Characterization of the Aerodynamic Roughness Length of the Marine Atmospheric Boundary Layer in Coastal Regions Using Doppler LiDAR Measurements

The mutual interaction of the waves and wind field at the sea surface plays a crucial role in the marine and coastal environment. To model transfer of heat, moisture, and momentum fluxes between the sea surface and the wind field, it is important to take into account turbulence dynamics occurring in the wind field, wave motion, and their mutual interactions [1]. The distribution of turbulence eddies within the marine atmospheric surface layer (MASL) is more complex than that occurring in the onshore atmospheric surface layer [2]. Air-sea interaction is a complex dynamic process that occurs between the lower part of the MASL and the ocean surface. In this region, the dissipation rate of turbulence kinetic energy is expected to be larger than turbulence production. This imbalance is due to the movement of large-scale eddies that are dominated by the pressure-wind term acting on the ocean surface [2, 3].
There is a noticeable gap in information regarding the influence of coastal effects on the MASL during both onshore and offshore wind conditions. The aerodynamic roughness length (z₀) is important for predicting the wind profile, specifically in terms of wind shear. This is one of the fundamental parameters in a boundary layer and needs to be characterized and modeled for improved weather prediction models. Moreover, friction velocity (u_*) is another important parameter for characterizing turbulence and the wind velocity field in the boundary layer. Since wave breaking is a common phenomenon in coastal regions, it has a significant impact on both parameters and can create notable differences compared to open-ocean conditions.
The model COARE 3.5 uses the Charnock model to predict z₀, and its constant (α) varies with different wind speed regimes [4]. Furthermore, the wind speed at 10-m height (U₁₀) is pivotal in influencing the variation of the α value. In the study on MASL wind field in coastal region, we aim to characterize and model α as a function of U₁₀ for coastal regions. Although several models for α exist in the literature for open-ocean cases [5, 6, 7], they seem not to be very accurate for applications in coastal regions. To address this knowledge gap, a comprehensive investigation in coastal regions, leveraging spatial and temporal measurements, is imperative. In this context, Light Detection and Ranging (LiDAR) emerges as a highly valuable measurement instrument for capturing wind variability. In this field study, we investigate the variability of the vertical profiles of streamwise velocity in the marine atmospheric boundary layer (MABL) collected at a site located along the U.S. coast of the Gulf of Mexico. Wind data were collected over an offshore area near the coastline with a scanning Doppler LiDAR, while simultaneously monitoring the corresponding open-ocean conditions through an instrumented buoy (NOAA buoy station #42035). The scanning Doppler LiDAR was deployed at a distance of about 100 m from the shoreline to perform fixed-point measurements over the water surface, thus collecting wind data relative to the mean sea level (MSL) in the shallow water wave region. The LiDAR measurements were performed along the mean wind direction measured at a 10-height through periodic DBS scans, sampling frequency of 2 Hz, elevation angle of 3.5°, and range gate of 18 m resulting in a vertical resolution of about 1 meter.
Observations from this study suggest that, in the open ocean region, the aerodynamic roughness length shows an increasing trend with increasing U₁₀, whereas in the coastal region, the roughness length decreases as U₁₀ increases. This observation highlights the influence of a physical process associated with shoaling waves in coastal region that contributes to the resulting aerodynamic roughness length. Moreover, a notably higher value of friction velocity is observed in the coastal region compared to the open ocean region. The typical models for the Charnock constant seem to not perform accurately for the coastal region. Therefore, we propose a power-law model for the Charnock constant as a function of wind speed for applications in coastal regions.

References
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