Compound Dry-Dusty Air Intrusions during the Genesis of Tropical Storm Kate (2021): Evidence 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 numerical simulation to examine the complexity of compound dry-dusty air intrusions during the genesis of TS Kate (2021). Passive atmospheric tracers 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.