wind analysis using an adapted version of the Variational Analysis Method (VAM) of Hoffman et. al (2003). The adaptations include a full forward model of the reflection problem (i.e., the observation operator), and calculation of a Jacobian of the sensitivity of modeled DDM power to ocean surface wind speed. With these aspects of the problem defined, it is possible to assimilate reflections directly into the ocean surface wind field. The assimilation methods developed will be described and sample results from DDM assimilation in the VAM will be presented. This research ultimately aims to assess the impact of assimilating derived CYGNSS wind vectors via regional and global model Observing System Experiments (OSEs), comparing the influence of CYGNSS wind vectors derived from VAM assimilation of DDMs versus VAM assimilation of CYGNSS retrieved winds.