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

Abstract

The influence of the Saharan air layer (SAL) on developing tropical cyclones (TCs) involves complex interactions between dynamic, thermodynamic, and cloud microphysical processes and thus remains highly challenging to forecast. This study leverages a unique set 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 Tropical Storm (TS) Kate (2021). The suite of CPEX-AW observations, featuring a multifrequency lidar, a precipitation radar and GPS dropsondes, provides a unique perspective of the interplay between multiple air masses in the environment surrounding TS Kate. We complement CPEX-AW observations with atmospheric tracers from the high-resolution coupled-model simulation to better understand the origins and transport of these air masses and their impacts on TS Kate. Two distinct intrusions are observed: one within the precursor easterly wave and one during the Tropical Depression (TD) stage. Within the precursor wave, low-${\theta }_e$ marine air associated with subsidence within the subtropical high undercuts the SAL as both air masses are entrained along two pathways: (a) lateral entrainment following the wave-relative inflow and (b) vertical entrainment downward into the boundary layer and subsequently upward within deep convection. Later, mid-tropospheric dry air from the subtropical high merges with the remnant SAL, resulting in strong radial ventilation of the TD above the boundary layer likely limiting any further intensification.