A simple approach to model optical turbulence for the unstable boundary layer at Cabauw (NL)

Atmospheric turbulence is a key factor when assessing the performance of earth-to-satellite optical communication links (Free Space Optical Communications, FSOC) as it impacts beam wander and intensity fluctuations of the laser beam. Models that allow to estimate vertical profiles of optical turbulence intensity, expressed as the structure parameters of the refractive index, C_n², either require costly simulations or are based on crude, empirical relations. In this work, we propose an efficient, physics-based method for estimating C_n² timeseries and vertical profiles in the convective boundary layer. We do this using the mixed layer column model CLASS (Chemistry Land-surface Atmosphere Soil Slab), which allows us to simulate surface fluxes of sensible and latent heat and the boundary layer height. Assuming linear profiles of the heat fluxes over the boundary layer, in line with mixed layer formulation, we derive profiles of C_T² and C_q², the structure parameter of temperature and humidity respectively, which together define the C_n² profile.

We will validate our approach for a reference case at the Ruisdael Observatory of Cabauw, the Netherlands (https://ruisdael-observatory.nl/cabauw/) against measurements of the surface energy balance terms, C_n² estimated from sonic anemometers and scintillometers, and the boundary layer height estimates from radiosondes and ceilometer. Next, we will present a sensitivity study of C_n² to surface and boundary layer properties and from these identify some typical cases representative for extremely favourable or unfavourable conditions for FSOC. The main conclusions from this work is that our approach fills a niche in C_n² profile modelling that better optimises model complexity versus model physics than the currently used approaches.