Aleksei V. Ermoshkin

This manuscript continues a series of papers devoted to the study of bio-optical characteristics of the Volga River waters in the context of development of regional bio-optical models. A particularly weak point in this effort is the limited knowledge of dissolved organic matter (DOM): its component composition, spectral absorption characteristics, and the lack of satellite-based assessment algorithms. Using excitation–emission matrix fluorescence spectroscopy, we examined the fluorescent fraction of DOM of surface water layer of the Volga River and its tributaries in the area from the Gorky Reservoir to the Volgograd Reservoir, a stretch spanning over 1500 km, in the period from May to September 2022–2024. Four fluorescent components were validated in parallel factor analysis. The ratio of fluorescent components was mostly stable, while their fluorescence intensities varied a lot. For example, the fluorescence intensity of the DOM of the Gorky Reservoir and the Kama River differed by more than 2.5-fold. The highest FDOM fluorescence was found in the Gorky Reservoir. Downstream, it decreased due to the inflow of the Oka and Kama rivers. The influence of small rivers such as Kerzhenets, Sundovik, Sura, and Vetluga was insignificant. It is demonstrated that neither conventional remote sensing techniques (LiDAR) plus in situ measurements of DOM with a probe nor DOM absorption at 440 nm allows probing all the fluorescent components, so their efficiency is determined by the correlation of fluorophore group content.

In this paper, we discuss the results of hydroacoustic sounding in the frequency range of units of kHz, conducted during voyage No. 90 of the research vessel “Akademik Mstislav Keldysh”. The employment of a vertical antenna array and electronic phasing to the recorded data made it possible to determine both the reflection coefficient from the bottom at incidence close to normal, and the diffuse scattering coefficients at oblique angles for the surface and the bottom. Based on the processing of experimental data, and with the help of computer modeling, an analysis of the structure of hydroacoustic signals scattered by the bottom and free surface of water was carried out. An approach combining the Green’s function and the scattering function was used to model the reverberation signal. The models of formation of the Doppler spectrum of the scattered acoustic signal were refined, taking into account the influence of sound propagation conditions in the marine environment. The comparison of the results of experimental studies of bottom reverberation in the waters of the Barents and Kara Seas with numerical calculations of the ray structure of the acoustic field demonstrates good agreement.

Data on the current structure in most reservoirs of the Volga Cascade were obtained in the last century and correctly describe the current structure only in general terms. The paper presents current structures at different distances from the HPPs in the Gorky, Cheboksary, Kuibyshev, and Volgograd reservoirs. The present work is a brief review of the existing database of current measurements in the water area of the Volga Cascade reservoirs. The data were obtained during field measurements in 2023 and 2024 using an acoustic current profiler. Longitudinal sections of the river part of the Cheboksarsky and Kuibyshev reservoirs, spatial structures of currents in the lake parts of the considered reservoirs are demonstrated. It is shown that the average current velocity in the river part of the reservoir decreases with distance from the HPP dam. The structure of currents in the lake part of the reservoir has high variability depending on the flow regime through the HPP. Local features of currents related to flow regulation at hydrosystems are presented, which include formation of whirlpools, changes in velocity and current directions.

The presented paper opens a series of articles devoted to studies of the bio-optical characteristics of the Volga waters in the context of creating regional bio-optical models accounting the transformation of the water optical properties from north to south. The preliminary results of field measurements carried out in 2023 at the middle Volga are presented. Measurements were performed with high spatial resolution — 3 m. The spatial distributions and variations of various hydro-optical and hydrological characteristics, including water brightness spectra and the content of dissolved and suspended substances that determine its color (chlorophyll a, suspended solids), data on water temperature, electrical conductivity, salinity, dissolved oxygen content, flow velocity and direction, were investigated at different spatial scales. The influence of different Volga tributaries was assessed, including the seasonal variability of bio-optical characteristics in the areas of the confluence of the Volga and Oka, and the Volga and Kama. Cross-correlation estimates were given. The obtained results expand an actual picture of the studied water area and will be used in the analysis of satellite data and the development of bio-optical models.

