High-resolution large-eddy simulations of the Riviera Valley: methodology and sensitivity studies

High-resolution simulations of the atmospheric boundary layer have recently become possible due to the increase in available computational power; however, the influence of parameterizations such as those for turbulence, land surface conditions and radiative forcing, and the configuration of initial conditions, lateral boundary conditions, and the numerical grids is highly situation dependent. Simulations are generally performed using "the best available" information and datasets. The situation is further complicated in the case of extremely complex terrain such as the European Alps, because the numerical parameterizations used in the simulation codes are largely based on theory and observations over flat terrain.

This paper investigates the steps necessary to achieve accurate simulations of the flow and temperature fields in the Riviera valley, located in the Alps in southern Switzerland. As our simulation tool we use the Advanced Regional Prediction System (ARPS), a non-hydrostatic, compressible large-eddy simulation code written for mesoscale and small-scale atmospheric flows. High-resolution large-eddy simulations (LES) are a powerful tool to investigate the effect of the multiple subgrid-scale parameterizations involved by providing a more detailed picture of the turbulence and flow structure.

We show that a straightforward grid nesting approach is not able to reproduce the valley winds and circulations observed under convective conditions during the MAP-Riviera project field campaign of 1999. Rather, careful initialization and the use of high-resolution land use and soil moisture data sets, among other considerations, are necessary to achieve satisfactory results. The sensitivity of the simulation results to changes in these numerical settings is explored by comparisons to observation data from soundings and surface station measurements on the Riviera Valley floor. It is found that even with strong local forcing, the onset and magnitude of the up-valley winds are highly sensitive to moisture and radiation processes in areas which are well outside the high-resolution domain. These processes are inadequately resolved on the coarser grid of the previous nesting level, and directly influence the flow structure in the high-resolution domain via its lateral boundary conditions.

The goal of this work is, ultimately, to accurately simulate, using all available data, the physical processes in this complex valley region to improve understanding of valley flow physics. This work focuses on the numerical aspects of the large-eddy simulation needed to accurately represent the Riviera Valley flow. A companion paper describes the physical and meteorological features of our simulated valley flows.