Representativeness of surface heterogeneity induced secondary atmospheric circulations in Large Eddy Simulations

Abstract:

Numerous studies have hinted at the key role of land surface heterogeneities for developing secondary circulations in the atmospheric boundary layer. Observational studies and large eddy simulation (LES) models have been used to understand the role of these circulations in boundary-layer development, cloud processes, and documented biases in measurements such as eddy-covariance energy fluxes. However, these studies have been limited in their ability to capture these events and robustly test hypotheses. In this study, we ask, to what extent can we combine a high-resolution observational data set with LES to overcome this gap and test how heterogeneity drives circulations?

Here, the Parallelized LES Model PALM was used to simulate the diurnal cycles for two days in early fall over a large (40x40 km) heterogeneous study domain. Observations to initialize, calibrate, and evaluate the forcings and circulations were taken from CHEESEHEAD19, a field campaign designed to intensively sample and scale land surface heterogeneity and the lower atmospheric response across a northern Wisconsin landscape.

The observations conducted during the field campaign were used to inform a land-surface model, coupled with soil, radiation and plant-canopy models. This approach, instead of using prescribed surface flux forcings, helps us to investigate surface-atmosphere feedbacks such as self-reinforcement of mesoscale circulations over the heterogeneous study domain. Augmenting the high-resolution observational data set, the LES can provide space-time continuous measurements virtual measurements that can help us diagnose and quantify these processes. The simulated spatial and temporal spectra and co-spectra are investigated for signals of larger scale, secondary circulations superimposed over the atmospheric turbulence. Ogives of the simulated co-spectra are analysed to estimate the contributions to surface-atmosphere fluxes at lower frequencies. The impact of these secondary circulations on the surface energy balance has to be studied further.