The Impact of Cloud-Radiative Forcing on the Boundary Layer Evolution for Landfalling Tropical Cyclones

This study investigates how cloud-radiative forcing interacts with continental thermodynamic environments to influence the evolution of the tropical cyclone boundary layer (TCBL) during landfall using high-resolution, full-physics numerical simulations. Four idealized landfall experiments representative of Atlantic continental environments are analyzed to examine how cloud-radiative processes respond to progressively stronger environmental thermodynamic forcing. The impact of cloud-radiative forcing is isolated via paired sensitivity runs that compare an all-sky RRTMG radiation parameterization against a modified cloud-transparent version in which hydrometeors are radiatively transparent. All experiments begin from the same initial modified Rankine vortex with a maximum wind of 12.5 m/s and a radius of maximum wind (RMW) of  75 km. Each vortex reaches a quasi-equilibrium state (characterized by steady minimum sea level pressure and maximum azimuthal wind) near 90 hours, and after 96 hours, a large-scale nudging technique imposes a uniform 5 m/s southerly flow that produces landfall within 48 hours.

Analysis of a 6-h window spanning 3 h before to 3 h after landfall shows that cloud-radiative forcing influences the TCBL through a coupled thermodynamic pathway in which cloud condensate modifies radiative heating and cooling, altering boundary-layer stability and equivalent potential temperature. These changes reorganize the secondary circulation, producing variations in hurricane boundary-layer depth (HPBL), turbulent mixing, and radial inflow. The strength of this feedback depends strongly on the surrounding continental environment. Under warm-moist conditions an extensive cloud shield generates longwave radiative forcing that broadens the inflow layer and deepens the HPBL. In warm-dry environments, dry-air intrusion fragments the onshore cloud shield, confining radiative forcing and its boundary-layer response to localized regions. Under cool-moist conditions environmental thermodynamic forcing increasingly competes with cloud-radiative forcing, shifting the secondary circulation rearward as cool continental air is advected cyclonically into the vortex. The cool-dry case is the limiting regime, in which cyclonic intrusion of low-θₑ air overwhelms the cloud-radiation–boundary-layer feedback, disrupts the secondary circulation, and accelerates vortex decay.

These experiments identify four distinct regimes of TCBL evolution during landfall and show that dominant control shifts from cloud-radiative to environmental thermodynamic forcing as continental forcing strengthens.