Compound Dry-Dusty Air Intrusions during the Genesis of Tropical Storm Kate (2021): Observations from the CPEX-AW Field Campaign and Coupled Modeling.

Despite decades of active research, the influence of the Saharan Air Layer (SAL) on developing tropical cyclones remains highly uncertain. In this study, we leverage a unique combination of in-situ aircraft observations from the NASA Convective Processes - Aerosols and Winds (CPEX-AW) field campaign and a high-resolution, fully coupled atmosphere-wave-ocean model simulation to examine the complexity of compound dry-dusty air intrusions during the genesis of TS Kate (2021). The unprecedented observations from the DC-8 aircraft with multi-frequencies lidar and radar as well as GPS dropsondes measurements in TS Kate during CPEX-AW provide a unique perspective of multiple air masses properties in the environment of Kate. We use the high-resolution coupled model simulation with passive atmospheric tracers to better understand the origins and transport of these various air masses and their impacts on TS Kate. The results reveal that low air associated with the SAL and with subsidence within the subtropical high is entrained within the disturbance along two pathways: i) lateral entrainment following the wave-relative inflow, ii) vertical entrainment downward into the boundary layer and subsequently upward within deep convection. Two distinct intrusions are observed: one within the precursor easterly wave and one during the tropical depression (TD) stage. Within the precursor wave, low marine air from the eastern Atlantic undercuts the SAL as both air masses are entrained. Subsequently, mid-tropospheric dry air originating within the subtropical high combines with the remnant SAL front, resulting in strong radial ventilation of the TD inner core to the right of the shear vector and limiting further intensification. The combination of CPEX-AW aircraft observation and high-resolution coupled model provides new insights into tropical cyclone-environment interaction over the tropical Atlantic, which have important implications on the need for better observing and modeling both TCs and their environment to advance TC predictions in the future.