African Easterly Waves and Tropical Cyclogenesis Observed by CYGNSS and ASCAT

The CYGNSS constellation of eight smallsats has been collecting ocean surface wind speed data for over seven years. Unlike the more familiar conventional scatterometers, CYGNSS passively relies on signals of opportunity to deduce wind speed: reflections of GPS signals off the ocean surface. The reflected signals contain information about the roughness of the ocean which is generated by wind blowing over it. Another difference is that the CYGNSS constellation is in a low-inclination orbit, reaching poleward to approximately 35°, while scatterometers such as ASCAT are in sun-synchronous polar orbits. These differences present both challenges and opportunities. The biggest challenge has been deriving a useful signal at higher wind speeds. At the 19-cm wavelength at which GPS satellites transmit, the sea state can be a combination of freshly-generated waves ("young seas with limited fetch") and more mature long-distance swells ("fully-developed seas"). Depending on the age of the sea state, different wind speeds could be present, and this ambiguity begins to appear above approximately 20 m/s, or roughly the minimum tropical storm intensity. Two significant opportunities presented by CYGNSS are its ability to retrieve wind speeds through precipitation (because of the longer wavelength) and its more frequent revisit times over a given area (because of its lower-inclination orbit). This study will utilize both CYGNSS and ASCAT data to look at African easterly waves at pre-tropical-storm intensities in the Atlantic "main development region". These early stages of a storm's lifecycle can be difficult for forecasters to analyze and to determine when genesis occurs. We will identify differences between the two data types and explore the benefits of CYGNSS filling in gaps in ASCAT coverage.