Coupled large eddy simulations of land surface heterogeneity effects and diurnal evolution of late summer and early autumn atmospheric boundary layers during the CHEESEHEAD19 field campaign

Observational studies and large eddy simulations (LES) have reported secondary circulations in the turbulent atmospheric boundary layer (ABL). These circulations form as coherent turbulent structures or mesoscale circulations induced by gradients of land surface properties. However, simulations have been limited in their ability to represent these events and their diurnal evolution over realistic and heterogeneous land surfaces. In this study, we present a LES framework combining the high-resolution observational data collected as part of the CHEESEHEAD19 field campaign to overcome this gap and test how heterogeneity influences the ABL response. We simulated diurnal cycles for four days chosen from late summer to early autumn over a large (49 x 52 km) heterogeneous domain. To investigate surface atmospheric feedbacks such as self-reinforcement of mesoscale circulations over the heterogeneous domain, the simulations were forced with an interactive land surface model with coupled soil, radiative transfer and plant canopy model. The lateral and model top boundary conditions were prepared from National Oceanic and Atmospheric Administration High-Resolution Rapid Refresh (HRRR) meteorological analysis fields. Comparing the simulated profile and near surface data with field measured radiosonde and eddy covariance station data showed a realistic evolution of the near-surface meteorological fields, heat and moisture fluxes and the ABL. The LES had limitations in simulating the night-time cooling in the nocturnal boundary layer. The simulated fields were strongly modulated by the imposed HRRR derived mesoscale boundary conditions, resulting in a slightly warmer and drier ABL. The simulations were run without clouds which resulted in higher daytime sensible heat fluxes for some scenarios.

Our findings demonstrate the capability of the PALM model system to realistically represent the daytime evolution of the ABL response over unstructured heterogeneity and the limitations involved therein with respect to the role of boundary conditions and the representation of the nocturnal boundary layer. The simulation setup and dataset described in this manuscript build the baseline to tackle specific research questions associated with the CHEESEHEAD19 campaign, particularly to address questions about the role of heterogeneous ecosystems in modulating surface-atmosphere fluxes and near surface meteorological fields as well as highlight the needed improvements in model representations of land-atmosphere feedbacks over vegetated environments.