Combining SAR and numerical modeling to reconstruct wind, waves and surface currents in Lake Garda, Italy

Water quality and ecology in lakes depend on mixing and transport processes, hence on hydrodynamics. Understanding these processes requires a conceptual model, usually supported by measurements and numerical simulations. In situ measurements are carried out using various devices, such as thermistor chains and ADCPs, which have high temporal resolution but are very local. Conversely, remote sensing can provide large-scale spatial information, but only in the surface layers and often at low frequency, as in the case of satellite imagery. As the main driver of lake motions is typically wind, knowledge of the meteorological forcing and its spatial distribution is also crucial and is often a major source of uncertainty in lake models. Numerical models can be used to integrate this information, which in turn allows them to be calibrated and validated.

In recent years, Earth observation products have become increasingly more used. Satellite imagery is often used in inland water quality studies (multispectral imagery), while radar products, such as those derived from Synthetic Aperture Radar (SAR), are mostly used in open waters such as oceans, seas, and very large lakes. In this contribution, we focus on the use of SAR imagery to reconstruct the wind field, surface currents, and wind waves, in a medium-sized lake (Lake Garda, Italy). For this purpose, we have developed a modeling chain consisting of three numerical models: Weather Research and Forecasting (WRF), providing the spatio-temporal distribution of meteorological variables; Delft3D, forced by WRF, simulating the three-dimensional flow field and heat and mass transport; SWAN (Simulating Waves Nearshore), modeling the surface wind waves with a two-way coupling with Delft3D. Following a common approach, Delft3D was calibrated against pointwise in-situ measurements (vertical profiles of velocity and temperature, floating drifters’ trajectories) and validated considering spatial patterns of temperature and turbidity obtained from multispectral imagery. 

As a novel element of the analysis, we used SAR backscatter amplitude and Doppler anomaly obtained by COSMO-SkyMed (CSK) to reconstruct wind speed and Surface Radial Velocity (SRV), respectively, by applying a Geophysical Model Function (GMF). The wind field reconstructed for Lake Garda with this approach shows consistent spatial distribution and magnitude when compared to the WRF results. The SRV obtained from CSK, on the other hand, shows a qualitative agreement with the results obtained from Delft3D+SWAN, but an overestimation of the magnitude of the flow current. 

We are now planning an intensive field campaign in winter 2025, with in-situ measurements in parallel with satellite SAR and ground radar observations, to revise the GMF and further improve the understanding of how wind waves and bulk currents contribute to the SAR signal. The final goal is to derive detailed information of wind speed, wave amplitude and surface currents directly from Earth observation even in medium-sized lakes.