Simulation of turbulence recovery in the stable boundary layer over Pampa grasslands using the PALM model

Observations from the Pampa-2016 field campaign over southern Brazilian grasslands documented nights with a distinct nocturnal transition between two stable boundary layer (SBL) regimes a Very Stable Boundary Layer (VSBL) shortly after sunset, followed by a transition to a Weakly Stable Boundary Layer (WSBL) after midnight. This work investigates the physical mechanisms and forcing components required to reproduce such regime shifts using Large-Eddy Simulations (LES) with the PALM model. The simulations are initialized using convective boundary layer (CBL) profiles observed during the campaign and integrated through a complete diurnal cycle to resolve the evening transition. The numerical domain assumes horizontal homogeneity, representing the natural grassland footprint of the 30 m flux tower. The SBL regime transition is analyzed through the relationship between wind speed (V) and turbulence intensity diagnosed from the square root of the turbulence kinetic energy (VTKE). Model performance is evaluated against tower measurements at 3 m and 29 m and 3-hourly radiosoundings. The comparisons focus on turbulence quantities and in the vertical structure of potential temperature, including inversion strength and depth. Finally, the individual terms of the TKE budget are analyzed to assess the relative roles of radiative cooling and shear-driven mixing during the transition from a radiation-dominated VSBL to a turbulence-driven WSBL.