Diagnosing Radial Ventilation in Dropsonde Observations of Hurricane Sam (2021)

Abstract

Abstract This study presents a method to diagnose radial ventilation, the horizontal flux of relatively low-θe air into tropical cyclones, from dropsonde observations. We used this method to investigate ventilation changes over three consecutive sampling periods in Hurricane Sam (2021), which underwent substantial intensity changes over 3 days. During the first and last periods, coinciding with intensification, the ventilation was relatively small due to a lack of spatial correlation between radial flow and θe azimuthal asymmetries. During the second period, coinciding with weakening, the ventilation was relatively large. The increased ventilation was caused by greater shear associated with an upper-level trough, tilting the vortex, along with dry, low-θe air wrapping in upshear. The spatial correlation of the radial inflow and anomalously low-θe air resulted in large ventilation at mid- to upper levels. Additionally, at low to midlevels, there was evidence of mesoscale inflow of low-θe air in the stationary band complex. The location of these radial ventilation pathways and their effects on Sam’s intensity are consistent with previous idealized and real-case modeling studies. More generally, this method offers a way to monitor ventilation changes in tropical cyclones, particularly when there is full-troposphere sampling around and within a tropical cyclone’s core. Significance Statement Ventilation, the injection of relatively dry and/or cool air into a tropical cyclone, may weaken a storm. In contrast, the lack of ventilation is favorable for intensification. The purpose of this study is to present a method to diagnose ventilation using aircraft dropsonde observations. Using dropsonde observations collected in Hurricane Sam (2021), there was a period of increased lateral ventilation in two regions around the storm that coincided with when the storm rapidly weakened. The results suggest that monitoring ventilation from dropsonde observations, when available, may be useful for anticipating ventilation-induced intensity changes in tropical cyclones and further studying ventilation pathways.