Ocean mixed layer depth estimation using airborne Brillouin scattering lidar: simulation and model

Abstract

The potential of Brillouin scattering lidar for detecting the mixed layer depth (MLD) was studied. We simulated the Brillouin scattering lidar signal in various water environmental parameters and developed an MLD retrieval model for Brillouin scattering lidar data. We first analyzed the theoretical maximum detectable depth for Brillouin scattering lidar in low-latitude sea regions based on the multiple scattering lidar equations. Subsequently, a theoretical method for calculating the Brillouin scattering frequency shift and linewidth was derived based on the international thermodynamic equation of seawater-2010 and the coupled wave equations. Then we used the theoretical method and the temperature-salinity (T-S) profile of the global Argo data in low-latitude regions to simulate the vertical profile distribution of the Brillouin scattering frequency shift and linewidth. Furthermore, we used a maximum angle method to estimate the ocean MLD in low-latitude regions based on the vertical profile distribution of the Brillouin scattering frequency shift and density in seawater. They are well correlated, which indicates that the frequency-shift component of the Brillouin scattering lidar signal for estimating ocean MLD is feasible and reliable. It appears that airborne or spaceborne Brillouin scattering lidar technology provides great potential for high-efficiency, large-area, and long-term monitoring of the global ocean MLD and upper-ocean water bodies.

Ocean Temperature Profiling Lidar: Analysis of Technology and Potential for Rapid Ocean Observations

“…The complete set of daily mean fields on the ECCO Data Portal: https://archive.podaac.earthdata.nasa.gov/ podaac-ops-cumulus-protected/-ECCO_L4_TEMP_SALINITY_05DEG_DAILY_V4R4/ and the complete set of daily MLD estimates are at: https://archive.podaac.earthdata.nasa.gov/podaac-opscumulus-protected/ECCO_L4_-MIXED_LAYER_DEPTH_LLC0090GRID_DAILY_V4R4/, Data Accessed: 1 September 2022. Global Ocean Heat Inventory Data: The global ocean heat inventory data [17] used for comparison with the ECCOv4r4 OHC (Figure 2 in [17]) was obtained from the World Data Center Climate data repository (http://www.lwdc-climate.de (accessed on 8 January 2024)), Identifier: doi:10.26050/WDCC/GCOS_-EHI_19602020_OHC, filename: GCOS_EHI_1960-2020_Earth_Heat_Inventory_Ocean_Heat_Content_data.nc. SIO Argo Data: The Argo program is part of the Global Ocean Observing System, and its data are made are freely available by the International Argo Program and the national programs that contribute to it.…”

“…Theoretical estimates of ocean sound velocity profiles obtained using World Ocean Atlas 2018 data [48] and an empirical expression for sound velocity [49] compared well (difference 1-2 m/s) with direct laboratory measurements of Brillouin scattering using a benchtop lidar instrument in several studies [50,51]. Brillouin lidar instruments could potentially be developed for either ship-based, aircraft, or satellite to carry out large scale observations of the upper ocean thermal and salinity structures and mixed layer depths [17,47].…”

“…Near real-time measurements of ocean Mixed Layer Depth (MLD) are important for providing information on ocean heat content that fuels hurricanes [15]. Yet the sparseness in the temperature observations necessary for adequate MLD estimation has necessitated the development of alternative methods to measure MLDs in indirect ways [16], as opposed to the more direct method that a remote sensing instrument, such as a Brillouin Lidar [17], could provide.…”

“…Airborne lidars can retrieve data down to 3-4 optical depths, effectively allowing for profiling through the ocean thermocline in 70% of the ocean [46]. Such lidar systems have already been proposed for aircraft [17,36,46] and satellite [47] remote sensing applications to detect Brillouin scattering at depth. Theoretical estimates of ocean sound velocity profiles obtained using World Ocean Atlas 2018 data [48] and an empirical expression for sound velocity [49] compared well (difference 1-2 m/s) with direct laboratory measurements of Brillouin scattering using a benchtop lidar instrument in several studies [50,51].…”

“…C [36], while another lab study demonstrated Brillouin lidar can retrieve sound velocity profiles with accuracy of 1-2 m/s [51]. Another instrument simulation study concluded that the detection of ocean MLDs using Brillouin lidar was feasible and reliable [17]. The expected global ocean variability of Brillouin lidar frequency shift was calculated using World Ocean Atlas 2018 data and a simulated Brillouin lidar [50].…”