Surface Wind Speeds and Enthalpy Fluxes During Tropical Cyclone Formation From Easterly Waves: A CYGNSS View

We examined the Cyclone Global Navigation Satellite System (CYGNSS) retrievals of surface wind speeds and enthalpy fluxes in African easterly waves that led to the formation of 30 Atlantic tropical cyclones during 2018-2021. Lag composites show a cyclonic proto-vortex as early as 3 days prior to tropical cyclogenesis. The enthalpy flux distribution does not vary substantially before cyclogenesis, but subsequently, there is a marked increase in the extreme upper values. In the composites, a negative radial gradient of enthalpy fluxes becomes apparent 2-3 days before cyclogenesis. These results-based on novel data blending satellite retrievals and global reanalysis-support the findings from recent studies that the spin-up of tropical cyclones is associated with a shift of peak convection toward the vortex core and an inward increase of enthalpy fluxes.

Automated summary

We investigated the surface wind speeds and enthalpy fluxes of African easterly waves using the Cyclone Global Navigation Satellite System (CYGNSS), which led to the formation of 30 Atlantic tropical cyclones from 2018 to 2021. Lag composites revealed the presence of a cyclonic proto-vortex as early as 3 days before tropical cyclogenesis. The distribution of enthalpy fluxes did not show significant variations before cyclogenesis, but there was a noticeable increase in extreme upper values afterwards. In the composites, a negative radial gradient of enthalpy fluxes became evident 2-3 days before cyclogenesis. These results, based on a novel combination of satellite retrievals and global reanalysis, provide support for recent studies indicating that the spin-up of tropical cyclones is associated with a shift of peak convection towards the vortex core and an inward increase of enthalpy fluxes.