This paper theoretically and experimentally considers the problem of determining the significant wind wave height based on coherent radar sounding of the sea surface. It is shown that the modulation of the velocity of radio wave scatterers by the orbital velocities of energy-carrying waves makes it possible to find an unambiguous relationship between the Doppler velocities measured with a coherent radar and the significant wave height. The problem of the influence of shading of sea surface areas by wave crests during sounding at small grazing angles is studied depending on the radar installation height and the significant wave height. The paper uses an original empirical model of the Doppler velocity of microwave radio wave scatterers on the sea surface, which demonstrates a good agreement with the experimental data. As a result, a remote sensing method for determining the significant wind wave height is proposed, which does not require calibration and is applicable from both fixed bases and moving carriers. The accuracy of the proposed remote sensing method is estimated based on experimental data obtained at a stationary oceanographic platform in the Black Sea in 2021. The correlation coefficient between the significant wave height according to contact and remote measurements was 0.9 at a root mean square error of 15 cm.

This work presents the optical methods for measurements of physical parameters on sea surface also covered by ice by using stereo cameras. New method of images with sea ice processing is offered. It can detect areas of open water and sharp edges on ice, which influence on radar signal refraction. This method is based on area of interest detection on optical images, statistical parameters for each area calculation, classification and local level definition to recognize needed structures. By using stereo system perspective distortions can be corrected and physical parameters of wind waves and sea ice can be calculated. This method was probe on stereo images obtained from 90th voyage of R/V «Akademik Mstislav Keldysh» in the Laptev Sea.The results of ripple waves velocity measurements in case on open water by using development previously stereo method is presented. Comparison with Doppler shift of microwave signal is made.The methods presented in this paper are of interest in field experiments simultaneously with the use of coherent radar stations for the quantitative interpretation of radar data, as well as for remote monitoring of the sea surface and ice conditions methods development.

The article describes an original model of the Doppler velocity of microwave radio wave scatterers on the sea surface. Theoretical modeling in a wide range of hydrometeorological conditions demonstrated the possibility of reconstructing the significant wave height, current velocity, and direction based on the Doppler velocity data obtained during coherent radar sensing of the sea surface. It is shown that information on the current velocity and direction is contained in the average value of the Doppler velocity. Variable values of the Doppler velocity arise due to the modulation of the radio wave scatterers velocity by the orbital velocities of the energy-carrying waves. An unambiguous relationship was found between the Doppler velocities measured by the coherent radar and the significant wave height, current velocity, and direction. As a result, a remote method for determining the significant wind wave height, current velocity, and direction is proposed, which does not require calibration and is applicable from both fixed bases and moving carriers. The experimental testing of the proposed remote method was carried out based on experimental data obtained at a stationary oceanographic platform in the Black Sea in 2021.

This paper is devoted to a series of the first field subsatellite experiments conducted in the waters of the Kuibyshev Reservoir (Kama estuary) in 2023. Simultaneously with ship-based measurements of current and wind fields, as well as chlorophyll-a concentration, two high-spatial-resolution satellite scanners surveyed the study area of the reservoir. From sequential images, current fields were reconstructed using the standard maximum cross-correlation (MCC) method, which were then compared with measurements from the Acoustic Doppler Current Profiler (ADCP). In certain parts of the water area, satisfactory agreement was obtained between the reconstructed currents and direct measurement current data. And in those parts of the water area where a significant discrepancy between the ADCP and MCC data was recorded, the possible reasons for the discrepancies were analyzed. Preliminary estimates of the parameters that have a significant impact on the possibility of reconstructing currents using the MCC method in inland eutrophicated water bodies have been made, and some limitations of the MCC method as a whole have been identified. Possible ways of further development of the method are analyzed.

The key area of the Arctic Ocean for atmospheric venting of CH₄ is the East Siberian Arctic Shelf (ESAS). Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate. The development of remote sensing techniques for gas seepage detection and mapping is crucially needed for further applications in the ESAS and other areas of interest. Given the extent of the seepage areas and the magnitude of current and potential future emissions, new approaches are required to effectively, rapidly, and quantitatively survey the large seepage areas. Here, we consider the main features of gas seep detection on the sea surface in the characteristics of wind waves and radar signals. The kinematics of wave packets based on the kinetic equation for the spectral density of the wave action of surface waves is described. The results of a full-scale experiment on the remote radar observation of a model gas seep to the sea surface in the radar equipment signals are considered. The characteristic radar signatures of the gas seep in a wide range of hydrometeorological conditions, the parameters of which were recorded synchronously with the radar mapping, were determined. The results of the first radar observations of natural methane seeps on the ESAS are presented, and their radar contrasts are evaluated. The theoretical conclusions are in good qualitative agreement with the results of the model experiment and field studies and can be used for further research in aquatic areas with potential gas seepage, both of natural or anthropogenic origin, such as bubbling release from broken underwater gas pipelines.