Coastal boundary layer gravity wave characterization by scanning lidars and a microwave radiometer

Since the last two centuries, gravity waves have been an atmospheric enigma that scientists strive to fundamentally define. These events occur throughout the earth’s troposphere to the mesosphere, but full physical understanding of this phenomenon within the boundary layer remains out of reach. Generally, gravity waves are caused by airflow within stable atmospheric conditions losing stability due to a jump in air density and/or wind speed (Lyulyukin et al., 2015). Because this trigger of instability occurs in hydrostatically stable conditions, the flow becomes dynamically unstable resulting in continuous oscillations in unstable vertical arrangements that can last for long distances and periods of time (Mayor, 2017). Buoyancy and shear induced gravity waves induce vertical transport of heat, momentum, energy, and water vapor. This redistribution of key atmospheric components yields important consequences within the boundary layer. Yet, because they evolve in mostly stable atmospheric conditions and can occur at the sub-meso scale, they are considered less critical for atmospheric models and thus not precisely represented.

Boundary layer measurements of these gravity wave events provide insight for improved parameterizations of numerical solutions. As these events require specific atmospheric conditions to occur, however, it proves rare to obtain continuous and sufficient data when and where these events happen. Thus, most atmospheric observations lack substantial information to derive key wave information (Mahrt, 2014).  

 The Centre for the Testing of Environmental Sciences Technology (C-TEST) conducted a recent remote sensing measurement campaign in 2025 on the coast of Blyth, where gravity wave events were detected. Four significant gravity wave events were captured between 300-2000m in altitude from its suite of instruments, including a MWR (microwave radiometer) and two scanning lidars. One lidar was continuously measuring vertically, providing consistent information on vertical wind speed and backscatter content. The second lidar alternated every 5 minutes between vertical measurements and DBS (doppler beam swinging), providing horizontal wind speed and direction.

This analysis aims to promote remote sensing measurements to provide insight on gravity wave characterization and evolution throughout the boundary layer. The analysis is still ongoing, but preliminary results show interesting characterization of the events. Buoyancy and shear regimes have been defined by evaluating the backscatter, wind speeds, and potential temperature profiles. For some events, the Taylor Goldstein linearization relationships are applicable as the horizontal and vertical wind components are in phase, whereas the backscatter density is 90˚ out of phase (Nappo, 2012). The combination of continuous temperature, humidity, wind speed and backscatter profiles from all instruments give insight into the complex coastal conditions that lead to the development and dissipation of these gravity wave events. Quantification in momentum flux, turbulent kinetic energy, and other important atmospheric profiles additionally contribute valuable information for improving numerical modeling and physical understanding.